Warm air furnace installation, troubleshooting, repair FAQs:

Questions & answers about how to repair a home heating furnace - force warm air heat troubleshooting.

This article series answers just about any question about forced air or warm air furnace central heating system troubleshooting, inspection, diagnosis, and repairs in residential buildings and homes. Sketch at page top courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

Warm Air Furnace Troubleshooting or Inspection FAQs

These questions and answers about warm air heating system furnaces were posted originally

at FURNACES, HEATING - home. That's a great place to start reading if you're having trouble with your furnace.

If you have no heat a all and your heat is by forced or gravity warm air,

see HEATING LOSS DIAGNOSIS-FURNACES.

If you don't know what kind of heat your building uses, we explain how to figure out the answer in more detail

at HEATING SYSTEM TYPES.

 

Should we draw outside combustion air for our furnace?

Should the intake pipe for combustion be vented to draw air from outside? On 2020-11-25 by Scott -

Reply by (mod) -

Scott,

As I am not sure what you mean by intake pipe perhaps you could use the add image button to post a photo so that I can see the situation.

If for example the area from which combustion Air Supply to your heater is too small for your building is very tight you may indeed need to provide an outdoor combustion air source.

See details at COMBUSTION AIR REQUIREMENTS

Tell me the difference between the Weil-Mclean "CGa Gas Boiler Series 3" and "CGa Gas Boiler Series 2"

Can anyone tell me the difference between the Weil-Mclean "CGa Gas Boiler Series 3" and "CGa Gas Boiler Series 2" product lines? They seem to be very similar with just slightly different specification.

Even the description and info on the vendor website seems basically the same. I'm just trying to understand what the differentiator(s) are between these two series. Thanks much! -On 2020-11-21 by Bill

Reply by (mod) - difference between the Weil-Mclean "CGa Gas Boiler Series 3" and "CGa Gas Boiler Series 2" product lines?

Bill:

The two heaters have different dimensions, and slightly different efficiency ratings and some component differences.

For example the -2 is shorter, at 31 1/2"high than the -3 at 35" high
and the -2 has input BTUH ratings (varying by model) from 52 - 245 while the -3 has MBTUH ratings of 38-233

Both are cast-iron boilers;

Summarizing the -3 series: Water | Natural or Propane Gas | 38–233 MBH | 7 sizesSeries 3 | Atmospheric / Natural Draft | 84% AFUE

Summarizing the -2 series: Water | Natural/Propane Gas | 52-245 MBHSeries 2 | Natural Draft | Efficiency 82%-84%

Here are two specifications documents that will let you make a detailed side-by-side comparison of the two gas boilers from Weil-McLean

Weil-McLain CGA Series-3 BOILER SPECIFICATIONS [PDF] at https://inspectapedia.com/heat/Weil-McLain_CGa-Series3-Brochure.pdf original source: WeilMcLain at www.weil-mclain.com/products/cga-gas-boiler-series-3

Weil-McLain CGA Series-2 BOILER SPECIFICATIONS [PDF] at https://inspectapedia.com/heat/Weil-McLain_CGa-Series-2-Brochure.pdf original source: WeilMcLain at www.weil-mclain.com/products/cga-gas-boiler-series-2

Followup by Bill

Thank you danjoefriedman, I had already seen those documents, which is what caused my confusion. As I look at them I don't see any real differences between the two units.

I continued researching, and I think I figured it out. The Series are used when they change that base model, over time.

So there seems to have been a CGA Series 1, then CGA Series 2, and now CGA Series 3.

Not a lot has changed from Series 2 to Series 3, but it is a different enclosure (the tan moving to gray), and the control module is slightly different, adding an LED to support the new built-in LWCO now included )used to be an add-on the installer had to implement if needed for code).

They added the functionality to the existing temperature sensor into the water chamber, and it now appears to have the ability to detect a low water condition as well. Appreciate you staying with me on this, maybe this will be useful to a future reader. Thanks.

Reply by (mod) - heater serise numbers often indicate different manufacturing dates or periods

That's helpful; clearly the products evolved as the capacities described overlap one another.

Are there publication dates on those brochures? Perhaps encoded:

Series-2 = WM1905_BRO_034 CGa - maybe May 2019

Series 3 = WM2008_BRO_034 CGa - maybe August 2020 ? or 2008 ?

Thanks for the follow-up as that'll help others.

Reader Question: My furnace gas burners go on but the blower does not start and then the flame goes off. What's wrong?

I

have a american standard furnace Freedom 80 the burners go on but the blower fan does not and then the flame shuts off what's wrong - Robert

Reply: as a safety feature on furnaces, the fan limit switch turns off the burner if the blower does not start

Robert, if the furnace blower fan will not start, the limit switch on your furnace will turn off the burner in order to avoid overheating and damaging the heat exchanger.

You'll need to inspect and fix the blower fan problem. If your furnace blower uses a drive belt to connect the fan to the driving motor check that the belt is in place and not slipping. Other blower fans use a direct drive motor whose shaft spins the fan assembly. IN either case check that the motor is starting;

See BLOWER FAN OPERATION & TESTING for our article on diagnosing and fixing these problems.

Is a burning smell from our heater dangerous?

I’m worried cause last night the heater ,fan and all just stopped, you could smell like burning something just before it stopped but I’m worried cause after all day and a few hours of last night it stopping working, it just made a air release like noise and the thermostat is completely turned off / it did this for maybe 30 seconds , but’s quiet now , could it be dangerous to us? On 2018-03-07 by Shirley Nixon -

Reply by (mod) - If you smell burning at heating equipment it may be unsafe: turn it off and call for repair

Shirley

Watch out: Any time you smell burning at heating equipment there is a risk that the system is unsafe - it should be left off and of course you'd call your heating company for emergency repair.

What's causing a knocking sound at our furnace?

Gas warm air furnace located in attic of townhouse. Entry is in 2nd floor laundry closet above washing machine connections.

Temps are unusually cold at 23 Fahrenheit. Heat set on 71. When air is blowing I hear a knocking noise coming from laundry closet.

Thought perhaps water connections for washer but no change when I turned them off.

No other issues other than frequency of how often unit runs but assuming that is due to weather. It turns on every 15 minutes or so and stays on about 3-5 minutes. - On 2018-01-02 by L

On 2018-01-02 by (mod) -

Is the knocking sound is continuous, look for a loose blower or squirrel cage fan or fan bearing.

By the way a washing machine that's out of balance or has its feet set un evenly on the floor will wobble and make horrible knocking sounds.

See HVAC NOISE-1 BANG BUZZ VIBRATE - banging, bearings, buzzing, thumping

Furnace Gas burner won't stay lit

My gas furnace ignites but won't stay on for more than a few seconds before it goes out again. The induction fan and blower seem to be working fine.

The furnace just seems to go through a series of igniting then going out right away.

Sometimes, after a while it will stay on and heat to set temp and shut down appropriately, but then after the temp drops it will go back to a series of igniting then then flame out. - On 2017-02-27 by Miguel

On 2017-02-27 by (mod) - gas furnace burner ignites but won't stay lit

Check for a bad thermocouple or flame sensor

Check for CO or flue gas spillage at the draft hood

Check for inadequate combustion air

Watch out: these conditions are unsafe and risk fatal carbon monoxide poisoning

Check for a fuel delivery problem: blocked gas line, blocked pilot or burner orifices, spiders in the igniter, (really), lack of combustion air

Followup by Tim - found a bad ignition controller

Just had that happen to ours last month...

in our case it was a bad ignition controller. Our furnace (circa 1980) has an intermittent pilot, meaning when you get a call for heat, it first turns on gas to the pilot and then ignites it with an electric spark, then once it senses the flame, turns on gas the main burners, which get ignited by the pilot.

Our old controller had failed so that as soon as the main gas valve came on, voltage would drop to zero, which would shut it off, then this would happen 6 or 8 times before the controller went into fail-safe mode and shut down.

Reply by Mod

Thanks Tim I should have included that part in my suggestions - I was thinking of older gas fired appliances that don't use a solid state control module.

Your feedback will help other readers.

Usually a bad ignition controller will prevent the gas flame from igniting at all. That's not the same issue as a flame that ignites but then goes out.

Engineer says old Johnson Starley Furnace is illegal because no outside air vent is provided - right?

Hi. I have inherited a Johnson & Starley warm air heating system. I asked a local engineer to service it and he advised that it was illegal because there was not outside venting in the utility room it is housed in. Is he correct?

On 2016-06-02 by Chuck Milne -

Reply by (mod)

Chuck It would be a bit of a stretch for me to pronounce legality having almost no information about your system, fuel, venting, etc. (and technically it's your local code inspector who declares it "legal" or not under building codes.

But certainly if the system is fossil fueled (oil or gas) its exhaust needs to connect to a working chimney and vent to outside.

Speculating and guessing all over the place since your question didn't give much information, if your engineer is referring to inadequate combustion air, that she thinks would be solved by providing outside air, that's a critical safety observation. Search InspectApedia for COMBUSTION AIR REQUIREMENTS to read details.

Followup by Chuck Milne

Hi. Sorry, yes my question was basic but just to start the conversation. The system is gas fired warm air and has venting going from the unit to a chimney on the roof. The unit is in a large utility room which has a good sized window which has an air vent.

The engineer has suggested I need to get a builder to punch a hole through the granite wall and install a vent. It has been in the building (Converted Church Hall) for up to 20 years but seems very efficient and we've experience 2 Aberdeen winters. I just thought it prudent to have it serviced but it seems to have opened a can of worms. Any comments would be appreciated

Reply by (mod) - is your engineer really a heating expert?

Is your "engineer" actually a P.E. ?

Does the engineer actually have expertise in gas fired furnaces, chimneys, combustion air? P.E.s usually have something better and more profitable to do than servicing furnaces.
How did the engineer determine that the furnace lacked adequate combustion air.

Did you search InspectApedia for COMBUSTION AIR REQUIREMENTS to read details about estimating combustion air needs? It's true that gas fired equipment is **VERY** dangerous if there is inadequate combustion air as you'll produce high CO.

Followup by Chuck Milne

Thanks again for your comments. I did study the link but couldn't determine a definitive answer.

Unfortunately it's all getting a bit to technical for me as I'm not an engineer and I don't understand the jargon.

Your efforts to help is much appreciated but I think the best way forward is to ask for a second opinion from another company. Kind Regards. Chuck

Reply by (mod) Agree on getting an actual on-site expert to check the safety of your system

It makes sense to be careful where life safety questions are involved. I agree with bringing in a trained, experienced gas heat service technician. Your gas supplier should also be able to help.

Basically if a heater is installed in a small space it may lack enough air for the gas burner to completely consume it's fuel.

Complete combustion produces just CO2 (carbon dioxide) and water vapor.

Incomplete combustion Occurs if the gas burnerlacks enough oxygen (in air) so it then produces CO (dangerous Carbon monoxide)

An expert tests the exhaust for unsafe levels of CO using an instrument.

That could be due to improper chimney draft, bad burner adjustment, or insufficient combustion air.

In the last case one may add air from outside.

There are also simple rules of thumb about burner size in BTUs and room size that can suggest inadequate combustion air.

A pro can sort out the question for you.

Your immediate safety action must be to be sure your home has properly installed, located, tested Carbon Monxide detector alarms. If an alarm sounds, get everyone out right away and call emergency services.

Please keep me posted on what you are told.

Followup by Chuck Milne - installed CO detectors

Good advice about the CO detector. I fitted three 18 months ago, in strategic places i.e. in the utility room the furnace is in, another outside the room, and yet another upstairs in the gallery bedroom.

The property is unusual in that it is a voluminous cavern of a living space (100'x 30'x 35' open plan) so my guess is the experts will take it at face value and throw away the standard rule book. I'll keep you posted.

 

Exhaust smells come through the ductwork of our furnace heat system

My fiancee's house (manufactured) has forced hot air oil furnace. During times he's not there, the thermostat is set at 58. When returning home, the heat is turned up to 72. While the furnace is running, it smells like exhaust coming through the ductwork. The furnace was cleaned 4 months ago.

There is no basement in the home, only a crawlspace. This is where the oil tank is located. There is a woodstove set up in another room but is not using the same chimney.

What can be causing this problem?
The man who cleans the furnace doesn't find anything. On 2016-01-31 by JaPete -

Reply by (mod) - fatal carbon monoxide warning

Watch out: You are describing what may be a very dangerous condition. If the furnace heat exchanger is cracked, rusted, or has a hole in it then you may be venting dangerous flue gases including potentially fatal carbon monoxide into the occupied space.

My suggestion is to ask for an inspection by a trained heating service company technician.

Question: how to find the age of my Lennox GN1-105C Gas Furnace

I am trying to find the age of my Lennox model GN1-105C gas, gravity fed air, furnace which the only thing I have had to do to it in the last 40 years besides cleaning is change the thermo-couple. (Nov 23, 2011) jack

Reply:

Jack, in the ARTICLE INDEX near the end of this page where you see our links to heating-related articles and at the ARTICLE INDEX found at CONTINUE READING I have added
AGE of HEATERS, BOILERS, FURNACES
please take a look at that article and if questions remain just ask and I'll do my best to assist further.

 

Question: why won't my furnace heat the house to the thermostat set temperature

My furnace will not reach the set temperature (winter now). I'd set it to 22c, the room temp starts at 17c, the furnace comes on, but can never hit 22c before it turns off again. I have no leakages. What could be the possible reasons? (Jan 4, 2012) patrick

Reply:

Check the building for unusual heat loss points;

Check the furnace BTUh rating against the building size and rough estimate of heat loss

Check the furnace for overheating in the heat exchanger by watching the fan limit switch dial to see if it's cutting off at high temperature

 

Question: musty odor from heating registers

I have musty smell coming from registers. Furnace is on concrete in basement, but basement is dirt floors and dirt wall. Any ideas how to fix this. I was told when they put it in the basement that the basement smell would not be smelled through duct work. Otherwise, I would have put furnace in garage. (Jan 17, 2012) Musty smell

Reply:

Musty, there is not enough information in your note to reliably diagnose and cure your musty smell complaint, but I can suggest some places to start investigating:

1. leaks into the return duct from unconditioned spaces such as dirt floor and wall basement

2. improper return air source register location picking up air and odors from #1

3. water or moisture sources in the duct system or air handler contributing to mold growth

4. a mold reservoir in the building that has not been found, telegraphed via the HVAC system

 

Question: furnace motor runs for no reason

My furnace "motor" runs on its own and for seemingly no reason. For example the temp in the house today is 67 and the thermostat is set at 58. The motor will run for hours with out the burner or blower turning on. To clarify; I only have heat and not ac. (May 7, 2012) MB said:

Reply:

Check for shorting wires controlling the furnace or fan or thermostat

 

Question: can a heat exchanger be cracked

(Nov 10, 2012) Eddie C said:
My furnace its going on and off. The repair man upon inspetion said: the heat exchange it's cracked and need to be replaced. Is that possible? It is an upflown system fairly modern, i'm unable to tell the brand for i'm away from home. Please advise thanks.

Reply:

A cracked heat exchanger is unsafe and should be replaced. I can't guess why that'd cause the furnace to cycle on and off; they may be separate problems.

Question:

(Dec 24, 2012) Robin G said:
I have a wood stove in my basement (family room). This stove heats 2/3's of my house nicely (via convection up the stair way. However, the family room is unbearably hot and the far end of the house is 5 degrees cooler than desirable.

I am thinking of running some duct work with an inline fan (air boost fan) to pump the hot air from my basement over to the other end of the house. The duct inlet temperature will be approximately 120-130 deg. F. The duct insulation will be R5. Length of duct run is approximately 90 feet.

Question: How many CFM and at what static pressure should the fan be rated for? (room size = 14 x 14 ft)

 

Question: OK to leave the furnace shut off?

I have heard that it is not good to leave my oil furnace shut off for extended periods of time...I am heating with wood & would like to keep my furnace as a backup but do not want to keep it on for the sake of keeping it on. (Dec 29, 2012) William Wotton

Reply:

The concerns are:

1. moisture causing rust in a furnace that never is heated to dry out - more of a concern for steel furnaces than cast iron units

2. solidification of soot and creosote deposits making the unit hard to clean

Have the unit thoroughly cleaned before shutting it down; inspect for and fix any unusual moisture sources.

Question: Lennox Conservalor III forced hot air furnace shuts off too much

I have a gas Lennox Conservalor III forced hot air furnace. Sometimes it shuts off and the temperature drops quickly. When I reset the Carrier thermostat installed by Con Ed, the fan will go on, but no heat. When I go down to the furnace I can hear/see the electric starter spark, but the furnace does not ignite.

I then shut off the switch next to the furnace for a couple minutes and then switch it back on and the spark begins again and usually the furnace fire up, but sometimes it takes a couple of times.

This problem occurs sporadically, but during this cool spell it has happened twice in two days. Obviously, if we ware on vacation then we would have suffered broken water pipes etc.
One addition point is that the ‘clean air filter’ message is always present even though I change the filter.

I did inform a company to come in to check it. They went through the same procedure and then just opened exhaust pipes and cleaned them out, but after awhile this problem reoccurred.
Is there something between the spark and ignition of the gas that could be the problem? (Feb 15, 2015) RLongfield3rd@hotmail.com said:

Reply:

Sure; it's time for a detailed inspection by your heating service technician. If the unit is not igniting there may be a burner, fuel, or control problem. Don't force the system to ignite as you may blow yourself up.

...

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Recommended Articles

• FURNACE CONTROLS & SWITCHES
• HEATING FURNACE INSPECTION GUIDE
• NO HEAT - FURNACE
• FURNACE OPERATION DETAILS

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