Gas Water Heater Vent Code & Standard FAQs

Q&A water heater venting codes

Safe & Proper Installation Standards

New installer of water heater did not seal the flue connection

I recently had a new water heater installed. They changed out the flue that goes into the chimney. Is it the installation companies responsibility to seal around the flue pipe that leads into the chimney? 2023-02-13 by Krista

Reply: the water heater installer must include proper flue-to-chimney seal

@Krista,

Yes. That would be part of a complete and safe installation and would typically be included in the installation directions for your water heater.

If you tell us the brand and model of your water heater, we can research this further.

Can I use existing power exhaust for another device to also vent my new gas water heater?

I’m replacing a 50 gal 40,000 btu NG water heater with another same size. Can I take the “power exhaust” off old same sized old unit just four screws

Then install on new “natural draft” unit of same size? Saves me $400 2022-09-27 by Zehringer David

Reply: don't vent a natural draft water heater through a power-exhaust vent system

@Zehringer David,

Sorry but I don't understand how you could possibly vent a "natural draft" water heater out through what was a power-exhaust vent system.

Typically a power-exhaust vent uses a fan to blow the combustion products out through a side-wall.

Watch out: For a natural draft water heater, excessive draft causes by a power vent risks improper and unsafe water heater operation.

A natural draft gas appliance relies on the draft developed in a properly-sized metal (or other) chimney to safely vent combustion products..

Just venting out through a wall or through a too-short chimney is very dangerous, risking backdrafting and dumping fatal carbon monoxide gas into the building.

Check the installation manual for your new water heater and follow its venting instructions.

Follow up:

I currently have a 50 gal 40,000 btu NG hot water heater And I have a unused 180,000 btu. Furnace with a built in water heater the same furnace unit that STATES it will also produces 135 gals hot water per hour, are these two output comparable outputs given different types of units? Or does the tank unit out produce hot water in a long use application.

There are two bathrooms, two kitchens and two laundry rooms currently handled fine with the 50 gal tank. Can the tankless 135 gal per hr keep up like the 60 gal tank unit ?

Reply: can I take old “power exhaust” off then install new “natural draft” unit?

@Zehringer David,

Since we don't have the brand and model of your heating boiler, and just using the data that you've provided, I think it's likely that your heater, that is the one that you've not used, is including a tankless coil that can produce hot water at the rate of about two and a half gallons per minute. That's a flow rate not necessarily a total quantity even though that total quantity was given.

Follow up:

The power vent system must draw a specific amount of combustion air through the burner, for a 40,000 btu burner a constant, X this is in-order to properly fire a 40,000 btu heater, this correct.

Therefore, changing the burners should not create any excess or any deficiency in the air flow from unit to unit, if the units ratings are properly stated and correct.

Therefore changing the water heater unit shouldn’t present any back draft hazard then should it.

Power vented units are used primary where 2000 square feet of available air space is not available to draw on for combustion air and B. a side vent is needed.

Reply:

@Zehringer David,

Thanks for the discussion - though I'm not sure we have all that's been said quite correct.

I agree that a system may use a blower to obtain combustion air but as you doubtless know there are many designs that vent out through a sidewall and plastic venting whose exhaust vents could not then be used safely for an atmospheric vented gas water heater.

Watch out: to be safe, Let's name the water heater brands and models involved so that we or anyone can look at their installation requirements as given by the manufacturer.

Gas water heater vent slopes "down" to the chimney

My slope is 2” downward before going upward is that dangerous on my gas water heater. 2022-08-30 by Dell

Reply: is 2” downward before going upward dangerous on gas water heater

@Dell,

Watch out: It's very possible that your heater isn't venting correctly.

Check the draft hood for burn marks or to see if you feel exhaust spilling out there. If you do, it's definitely unsafe.

Watch out: even then, I worry that in changing weather conditions, such as in cold weather, the heater may not vent properly. The risk is fatal carbon monoxide (CO) gas poisoning.

 

Is this water heater vent pipe installed correctly?

Is this installed correctly? 2022-05-17 by Laura

Mod reply: check owner's manual for proper installation of this gas water heater

@Laura,

We can't say that a water heater is installed properly from just a partial snap of a portion of the equipment.

If your only question, considering you're posting on a water heater vent page, is about the vent - we can't see all of it, and can't even see underneath that foil tape to assure that the metal exhaust flue vent connector is properly mounted to the draft hood over the water heater.

When the heater is running do you feel hot exhaust spilling out at that vent opening? If so, the vent isn't working properly.

Do you have working CO and smoke detectors properly installed, located, tested in your home?

Do you have a copy of the installation and owner's manual for your water heater? If so, you can certainly check the installation requirements given there.

see WATER HEATER AGE & MANUALS - for all brands - live link at Recommended Articles on this page.

Beyond that,

What I can see doesn't raise a serious concern though I'd want to know

• that there are appropriate shutoff valves
• that the vent is completed properly and safely
• if you're in an earthquake zone that seismic restraints are in place
• that the heater temperature is set to a safe level
  etc.
  

Can I vent both a LP water heater and LP furnace through the same vent?

Can I vent both a LP water heater and LP furnace through the same schedule 40 PVC sidewall vent (chimney) if both appliances are located on the same floor (basement) and in close proximity to each other? 2022-04-01 by Matt

Reply: each direct vented appliance should have its own intake and exhaust vents

@Matt,

I wouldn't try that.

Typically each direct vented appliance expects to have its own intake and exhaust vents.

You don't give a diameter or length or other features of your exhaust system but it's possible that you would not be providing adequate the combustion air or exhaust, either of which could cause unsafe conditions even risking a carbon monoxide poisoning hazard.

We are pretty much obligated to stick with a manufacturer's installation instructions.

 

Are the section-connections in my water heater flue vent safe?

Water heat 4" venting pipe connected to elbow appears short, only about 1/2" is inserted into elbow and 3 screws holding. Seam is covered with foil tape. The overall vent pipe is secure with ceiling clamp and does not move. Question is whether the 1/2" insert into elbow is okay with 3 screws and foil tape?
please advise
Thanks
2021-03-24 by Bob

Reply:

@Bob,

With the obvious qualifier that no one can perform a safety inspection by text alone, what you describe sounds secure to me with three screws and foil tape holding the connection between two metal flue vent Parts together.

 

Some water heaters are designed to vent through a side wall, others, not.

We have a propane hot water tank in our basement that the previous owner vented out the side of the basement wall.

Apparently the hot water tank is one that is suppose be vented through a chimney. Is there a replacement propane water heater that we can get that vents through the wall and if so what would you suggest and what would we need to get it installed properly? 2021-03-13 by Alesia

Reply:

@Alesia,

Certainly, there are both tankless- and tank-type propane and natural gas (shown below from AO Smith as its Signature 40-Gallon Short 6-Year Limited 38000-BTU Natural Gas Water Heater) that can be vented directly out through a side wall.

Check with your local plumbing or building supplier

 

Venting a gas water heater inside of an enclosure is dangerous, could kill someone

For a a natural gas fired water heater installed outdoors in a galvanized water heater enclosure, are you required to cut through the top of the enclosure and vent above the roofline? Or is it acceptable to allow it to vent inside of the outdoor enclosure? 2021-02-02 by MBriggs

Reply:

MB

Watch out: Venting a gas water heater inside of an enclosure is dangerous, could kill someone, and even if nobody is inside the enclosure to be killed, risks improper and unsafe operation, inadequate combustion air, production of fatal carbon monoxide, and perhaps other hazards.

A gas water heater must be vented to the outdoors.

If cutting through the roof is inconvenient or difficult, probably possible to vent out through a side wall and then up to an adequate height above the roofline.

Can you share common exhaust vent for two direct vent gas water heaters?

Can you vent two water heater through one single PVC pipe? These are power vented water heaters. 2021-01-25 by Mike

Reply:

Mike:

A condensing or high efficiency heater that vents directly out through a side wall might use PVC vent materials as approved by the manufacturer, but none of those vents that I have seen or installed are shared; further, there is a required separation distance between them.

However the final word on your question should come from the manufacturer of your specific water heater brand and model.

For other readers: A conventional oil or gas fired water heater vents through a metal flue vent connector and a UL-listed chimney (metal such as a B-vent, or masonry) never through plastic.

To 'seal' a hot water flu vent pipe where it runs through the roof

To 'seal' a hot water flu vent pipe where it runs through the roof may I safely use a 3M foil tape with the temperature rating of either 225 degrees Fahrenheit or 300° f? Or, wood the heat of flue vent pipe make that foil tape into a combustible product? Thank you! 2020-10-22 by Mark

Reply:

Mark:

If you are sealing a gas fired water heater flue and are so far away from the heater (presumably on a basement or first floor) as to be at the roof, then sealing a metal flue joint with foil tape ought not cause an issue; I'm not sure why that's needed if you're using a UL-listed B-vent as those sections normally lock together.

Vent a standard gas water heater horizontally through the sidewall of house provided

forgot to mention it is a Gas heater

can I vent a standard type water heater thru the side of house. 2020-09-25 by tom

Reply:

Tom

You can vent a standard gas water heater horizontally through the sidewall of house provided it's venting to a chimney of proper materials and height OR it is connected to an approved direct vent system.

The proper chimney material and height are absolutely critical.

Watch out: The danger if you simply vented a conventional water heater exhaust straight out through the wall and stopped, or stopped with a short stub of a chimney is that you would have inadequate draft. In turn that risks dumping fatal carbon monoxide into the building - which could kill someone.

Follow up:

Thank you ...I will need to find out the proper materials and height...at the moment, I don't know.

Reply: need a B-vent that extends sufficiently above the roof

Tom,

Typically you need a B-vent that extends sufficiently above the roof

TYPE B-VENT CHIMNEYS describes that material, and links at the end of that article address necessary height.

What is the horizontal run on a vent

What is the horizontal run on a vent? 2020-09-01 by Patrick

Reply:

Patrick,

You're going to want to find the manufacturer's installation specifications for your specific water heater brand and model. The manufacturer will tell you the maximum horizontal run that they allow. It's not the same for all products.

Can a vent system be installed which has a 3’ elevation drop be corrected by adding an in-line fan?

Can a vent system be installed which has a 3’ elevation drop be corrected by adding an in-line fan? 2020-07-23 by Denny

Reply:

Yes you can add a power vent system where necessary, provided your water heater manufacturer approves.

Watch out: A more subtle concern is creating excessive draft that causes improper gas water heater operation that could then be unsafe.

Backdraft issues when sharing a chimney with a natural draft water heater and a fan assisted furnace

Hi, I live in NJ and had backdraft issues with the venting of my fan assist 140,000 BTU gas furnace and 50 gallon 40,000 BTU gas natural draft water heater. We had a Forever Flex 30' 5" liner installed, and I have been smelling fumes in my home ever since. The installing company insists they put the correct liner in.

I had an inspection by a certified chimney inspector, and he said the liner should be 6". He also said that because one of the units is fan assist and one is natural draft, they should be joined together in the chimney, and not before they enter the chimney which is what the installer did. Previously, they were joined in the chimney.

I obtained a permit for this work and had it inspected, but there seems to be a difference of opinion.

All I know is I smell fumes, and I shouldn't. Now my water heater is leaking, and my plumber wants the venting issues fixed before he installs. Any help would be greatly appreciated. Thank you. 2020-07-02 by Pamela Nardone

Reply:

Pamela

Because one of the units is fan assist and one is natural draft, they should be joined together in the chimney, and not before they enter the chimney which is what the installer did.

That sounds important to me.

Watch out: But lets get someone to actually measure the draft so we can get past the speculation - otherwise your heating system is unsafe and risks dangerous carbon monoxide poisoning or an over-draft that causes the water heater to work improperly.

Possible to share a flue with two gas fueled appliances?

Draft venting water heater in basement venting stainless liner in chimney.

Chimney was taken down below roof line and roof sealed. Is it possible to connect liner to rigid b-vent in attic (seal top of chimney around connection with metal flashing/cap) and wye into existing furnace (in attic) b-vent through roof approximately 10 feet away from chimney entrance point in attic? 2020-04-30 0 by Geoff M.

Reply:

Geoff

Forgive you but I am a little confused about this question.

If you're asking whether it's possible to share a flue with two gas fueled appliances, the general answer is yes provided the vent has adequate capacity.

Follow up:

thanks for the response. That's the first part of my ask. I guess the second part is can you transition flexible chimney liner to rigid b-vent to make the 10' connection from where the chimney now ends to where the existing furnace vent pipe goes through the roof.

Reply:

I would identify the brands involved and ask the manufacturer for a listed, approved connector.

Forcing a gravity-vented gas-fired water heater to vent "down" is defying gravity, and increasing fatal risk

If I install a gas hot water heater that has the outlet flue at 49 and one-half inches above the floor, can I attach to a chimney flue that opens at 50 inches above the floor? The connecting pipe would be tipped slightly down going to the chimney. 2020-04-25 by Monte

Reply:

Not really, Monte.

Forcing a gravity-vented gas-fired water heater to vent "down" is defying gravity, impeding the draft, and risking dumping fatal carbon monoxide gas into the building.

Solutions:

- a shorter water heater
- moving the chimney flue vent connector thimble up higher (watch fire clearances from nearby wood or other combustibles)
or
- (only if the heater manufacturer agrees it's safe ) using a power venting system

Frankly if my water heater vent connection to the chimney sloped down 1/4" across a foot or two I'd not panic, but from your description, if the outlet at the top of the water heater is 49 1/2" above the floor you cannot possibly connect it safely to a chimney opening whose bottom is essentially at 50 inches, because you still need the additional height of the draft hood and the height required for the 90 degree elbow to make the bend to head over to the chimney.

Reply: gas fueled appliances vents and chimneys must slope up not down

...

Not really, Monte.

Forcing a gravity-vented gas-fired water heater to vent "down" is defying gravity, impeding the draft, and risking dumping fatal carbon monoxide gas into the building.

Solutions:

- a shorter water heater

- moving the chimney flue vent connector thimble up higher (watch fire clearances from nearby wood or other combustibles)
or

- (only if the heater manufacturer agrees it's safe ) using a power venting system

Frankly if my water heater vent connection to the chimney sloped down 1/4" across a foot or two I'd not panic, but from your description, if the outlet at the top of the water heater is 49 1/2" above the floor you cannot possibly connect it safely to a chimney opening whose bottom is essentially at 50 inches, because you still need the additional height of the draft hood and the height required for the 90 degree elbow to make the bend to head over to the chimney.

You can see from this example of a 3-inch diameter metal flue vent 90 degree elbow from PelletVent that the elbow alone is going to need more than 5 1/2" of height to make the bend from vertical to horizontal - actually it's more like a total of 7" from the top of the heater.

Add to that the height of the draft hood,

Vent gas water heater out through side wall with PVC?

Can I vent a gas water heater designed to be exhausted through PVC pipe out the side of the house up the chimney that my furnace exhausts from? 2019-11-07 by Harry

Reply:

Watch out: No. Venting a conventional or typical gas fired water heater straight out through a sidewall using plastic pipe would be failure to follow the manufacturer's instructions and most likely would be unsafe.

Gas water heater be placed in a small closet?

Can our gas water heater be placed in a small closet? 2019-05-08 by Bill

Reply:

Bill,

I don't know about your specific water heater as we don't know it's brand and model, but there are some gas fired water heaters approved for installation in a closet or similar small area such as some mobile home or trailer heaters.

Watch out: Of course you need to provide combustion air. And you don't want a gas water heater that can vent into or consume oxygen from a sleeping area.

Steel brackets that are sold for securing metal chimneys and vent pipes to building walls

how do I support the double walled vent pipe? do they make some type of bracket? Thank you for the article, very helpful. 2019-04-26 by Jim Bunton

Reply:

Jim

Indeed there are steel brackets that are sold for securing metal chimneys and vent pipes to building walls. If you need to hang from overhead you may find the length of perforated steel strapping will also be adequate.

Remember also check your fire clearance distances.

Is it safe and to code to place a vent cover on the outside end of the vent?

I have a gas water heater vented through a side wall. When the wind blows through the vent it blows out the pilot light. Is it safe and to code to place a vent cover on the outside end of the vent? 2019-01-23 by Chris Hodge

Reply:

Chris

Probably not. Any obstruction close to the exhaust vent risks of fatal carbon monoxide Hazard inside and other improper operation of your heater.

the wind problem is one that's been reported before. And in fact we have an article on the topic here at inspectapedia.

Sometimes we can create a Wind Block that's sufficient distance away that the manufacturer will accept it.

Let's get the brand and model of your direct vent system and see what the manufacturer considers safe.

PLEASE see this article

DIRECT VENT WIND PROBLEMS

where we address your question in more-detail.

Replace And/or Repair Of Venting

Replacing our 28 year old Lennox Whisper Heat 80% gas furnace

I am considering replacing our 28 year old Lennox Whisper Heat 80% gas furnace but must go high efficiency. I would like to keep my conventional gas hot water tank which is common vented with the furnace into a 6” B-vent to the roof 30’ above.

I believe the B-vent is too large for just the tank vent.

My question, is there an affordable option of 3” or 4” metal flex vent I can run down the B-vent as chase to my tank? 2023-01-22 by Brian

Reply: risk of inadequate chimney draft with just water heater venting into chimney

@Brian,

Smart question. My opinion is that even at a smaller diameter vertical flue for just the water heater, the total chimney height might be too much for the heater to develop a draft.

So I would start by identifying the water heater brand and model, grab its installation manual (we provide hundreds of them here as free PDF down loads), and see what venting distances and sizes the manufacturer says are acceptable.

Also you might look into converting the water heater to a direct vent exhaust if the model permits that.

Watch out: insufficient draft at a gas fired water heater risks dangerous carbon monoxide release and poisoning of building occupants

Follow up:

@InspectApedia Publisher, thank you. If I go the route you suggest, could I use the then abandoned 6” b-vent to supply combustion air to the new furnace and water heater? I would like to minimize the number of holes cut through the building envelope.

Reply: use abandoned water heater vent to provide combustion air to new furnace?

@Brian,

Probably, yes, but I would want to be sure that not just the diameter but the length of run and its orientation (vertical, bends, etc.) will never short-change the equipment of combustion air. That's why I want to start with the specific brand and model of equipment.

Standard calculations look at input BTUh when calculating chimney and vent sizes and combustion air requirements - varying of course by equipment type.

Has improper venting made my water heater unsafe or shortened its life?

Bought a new house with a 50 gallon power vented Bradford White gas water heater.

However, the installer failed to install the manufacturer recommended condensation drain tube so condensation flowed back into the fan and tank and percolated around the hose clamps on the vent pipe corroding the vent fan chassis and the exterior of the tank all the way down to the pan.

After inspection by another licensed plumber I contacted the installer to correct and he simply added the condensation drain tube that was initially omitted.

My questions are 1) has the usable life the unit been shortened from a year of operation after improper installation, and 2) has it exposed us in any way to chronic low level CO due to the improperly vented appliance? 2021-11-17 by William in Missouri

Reply: condensation of the water heater controls, components, tank, vents, piping, etc means water heater's life has been reduced

@William in Missouri,

Shown here is a Direct-Vented Bradford White gas water heater.

Thank you for an important safety and water heater life question on the effects of mis-handled condensate.

My general OPINION is that any condensation of the water heater controls, components, tank, vents, piping, etc. certainly introduce at least a reasonable risk that the heater's life has been reduced, but a more-accurate answer ought to come from a careful inspection of the equipment to see its actual condition.

The possible effects of the improper condensate drain on the heater range from trivial-cosmetic to serious, shortened life or unsafe equipment.

So let's base an answer on a bit of objective data: an inspection.

About the low-level CO exposure, we have no basis yet for claiming that there was ANY abnormal CO leakage or exposure from your water heater. To make that assertion you'd need inspecting and testing that found that such leakage was occurring.

If you had a CO detector properly installed, located (nearby) and tested to be sure it's been working, and if that device never sounded, then it's not likely that there was a measurable hazard.

A typical home / residential CO detector (Carbon monoxide detector) uses a sensor that detects very low levels of CO, down to perhaps 10 ppm.

That's to enable the detector to sound an alarm at different CO levels depending on the duration of the presence of CO that the device has measured.

Kidde, a major vendor of fire and safety equipment gives this good representative data of the response time/level of CO detectors:

40 PPM CO for 10 hours
50 PPM CO for 8 hours
70 PPM CO for 1-4 hours
150 PPM CO for 10-50 minutes
400 ppm CO for 4-15 minutes

For a healthy adult OSHA sets the maximum allowable concentration for continuous exposure for healthy adults in any eight-hour period at 50 ppm of CO.

See details at

CARBON MONOXIDE - CO

It would be helpful to know the specific water heater model about which you raised the condensate question so that we could look at the manufacturer's instructions.

Follow up:

@Inspectapedia Com Moderator,

The model number is RG2PV50T6N19

I would provide the link but I don't want to impede delivery of this message

Thank you for your attention.

Reply: How does mishandled condensate in gas water heater installation affect heater life expectancy and potential CO exposure?

@William in Missouri,

For other readers, see this

• Bradford White RG2PV50T6N19 Water Heater INSTALLATION MANUAL [PDF]

and this

• Bradford White PV-SERIES WATER HEATER SERVICE MANUAL [PDF]

or see all of our Bradford White water heater manuals

at BRADFORD WHITE WATER HEATER AGE & MANUALS

Continuing: from the IO manual we have

Venting System Condensation

Condensate formation does not occur in all installations of power vented water heaters but should be protected against on installations where condensation can form in the venting system

Formation of condensation in the venting system of power vented water heaters is dependent upon installation conditions including, but not limited to:

• ambient temperature and humidity of installation location;
• ambient temperature and humidity of venting space;
• vent distance and slope;
• and product usage.

In order to effectively control condensate from adversely affecting the mechanical components of the water heater several methods may be employed:

1. For horizontal installations the vent pipe can be installed with a downward slope (not less than 1/8" (3 cm)) and away from the blower.

2. In order to prevent condensate from draining back into the blower (vertical or horizontal runs), an optional condensate kit is available as a service part (Condensate kit, p/n 239-45875-00).

A molded exhaust adapter with drain outlet is secured with one hose clamp, to the vent pipe.

Tubing is provided to drain any accumulated condensate away from the water heater and to a suitable drain.

The kit comes complete with instructions for proper installation.

What I take from this is that the company knows that in some installations, failure to adequately direct condensate out of the heater can be a concern - hence their condensate drain hose kit is provided.

A read of the IO manual and of the service manual don't give further warning about this problem nor the possible safety concerns that could arise.

We're back at my earlier advice:

The impact on the remaining life of your water heater from previous condensate backflow over some unspecified period and at an unspecified rate, isn't possible to assess from text or discussion.

But an experienced technician could certainly make a visual inspection of your water heater and could express an opinion about the nature, location, and specific components affected by any corrosion she can find.

If those are light-superficial then there's probably no measurable impact on the heater's remaining life.

Do I need to replace the single wall pipe with 4-inch type B tubing?

I'm replacing a natural gas fires water heater with a new natural gas water heater.

I have about four feet of 4-inch,, single wall vent pipe that connects the water heater flue to a Type B vertical exhaust stack.

Do I need to replace the single wall pipe with 4-inch type B tubing? There are no combustible materials. 2021-08-27 by Dan

Reply:

@Dan,

The final legal authority is your local building inspector.

The final technical authority are the manufacturer's installation instructions for your specific water heater brand and model.

That whining and warning out of the way, no, you can use single wall metal flue vent connector pipe between the heater and the chimney as long as you're meeting clearances from combustibles.

But I think I see aluminum - if so, that's not usable; use steel.

Gas water heater got wet in flooded cellar

Gas water heater got wet in flooded cellar try lighting it lights but burns out notice when remove burner to out side works fine not getting air vent on bottom is problem. Any ideas to repair thanks. 2021-01-22 by Mack

Reply:

Mack:

Depending on just what got wet, I'm concerned that a previously flooded water heater could be unsafe.

I'm unclear how you could possibly move the burner of a gas fired water heater outdoors;

But I agree it sounds as if there is an air flow or combustion air or gas orifice clog problem.

Inadequate combustion air at a gas water heater is very dangerous

Im living in an apartment with a direct vented tankless water heater (State GTS-540-NEH) , located in a bedroom closet. The water heater keeps shutting off with error code: 391 "Air-fuel ratio rod failure." The bldg manager is ordering a new AFR rod assembly.

I later noticed that if I keep the door cracked open the water heater works fine. My worry is that the real problem is a blocked combustion air inlet since keeping the door open seems to fix the problem.

Also the closet is not sealed with a 1/2" gap under the door. Apparently not enough to avoid the shut off but also worrisome regarding CO and fumes even if the water heater is fixed. Does the code require a sealed closet for a direct vent system?
thanks
2021-01-10 by Bob Allen

Moderator reply:

Bob:

Watch out: this sounds unsafe to me; if there is inadequate combustion air the risk is fatal carbon monoxide poisoning - death of building occupants. Be sure you discuss that with the building manager and give that warning in writing as well.

And be sure that there are properly located, installed, tested CO detectors as well a smoke detectors in your apartment.

Follow up:

I just discovered an air leak from the 4" PVC exhaust pipe where it fits into the heater. Waiting on an HVAc guy to come and seal it. I wonder if the water heater has a CO monitor shutoff and that was the issue or maybe there is some other problem like with combustion air as you say.

Reply: check for a flue gas spill switch

Thanks for that interesting follow-up, Bob.

Sure some water heaters have a flue gas spill switch that can shut down the heater. Attach a photo of the heater's draft hood and we may be able to point out the sensor and its wiring.

Follow up:

Watch out: Just to follow -up, Turns out that in addition to the exhaust leak, the heat exchanger had a small leak so they are going to replace the whole unit.

The tech said the water dripping on the AF rod or near it was causing the problem.
thanks for your help.

Reply: leaks found in gas equipment: unsafe!

Thanks Bob that detail will help others.

Apologies for, like the Dickens character, asking for still more, but do you know where the drip originated?

Follow up:

No worries Oliver lol
I didnt actually see it but the tech said that the there was some pitting on the upper end of the heat exchanger tube. I dont think he actually saw a hole but said that it likely would leak when it was heated.

Either way Im happy to get a new system since its a rental
thanks again for the help
Bob

Horizontal vent for gas water heater?

I have a rheen 73 gallon Natural gas water heater I have it plumbed in I need to horizontally vent the old one had exhaust fan that plugged into water heater the new one has no fan can I just use metal to vent to outside? 2020-03-28 by Darrold

Reply:

Darrold:

What is the horizontal venting distance?
What do the manufacturer's installation instructions say about venting the new water heater?

I worry that the distance is great enough that the prior unit would not vent properly and needed a vent booster fan.

Watch out: when a water heater can't vent properly there is risk of fatal carbon monoxide poisoning.

Cold single wall metal vents may not vent the water heater properly

In the process of replacing 40 gal gas water heater. I have had 3 plumbers come due to well....some lacking professionalism among other things. Today I was told the vent pipe needed to be replaced. They stated it was not code to use single wall vent pipe.

The only issue I have with that statement is, the vent pipe runs from water heater up and over about 2 1/2 feet through cinder block wall to exterior.

The entire vent pipe is on the exterior besides the entrance through exterior cinder block wall. I can’t seem to find information saying this is not up to code. I understand it would need to be double wall if used on interior....any thoughts? 2020-03-18 by George

Reply:

George

Watch out: Even if you don't face a fire hazard, a single wall vent might, for some heaters, be too cold and thus the heater might not vent properly in some climates/locations.

Let's start by identifying the brand and model of your water heater, then take a look at the installation manual for that specific heater - as the manufacturer's word will be authoritative.

Roof Clearances

Gas vents that are 12 inches or less in size and located not less than 8 feet from vertical wall or chimney

This question is in reference to a gas water heater that needs to be vented through the roof and is located near an existing chimney (cannot vent through chimney).

G2427.6.4 (503.6.5) states that "Gas vents that are 12 inches (305 mm) or less in size and located not less than 8 feet (2438 mm) from a vertical wall or similar obstruction shall terminate above the roof in accordance with Figure G2427.6.4."

Does "vertical wall or similar obstruction" apply to an existing chimney that stands 18 inches high with and 18 inch flue extension?

Or, as an alternative way to think of this question, can a gas water heater be vented through a 4 inch pipe at the the minimum height according to Figure G2427.6.4, which would be 1 ft in my application, if it terminates within 8 ft horizontally from the existing chimney (18" by 18" and terminates 18" above roofline?

See https://up.codes/s/gas-vents for Figure G2427.6.4.

Thank you,
2023-02-24 by Adam

Reply: distance allowed from water heater to chimney connection

@Adam,

You say that your gas heater vent is "within 8 ft horizontal distance" from the chimney, but not how close to the existing chimney it actually-is. So your question is a bit tricky.

In my OPINON at 7 ft. 6" (within 8 ft) the nearby chimney might not be considered an obstruction, while if the two are quite close - say inches or a foot apart, the chimney might be an obstruction.

Your local building code inspector is the final legal authority on this gas water heater vent question, but my OPINION is that a chimney of typical dimensions above the roof line MIGHT count as a "vertical wall or OTHER OBSTRUCTION".

Is this the right height of water heater vent above roof?

I have a water heater vent penetrating near the eve of a 12/12 pitched roof. If I hold to the 2’ above any part of the roof 10’ away, this means the pipe will be 12’ tall above the penetration. is that correct? 2022-11-19 by Jim Y.

Reply: required gas water heater vent height above roof

@Jim Y.,

I'm not sure we have the same view of your situation - maybe you can attach a photo or sketch.

In general the clearance is the horizontal distance from the top of the chimney to the nearest surface.

You'll also want to read Charlie Buell's comments on this page.

Here is typical gas vent height language from the US Fuel Gas Code

Gas vents shall extend at least two feet above the highest construction, such as a roof ridge, parapet wall, or penthouse, within ten feet of the vent outlet whether the construction is on the same building as the chimney or on another building.

Here is a more-complete quote from New York's adopted version of the National Fuel Gas Code

503.5.4 Chimney Termination

Chimneys for residential-type or low-heat appliances shall extend not less than 3 feet (914 mm) above the highest point where they pass through a roof of a building and not less than 2 feet (610 mm) higher than any portion of a building within a horizontal distance of 10 feet (3048 mm).

Chimneys for medium-heat appliances shall extend not less than 10 feet (3048 mm) higher than any portion of any building within 25 feet (7620 mm).

Chimneys shall extend not less than 5 feet (1524 mm) above the highest connected appliance draft hood outlet or flue collar.

Decorative shrouds shall not be installed at the termination of factory-built chimneys except where such shrouds are listed and labeled for use with the specific factory-built chimney system and are installed in accordance with the manufacturer's instructions.

Reader comment: Safe Gas B-vent Clearance Distances Clarification

Daniel,

Watch out: while your drawing [above] is consistent with whirlpool drawings cited, it is not consistent with fuel gas code that Whirlpool says to follow for a persons applicable area.

Charlie Buell

Charles Buell Inspections, Inc, 206 478-7371 17123 22nd Ave. NE, Shoreline WA 98155 Email: charles@buellinspections.com Website: www.buellinspections.com, WA Licensed Home Inspector #220, ASHI Member #246514 ASHI National, Technical Review Committee Adjunct faculty at Bellingham Technical College

Moderator reply:

Thank you Charlie, we'll add your detail and helpful drawing in the original article.

Hot water tank is vented through my chimney and it is falling apart

My hot water tank is vented through my chimney and it is falling apart, it’s at the side of my house how far up from the roof can I go with the pipe because I’m taking the chimney down below the roof and I’m not going to build it back up.

The hot water tank is the only thing the chimney is using. 2020-05-24 by Tony

Reply:

Any chimney that's passing up the outside of a house needs to vent two feet above the Ridgeline if it comes out at the ridge or at the other chimney height requirements you'll see by searching inspectapedia.com for chimney height.

You also need to consider that if you are just venting a water heater it may not vent safely through a tall exterior single wall or all metal flue. Check the installation instructions for your specific water heater brand and model.

You want to look at a metal chimney like a "B-vent" for gas fired water heaters.

Extending the vent pipe higher at a mobile home

i have a mobile home home with a natural gas hot water heater that goes out on windy days. my vent pipe extends about 2 feet above roof and has a round cap on it. it does not appear to be as high as the peak of the house. could this be the reason the water heater goes out on windy days and what can i do to correct the problem. thank you. 2018-10-22 by steve

Reply:

Steve,

Thank you for an interesting question.

It's possible that extending the vent pipe higher will help with the downdraft problem if that is in fact what's blowing out your pilot. It's also possible that the cap on top of that metal chimney is not the proper one or is not properly designed to stop downdrafts.

Clearances From Other House Elements

Flue clearance from combustibles

House on market and buyer’s inspection report asks for 1” clearance around the flue vent in the attic. Hot water heater replaced about 2 years ago and we sprayed the stuff in picture to keep water from coming in.

Not sure we fully understand how to keep rainwater , bugs and small critters from coming in if we leave the requested gap. Is there some sort of cap or ring that should be there? 2024-08-11 by Melanie

Reply:

@Melanie,

By 1-in clearance from combustibles, no one is (or should be) saying that you need to have a 1-in hole in your roof that lets rain or insects in.

It is the case that where the roof flashing was installed the hole through which the chimney passes should have been cut large enough to leave an inch of clearance to the wood roof sheathing.

If that space was not provided, then it's a small job for someone to temporarily remove the chimney, and enlarge the hole, and then put the chimney and flashing back.

Besides that task, or if that's not what the inspector was referring to, there are two other things I can see in your photo.

First, if the foam that you sprayed wasn't fire block foam (your foam looks yellow) then to be meticulously accurate and safe you would pick out and remove that part of the foam that you can easily remove, and then spray a new with fire block foam.

It's a similar product, but orange in color, and specifically is rated for use at fire block openings or where there are issues such as the one raised about your gas vent.

Fire block spray foam insulation is readily available at your local hardware store or at building suppliers.

Second, although I can't tell for sure from your photo, if the vent is less than 1 in from that rafter, it's a technical violation even though chances are by the time flue gases has reached the roof they are so cool that it's practically ridiculous to be concerned.

To make that meticulously correct as well, if you have less than 1 inch of clearance, and if it is not easy to obtain the clearance by minor rerouting or adjustment of the metal flue itself, then you may need to install a simple non-combustible heat shield on the surface of the rafter.

That can be something as simple as sheet metal that's itself spaced away from the wood.

Honestly if this is the only concern raised by your buyer I would be doing handsprings of joy and I would be happy to do whatever made them happy about this gas vent.

Don't hesitate to let me know if you have further questions.

Daniel

Exterior installed direct vent water heater venting near an openable door or window

What are the requirements for an exterior installed direct vent water heater venting near an openable door or window of a dwelling unit? I could not find anything in the CPC regarding this. 2020-05-07 2 by Rob S.

Reply:

Rob,

GAS APPLIANCE / HEATER DIRECT VENT CLEARANCES

gives the required distances

Thank you for asking and for the safety question. You'll see there we have an entire but separate article on that topic. Don't hesitate to post follow-up questions or comments.

Clearance to combustibles for gas heater venting

Do I need to maintain 1 inch clearance to combustibles in a vertical wall for a direct vented gas conventional water heater installed in a full basement that uses 3"/5" concentric double walled metal vent pipe through the floor, vertical wall, and ceiling? 2020-03-10 by EngineerJohn

Reply:

You could (better) of course ask Reem directly, but my interpretation is that 1-inch clearance to combustibles means just that - 1inch throughout the routing of the vent.

Size Of Venting Pipe And Flue

What size flue pipe would handle this total BTU load connecting to a brick chimney?

Hi..... I have a building with 8 water heaters lined up, side by side, and located in the basement. Each water heater is 50 gal. 40,000 BTU, natural gas. What size flue pipe would handle this total BTU load connecting to a brick chimney? What would you recommend for this scenario? Thank you. 2025-02-20 by Ray

Mod reply:

On our gas type B vent chimney page TYPE B-VENT CHIMNEYS inspectapedia.com/chimneys/B-Vent-Chimneys.php

we include a number of PDF downloads including for example this sizing guide

Hart / Cooley TYPE B GAS VENT SIZING GUIDE [PDF] (2018) Hart & Cooley, LLC

inspectapedia.com/chimneys/B-Vent-Sizing-Guide-Hart-Cooley.pdf

Watch out: be sure to read the chimney sizing procedure for a chimney when multiple appliances are being vented together. In the example instructions we cite here that begins on p. 15.

You'll see that there's more to read and to include in your calculations, so ANY answer that I tried to give here would be wrong as we don't know all about your installation.

You'll see that you basically need to add up all of the BTUs of all of the appliances that are connected to the chimney to get the total flue size required.

Chimney height is also a consideration that you'll see in the tables.

Watch out: there are other restrictions and safety considerations discussed in our article and in the guides from the manufacturers listed on the B-Vent chimney page.

The assumption is that in the worst case all of your heaters are running at the same time.

8 x 40,000 = 320,000 BTUh

is the total BTU load from your water heaters and of course you need to be sure that nothing else is venting into that fluent connector and chimney.

In the tables you'll see that you will have to consider
- total input BTUH
- chimney total height
- lateral vent distances
- other factors

You'll also see that there are various chimney diameters that can work, depending on the other parameters (that you've not stated).

The illustration I include below is just one example and may not exactly fit your installation.

Daniel

Do I need a larger vent pipe if I add another water heater?

I have two 40 gal gas water heaters venting into a 4" vent pipe i want to add a third water heater. Do i need to increase the size of my main vent pipe which as said is 4". Thank you. 2022-06-09 by Declan

Reply: increase the size of main vent pipe

@Declan,

Almost certainly yes.

Usually with each additional water heater after one, each successive flue pipe is one size larger.

But of course the length of the run and then input btus are important.

Identify the brand and and model, pick up through IO manual, and see what the manufacturer says about shared flues. Or post that information here and we'll see what else we can find.

Watch out: as perhaps you already understand, inadequate venting can be very dangerous and can even produce fatal carbon monoxide gas in the building.

Venting Codes & Standards

Do these gas water heater vent codes apply to mobile homes?

Is this the same for mobile homes, too? 2022-11-16 by Jes

Reply: gas water heater codes for mobile homes

@Jes,

The codes for many aspects of mobile homes are found here

BUILDING CODES & STANDARDS FOR MOBILE HOMES
https://inspectapedia.com/Manufactured_Homes/Mobile_Home_Codes.php

and there you will find the link to

MOBILE HOME WATER HEATER CODES & STANDARDS

Looking for an authoritative source for the 12" minimum vent connector rise before the first elbow

I am having trouble finding an authoritative source for the 12" minimum vent connector rise before the first elbow.

While I understand this is best for priming the vent and establishing flow, it seems impossible to convey this to other plumbers, since our code book and the NFPA makes no mention of any minimum.

It seems very much a best practice, and not a hard rule. 2021-06-04 by A Plumber

Mod reply: building code requirement for 12 inch rise in gas vent before first elbow or bend

@A Plumber,

Thanks for a helpful question.

Please find it repeated and my detailed reply also now at:

BUILDING CODE REQUIREMENT to FOLLOW MANUFACTURER'S INSTRUCTIONS https://inspectapedia.com/plumbing/Gas-Water-Heater-Venting-Codes.php#ManufactCode

About the 12 inch rise of vent before the first elbow, the article above states:

This is a manufacturer-recommended detail to improve the flow of exhaust gases [we have not found this in the model codes].

But remember that as codes require the installation to comply with the heater's manufacturer's installation instructions, that makes the 12 inch rise a defacto code requirement as well.

This example text is an example from the Whirlpool gas water heater IO manual cited in the article above

Vent Installation

To improve the flow of exhaust gases, we recommend that a minimum of 12 inches of vertical vent pipe be installed on the draft hood prior to any elbow. A portion of the vent pipe (up to 75% of the total vertical height) can be horizontal, but the termination must be vertical.

Here is an example building code citation requiring that the manufacturer's instructions be followed.

This example is from the 2014 New York City Mechanical Code which is, of course, an adoption of the model codes like the National Mechanical Code

NYC Mechanical Code Section 304 Installation

304.1 General

Equipment and appliances shall be installed as required by the terms of their approval, in accordance with the conditions of the listing, the manufacturer's installation instructions and this code. Manufacturer's installation instructions shall be available on the job site at the time of inspection.

304.2 Conflicts

Where conflicts between this code and the conditions of listing or the manufacturer's installation instructions occur, the provisions of this code shall apply.

Exception:

Where a code provision is less restrictive than the conditions of the listing of the equipment or appliance or the manufacturer's installation instructions, the conditions of the listing and the manufacturer's installation instructions shall apply.

So you can see that by inclusion the manufacturer's instructions in fact become a "code requirement" where the "code" is less-restrictive,

therefore the 12-inch minimum vent connector rise before the first elbow is required by the model as well as adopted building codes even where it is not explicitly stated in the code itself, as long as it appears in the installation instructions for the gas-fueled equipment that you are installing.

Of course your local plumbing or building code compliance inspector is the final legal authority.

Requirements for venting a gas water heater in valencia county new mexico

what are the requirements for venting a gas water heater in valencia county new mexico. 2020-11-07 by louis chavez

Mod reply:

Louis:

With a few small exceptions New Mexico has adopted the Uniform Plumbing Code or UPC.

For a PDF copy please see CHIMNEY CODES & STANDARDS inspectapedia.com/Design/COPIES-of-BUILDING-CODES.php#Chimneys

Here are direct links to

The NEW MEXICO MECHANICAL CODE (2015) http://164.64.110.134/parts/title14/14.009.0002.html

and

The NEW MEXICO PLUMBING CODE (2015) http://164.64.110.134/parts/title14/14.008.0002.html

Requirements for a double-walled gas water heater vent pipe when exiting through the gable?

What are the requirements for a double-walled vent pipe when exiting through the gable? How far does it need to extend past the wall and/or roofline above? Does it need a 90-degree bend in pipe with cap? If there are gas water heater vented through separate pipes, what is the spacing requirements between the pipes, if any? 2020-11-06 by StephenNOLA

Reply:

Stephen

If we're discussing a B-vent metal chimney (your "double walled vent pipe") then the manufacturer as well as chimney codes will almost certainly expect you to extend the chimney up to the require height above the roof.

We organize the details of these chimneys and their requirements starting at

TYPE B-VENT CHIMNEYS - home https://inspectapedia.com/chimneys/B-Vent-Chimneys.php

How far off the floor do I need to raise a gas water heater in Oregon?

How far off the floor do I need to raise a gas water in Oregon?
Or do I need to raise it? 2020-01-08 by Dave

On 2020-01-08 - by (mod) - water heater height off of floor

Dave

There's no single "right" answer to gas water heater height requirements.

If the heater is located in a garage, most codes want it elevated 18" above the level of the garage floor.

In other locations, such as a concrete slab basement, the heater may sit right on the slab.

In some jurisdictions the local code official (city, town, county) may want a drip pan under the heater.

Have code requirements changed since installation

Hi, I am selling home with original water heater (1988). Buyers want exhaust/ vent pipe replaced because it is not "code". Have code requirements changed since installation and am I required to do modifications? 2019-09-16 by Tom

Reply:

If you are describing a gas fueled water heater that is not vented to the outside, that is unsafe.

That's more basic and immediate concern then the assignment of to code or not to code.

The cost to cure should be trivial compared with the value of a home. It might be that you want to fix it immediately for your own safety.

Other Gas Water Heater Venting Q&A

Small test instrument hole in a water heater flue or vent is not unsafe

I just discovered these holes in the venting/ chimney of my hot water heater. The chimney sleeve doesn’t cover the two bottom holes, and then there is seemingly a man made hole in the chimney itself. Is this a risk of improper ventilation? 2020-09-07 by Christina

Reply:

That hole is punctured or drilled in an exhaust flue vent connector by the heating service technician in order to permit the insertion of test instruments that are used at the time of service or maintenance.

It's not a safety concern though you could, if you like, cover it with a bit of aluminum-foil adhesive tape like the 3M product I show below.

Many technicians simply stick a short bolt into the opening.

Tree took down out heater pipe

Tree took down out heater pipe (side vent thru single block wall/exterior stucco). Plumber who installed heater 7yrs ago now says we're out of code & quoted $3-4K to replace pipe but I think that's wrong.

The photo is taken from is almost directly above gas furnace intake/exhaust pipes. PA code reads min 10ft from lot line (done) & 10ft above nearest grade (~10ft). Prior chimney never exceeded roofline. Sorry -- photo thru a screen. 2020-04-05 by Ceecee18

Reply:

If your prior chimney didn't extend above the roof line by at least 2 feet from nearest point of contact then it was improper and unsafe.

Follow up:

Sorry. I’m seeing images of non direct venting that are straight out the wall so it’s confusing. As noted, 4yr old gas furnace vent pipes run straight across walk out basement and out the rear wall, just below the window. Thanks.

Reply: route some metal chimney types out through a building side wall and then up above the roof line.

It's possible to route some metal chimney types out through a building side wall and then up above the roof line.

There are also high-efficiency condensing gas fired heaters that vent at a simple sidewall vent with no chimney.

From your photo I can't say what equipment is installed.

Question: what's the purpose of these fins on the gas water heater flue?

I have never seen this on a gas fired water heater, can you tell me what the heck it is? - Anonymous by private email 2019/01/23

Reply: "energy savings" or heat extraction fins on a gas water heater: Safety hazards

Those fins on the exhaust vent were popular starting in North America during the Arab oil embargo in the 1970s and are still sold and installed on occasion.

The function of these fins on the gas water heater flue vent connector is to try to increase the transfer of heat from the water heater exhaust into the surrounding area - maybe to make it warmer, in a fantasy that we're cutting heating cost by extracting heat, perhaps in hope of preventing frozen pipes nearby.

Watch out: There is, in my OPINION (there's no authoritative data I've found on this), a subtle potential safety hazard of the heat transfer fin add-on.

**IF** the gas fired water heater is already venting into a chimney flue that is big, and perhaps sometimes cold (as in old buildings) and perhaps tall, THEN the heater may have trouble venting under all weather conditions, risking back-drafting and carbon monoxide poisoning hazards in the building.

That problem could be present with or without the heat transfer fins but would be exacerbated by the additional heat that the fins remove.

If there were any doubt about the chimney, it would merit inspection by a certified chimney sweep, a check for signs of back-drafting. You probably already know those signs.

Thanks for the photo and question.

Watch out: reader MikeJC adds these additional safety hazards at your water heater:

On the photo with fins on vent pipe, that installation does not meet code for two reasons:

• Missing vertical riser: To vent properly that gas fired water heater needs 12" pipe extending vertically above H/W heater
• Inadequate flue vent connector slope: The flue vent connector's "horizontal" run appears to lack any incline - it needs to slope upwards from the heater to its connection at the chimney thimble ... "just saying" - Mike JC 2020/11/25

See also these examples of heat recovery methods using fins or devices attached to flue vents or other heater tubes:

• Cnossen, Andrew J., and Evelyn M. Yeager. "Auxiliary air heater." U.S. Patent 2,468,909, issued May 3, 1949.
• Jones, Robert J. "Waste heat utilization system." U.S. Patent 4,084,745, issued April 18, 1978.
• Segelken, Heinrich. "Retarder for flue-tubes." U.S. Patent 1,056,373, issued March 18, 1913.
• Trojani, Benito L. "Tube provided with inner fins and outer fins or pins, particularly for heat exchangers, and method therefor." U.S. Patent 4,306,619, issued December 22, 1981.
• Xu, Gang, Shengwei Huang, Yongping Yang, Ying Wu, Kai Zhang, and Cheng Xu. "Techno-economic analysis and optimization of the heat recovery of utility boiler flue gas." Applied energy 112 (2013): 907-917.
  

Venting a Tkagi tankless water heater

my son had a Tkagi tankless water heater installed what kind of venting should he have it looks like dryer venting was used. 2019-07-13 by davidponter

Reply:

Let's get the model number of the water heater - then we can make an authoritative remark or two by seeing what the manufacturer permits for that specific heater.

I agree that dryer duct, per se, is not likely to be proper nor safe. But there might be a flexible metallic vent pipe rated for use with gas heaters that could be ok.

Do you need high heat resistant paint for the vent pipe

Do you need high heat resistant paint for the vent pipe from the roof boot to the vent cap (including metal cap) on on the roof.
Example: Rustoleum high heat paint or engine enamel. Does the vent stack get that hot outside? 2018-11-13 by Brian

Reply: Rust-Oleum high temperature-resistant paint

Brian

The Rust-Oleum high temperature-resistant paint that I've used was a spray-on engine paint that bakes in to a durable finish beginning at 600 degrees F.

At the hottest point in a gas water heater flue - the flue-vent-connector right above the heater top - the temperature is not likely to rise above 350 degF.

It is reasonable to assume that higher in the flue temperatures are much lower. Not only do you thus not need a high temperature paint above the rooftop, but the high temperature paint may fail from not having ever reached a temperature high enough to "bake it" in to durable form.

Watch out however: if you are spraying a typical B-vent, until the surface has weathered and oxidized a bit the paint is not likely to remain adhered. Before painting a new galvanized steel such as at a B-vent chimney top I would

1. lightly wire-brush the surface

2. wipe down the surface with vinegar to give a slight etch to the surface

3. clean all that off and let the surface dry

4. paint it, perhaps first with a metal primer then the finish coat.

If your metal chimney is stainless steel you should eschew painting it at all as the paint won't adhere and the chimney will look horrible.

Follow up:

Thanks so much for the info.....this is very helpful. I will only be painting the vent above the roof boot and the vent cap.
I will paint the vent above the roof line, which will reasonably be below 350 degF with metal primer as you suggested
and plain common Rust-Oleum paint.

I would hope that this finish (primer/finish coat) would not blister with temperatures under 350 degF.

That was my main concern that the paint might blister or even for that matter catch on fire.
Thank you so much for all your detailed advice.