When is it acceptable for appliances or heaters to share a single chimney flue?

This article describes exceptions to the general case of prohibiting shared chimney flues, multiple heating appliances, fireplaces, woodstoves all using the same chimney venting path through a building and to outside.

Shared Chimney Flue Q&A

These questions and answers about acceptable and un-acceptable shared chimney flues were posted originally at x - be sure to see that article.

Shared flues can create fire and smoke spread hazards and carbon monoxide hazards in buildings. In some communities or jurisdictions for certain cases, it is permissible to share a single chimney flue among more than one heating appliance or fireplace.

But important safety constraints still apply.

[Click to enlarge any image]

Watch out: this arrangement sharing solid fuel (wood in this cases) and oil or gas fired heaters is flatly prohibited in many jurisdictions. For example per NFPA 31 and NFPA 211

Although two openings are permitted into a single chimney flue; NO combination of solid fuel-fired appliances are permitted to be vented into an oil-fired or
gas-fired flue.

On 2018-11-03 by (mod) - remove a woodstove that is not safely connected to a shared flue

Why not have the wood-stove removed completely?

About converting from oil to gas because ... "The oil burner keeps shutting off" ... I would first get help from an experienced oil heat service technician to diagnose and fix the problem with the present system. That is almost certainly far more-economical.

On 2018-11-03 by M.J.

We have an HS Tarm oil-fired boiler with an attached wood stove for hot water heat in the concrete slab floor. We have never used the wood stove and don't plan to.

The oil burner keeps shutting off, so we want to change it to a gas burner. The man from the gas company said we could not have gas and wood in the same unit.

If there was a gas explosion, everyone involved in the installation could be sued, insurance wouldn't pay, etc.

We may sell the house in the next 10 years and propose to remove the two access doors to the woodstove and replace them with heavy steel plates so no one else would use the stove. (Of course, they might retrofit it - no control after we are gone from here!)

On 2018-11-02 by (mod) - It's unsafe to have the same chimney opening onto two different floor

It's unsafe to have the same chimney opening onto two different floors. Aside from draft problems there could be fire hazards. My suggestion is to have a certified Chimney Sweep inspect the chimney and give you a bid on sealing or making it safe.

On 2018-11-02 by Dean

I live in an older house that used to have two stoves. The one in the basement was my heat last year, but it did not keep up. I want to tie into the chimney flue on the main floor.

There is an old access there. I am abandoning the basement stove and moving it upstairs.

Will the draft still be efficient? Do I need to worry about smoke coming in through the basement? I know that might be a ridiculous question. Thanks for any help.

On 2018-10-29 by Anonymous - ok to share coal and wood on same level with water heater?

You risk a chimney fire

On 2018-10-29 by Mat

Hi we have a coal aga and we’re wondering if it would be ok to fit a wood burner running off the same liner at the same level both would heat water flue liner is 6 inch straight up and out

On 2018-10-21 by (mod) - pellet stove shares flue with woodstove?

That might be possible if there was a way to ensure that both cannot possibly be run at the same time

. I don't mean just using common sense I mean you don't know what someone else might do with your property that results in burning the place down or smoking you out.

But I wouldn't make any such a change without reviewing it with both manufacturing of the two heating appliances

On 2018-10-20 by Randy

I have a wood stove venting into a six inch stainless chimney liner.

Can I put a pellet stove next to the wood stove and vent it into the same liner.
That way I can run the pellet stove on days I'm not home all day. I would run only one stove at a time.

On 2018-08-28 2 by (mod) - use old oil fired appliance flue for woodstove?

Maybe, Dan but not without a chimney inspection, probably cleaning, and depending on what the chimney inspector finds, there could be some flue repairs needed, or even a finding that the chimney materials and design would not be safe for use with a woodstove.

Watch out: On converting fuels the chimney must be cleaned and inspected for safety and adequacy in its new use.

On 2018-08-22 by Dan987

Hello,
I have a flue that had my oil furnace and gas water heater venting into.

The oil furnace has been replace with a high efficiency gas furnace that no longer vents into this flue.

Can I vent a woodstove into this flue? If not directly can I do this through a stainless steel liner?

Thanks,
Dan

On 2018-04-09 by (mod) - cannot share a single flue with heating appliances on different floors.

No, Halla

You cannot share a single flue with heating appliances on different floors. Doing so would be unsafe, cause improper heater operation, and is prohibited by model and NH building codes.

On 2018-04-09 by halladamp@gmail.com

I have one 8x12 chimmey a oil fired boiler inthe basement being vented into it

I want to install a pellet stove on the first floor with a 4 inch liner

can this be done in Hampstead NH

On 2017-08-15 20:53:34.932027 by Anonymous

@WCTinWY, BTW, the ss liner would be for the free standing wood stove, not the propane water heater. The water heater exhaust utilizes the old brick chimney alone.

On 2017-08-15 by WCTinWY

I follow the concept of two appliances not sharing the same flue but my proposed application seems a little different. Wondering if insurance would be okay with my concept.

Propane water heater in basement goes directly into an old brick chimney that used to belong to a fireplace ages ago(no longer in existence).

I am wanting to put a stainless steel chimney liner(insulated) in the same brick chimney to vent a new free standing wood stove on the main level.

The water heater exhaust would not be sharing the same path as the wood stove would be because it is its own with the ss liner (ChimneyLinerDepot).

The propane water heater would be venting around the ss liner. Would this work? Otherwise, I'm looking at a much larger expense to make all of this happen.

On 2017-01-11 by (mod) - cannot safely vent different heating appliances of ANY type into the same chimney from two different floors in a building.

Anonymous:

As I cannot see your house nor your fireplace, chimney, nor flues, I have to answer only in arm-waving generality.

You cannot legally nor safely vent different heating appliances of ANY type into the same chimney from two different floors in a building.

Each floor needs its own flue.

The fuel wouldn't change the problem.

If you can't see into your chimnney from inside or outside, I'd suggest hiring a certified chimney sweep to inspect and advise you on chimney safety and to tell you what flues are where.

On 2017-01-11 by Anonymous

I have a wood burning fireplace on the man floor, in the basement directly below there is a rough in for another fireplace. There is only one chimney flue outside.. Can I put a gas fireplace in the basement or does it need to be wood burning also?

On 2016-09-02 by (mod) - two gas appliances can share flue on same floor but not on different floors

If both heaters are on the same floor

and

if the chimney capacity and size will safely vent the appliances in all combinations of use

• unit a operating alone
• unit be operating alone
• both units operating at once

then yes.

On 2016-11-04 by Thomas colonna

Is it ok to share one flu

Two gas units in basement sharing fireplace chimney on above floor

On 2016-09-02 by (mod) - onsite expert chimney inspection needed

I just can't buy enough insurance to bet your life on promising that an unknown- and un-seen system is "safe" - you really want an onsite expert to take a look.

She will consider the total BTUs being vented, the distances, number of elbows or bends, total chimney height, flue size, composition, chimney cap, and perhaps other onsite details.

Generally if all the other factors (including combustion air in) are all working, one can vent two gas appliances out through a shared chimney.

On 2016-09-02 by Garrison

Hi,

I have a 75 gallon Gas water heater in the basement that is vented to a flue that runs straight up and through the upper 2 floors to vent out of the roof.

There is about 4 feet of lateral flue that runs from the water heater over to the vertical flue that goes up to the roof. I am finishing the basement and I would like to install a gas fireplace.

The ideal placement would be on the opposite side of the wall where the vertical flue is, less than 3 feet away.

Is it safe and in code (Maryland, AA County) to let the gas fireplace share the flue with the hot water heater?

Thanks in advance for any insights or advice you can offer!
Garrison

On 2016-02-09 by (mod) - some jurisdictions permit certain flue sharing

Cindy:

SOME (not all) jursdictions permit the flue sharing you describe. There are some installation details that are important such as venting the gas flue into the chimney above the oil flue.

If you feel flue gas spillage at the draft hood on the water heater (other than perhaps for a minute or less when the water heater first turns on) under any circumstance (such as oil furnace on or furnace off) then the system is unsafe.

You should be sure there are CO detectors (and smoke detectors) properly located, installed, working, and tested.

On 2016-02-09 by Cindy

Is it unsafe to operate a gas hot water heater and oil furnace venting into one chimney? Looking to rent a home and concerned about carbon monoxide poisioning.

On 2015-11-09 by (mod) - multi-fuel heater (wood + oil) venting into single flue may be unsafe

While I've seen multi-fuel heaters venting into a single flue I don't think it's a particularly safe installation.

I'm worried about a fire; you'll also find that the different heaters have different draft control requirements.

Do not install such a set-up before asking for help from your local fire inspector.

On 2015-11-09 by Anonymous

can a propane boiler exhaust and a a wood fired boiler( seperate units, same floor ) share the same flue? wood fired enters flue at oppisite side, approx 3ft above gas exhaust.

On 2015-11-04 by (mod) -

No caus: you cannot safely nor legally share a chimney flue with heating appliances on different floors in a building.

On 2015-11-03 by Causilgey

I have an old building with an open fire in the basement and in the livingroom above, can I vent the basement kitchen and a log burner above up the same chimney.

...

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