# Why can't a woodstove share a flue with an oil burner?



## morphosis (Nov 25, 2008)

My friend has his woodstove's 6" flue connected to his boiler's 8" flue, which then goes into an 8" x 8" clay lined masonry chimney. Despite my efforts to inform him that this is a dangerous setup, he insists that it's fine. He doesn't care what the building code says because it doesn't say why it shouldn't be done.

So what I need is some concrete evidence to disprove his theory. Can anyone help me? Thanks


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## Geez (Nov 25, 2008)

Setting aside the fact that carbon monoxide from the oil burner could find it's way into the living space through the woodburner, there is the problem of the oil burner's barometric damper ruining the draft for the wood stove. :monkey:


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## Dalmatian90 (Nov 25, 2008)

When he's running the stove, the barometric damper on the furnance will open. That will reduce the vacuum on the stove, so it draws in less air and increase the amount of "puff back" he'll have. Likewise the furnace will be a bit unhappy too.

It also opens up another path for CO to enter the living areas.

In the event he has a bad chimney fire that damages the main flue, he's looking at rounding up space heaters or draining his water pipes on a cold winter's night because he won't be able to use that flue for the oil furnance either.


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## Rookie1 (Nov 25, 2008)

Long story short. I ended up putting oilfurnace in 5" s.s.flue in 12x12 unlined brick chimney and woodburner in 6"s.s. flue in same cavity as other. looks a little stupid where they come out up top but they work fine.


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## AOD (Nov 25, 2008)

Oil furnace, we don;t need no stinking smoking oil furnace in this thread!


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## wistattman (Nov 25, 2008)

I don't know what the problem is either. When a heating contractor installed my parent's wood add-on furnace, they ran the wood and oil furnace together in one chimney. However, there was a stainless steel liner installed in the clay tiled chimney (tiles were badly cracked) that both the wood & oil uses. There has been no problems with the operation of either the oil or wood furnaces. 

Since installation of this wood furnace was done in 1981, I suppose building codes have been updated so building contractors / builders can make more money. 

Just my 2 cents.
Gary


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## Gooserider (Nov 26, 2008)

Rookie1 said:


> Long story short. I ended up putting oilfurnace in 5" s.s.flue in 12x12 unlined brick chimney and woodburner in 6"s.s. flue in same cavity as other. looks a little stupid where they come out up top but they work fine.



Technically, that is against code also - there is supposed to be a certain minimum separation, (4" of masonry IIRC)or both flues need to be Class A pipe in order to avoid the risk of a fire in one compromising the other.

As to the OP question, basically you have the answer in the prior posts - 
1. The HIGH risk of backpuffing smoke from one appliance into the other, and then into the living space. 

2. The Oil burner automatic draft damper causing problems for the draft on the wood stove.

3. There are some concerns about mixing the differing exhaust chemistries - potential for corrosion problems, and / or increased deposits.

4. Not mentioned, but each appliance will have it's own cross section area requirements, and a shared flue would need to be big enough to accomodate both running at the same time - but that would probably exceed the maximum allowed on the wood stove. (In the OP's case, he doesn't have the combined area, but the wood stove is marginally OK)

The rule for many years has been "One Flue / One appliance" - there are only two narrow exceptions that I know of - 

1. You can share a flue between a furnace and a hot water heater IF they are both the same fuel (gas or oil) and you meet certain requirements for placement.

2. There are a couple of multi-fuel (oil or gas and wood) boilers that are UL LISTED to share the same flue.

Bear in mind that the codes mostly aren't written to "make installers money" - they are written because people DIED doing it some other way... I had a friend (now deceased) who used to refer to the NEC as "The book of the dead" for that reason.

Gooserider


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## Rookie1 (Nov 26, 2008)

Goose I kinda knew that but to me that was the easiest safe way to add woodburner to basement near the oilfurnace(shh dont let AOD hear) without sharing chimney. Im sure there could be some issues but 3 years so far and no problems.


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## Crofter (Nov 26, 2008)

One of the main reasons it is not allowed is the flame out shutoff on the oil burner is not instantaneous and it can pump vaporised fuel for up to thirty seconds before it shuts down the pump. Probably enough to fill the chimney and then any spark from the wood burner can cause an explosion.

The float controlled flame out shut off mode in the non gun type oil burners can collect a lot of fuel in the bottom that could be ignited stuff falling down from the wood unit exhaust.

With gas fired, the same un ignited fuel is a problem but the draught issue is certainly a factor.

You could get away for a long time without having a problem so you will find lots of people who will say that there is nothing wrong with units sharing a flue but thar is a reason behind the rule.


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## CowboyAndy (Nov 26, 2008)

Okay, I have seen the barametric damper topic come up several times so far in this thread and I have to ask, whats the deal with combo units then? I have a wood/oil combo and have to have the barametric damper installed because of the oil. Is it having a negative effect on the wood burning? the furnace is a newmac cl86, and my chimney is 26'.


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## Crofter (Nov 26, 2008)

On the multi fuels is there not a mechanical interlock to prevent controls from one mode to affect the other?


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## KsWoodsMan (Nov 26, 2008)

morphosis said:


> My friend has his woodstove's 6" flue connected to his boiler's 8" flue, which then goes into an 8" x 8" clay lined masonry chimney. Despite my efforts to inform him that this is a dangerous setup, he insists that it's fine. He doesn't care what the building code says because it doesn't say why it shouldn't be done.
> 
> So what I need is some concrete evidence to disprove his theory. Can anyone help me? Thanks



When the wood burner is down to coals it is producing lots of carbon monoxide. It doesnt have much draft to carry them up the chimney quickly. If the second appliance ever has a negative draft that draws flue gasses from the pipe, deadly gasses will build up inside it and spillout into the living area of his home.

Death is pretty concrete, brain damage is not so much, but still undesirable from our standpoint. Not realizing there is a problem is much different than there not being one.


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## mtfallsmikey (Nov 26, 2008)

All great answers, no more to add except...walk briskly away from that kind of setup!:angry2:


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## A. Stanton (Nov 26, 2008)

I just had a chat with my local fire marshall about this subject. He said that this set up could allow for CO to enter the unused flue.


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## Zackman1801 (Nov 26, 2008)

heres my question im not understanding something, can you have a furnace and a stove connected to the chimney on 2 seperate flues, or is that still a no no?


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## Crofter (Nov 26, 2008)

Zackman1801 said:


> heres my question im not understanding something, can you have a furnace and a stove connected to the chimney on 2 seperate flues, or is that still a no no?



Depends what you call the chimney. Sometimes it refers to a single mass of masonry that can have one or more separate flues. I think each one to qualify as separtate has to have a minimum of 4" between it and its neighbour.

One flue cannot have more than one device connected to it even if on different floors.


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## Gooserider (Nov 26, 2008)

Crofter said:


> Depends what you call the chimney. Sometimes it refers to a single mass of masonry that can have one or more separate flues. I think each one to qualify as separtate has to have a minimum of 4" between it and its neighbour.
> 
> One flue cannot have more than one device connected to it even if on different floors.




Exactly... It's a matter of definitions...

_Chimney_ - a structure, usually made of masonry that *CONTAINS* one or more _FLUES_ which are passages for combustion byproducts to exit a structure... 

You can have as many flues in a chimney as you want, as long as each is compliant with all the relevant standards. While most of us think of a chimney as a masonry structure, and flues in a masonry chimney must be separated by at least 4" of masonry, it is also possible to build a wooden chase that contains multiple runs of Class A insulated pipe, and that would also be counted as a chimney....

In any case, with the exception of sharing a flue between a furnace and water heaters that are ON THE SAME LEVEL, and burn the same fuel, the rule is One appliance per flue. 

It is *ESPECIALLY* bad to share a flue between appliances on _different_ levels, as it's a good recipe for getting smoke backpuffing from the lower one.

Gooserider


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## Wood Doctor (Nov 26, 2008)

"It is ESPECIALLY bad to share a flue between appliances on different levels, as it's a good recipe for getting smoke backpuffing from the lower one."
-------------------
So, if your are going to do this and starving to death, I would recommend that the wood stove enters the chimney above the oil furnace entry. However I do not recommend flue sharing.

Remember that downdrafts are real occurrences. Totally separate but parallel chimney flues will often downdraft into each other. In the right conditions, the active fire can belch smoke all the way down the dormant flue and back into the house.


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## TallElf (Nov 27, 2008)

AOD said:


> Oil furnace, we don;t need no stinking smoking oil furnace in this thread!



I agree. The only reason i have a Naggy.. (Nat gas) is for the hot water baseboard that backs me up when I run away for more than 10 hours. 

Still thinkin of doin an OWB eventually, but love the feel of the wood stove inside. besides, can't persuade the wifey to go to a cook stove. NOT YET.


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## turnkey4099 (Nov 27, 2008)

Pretty well covered by all posters.

1. It does not meet code.

2. It will invalidate your insurance if any thing happens. My bet is that the insurance would not even pay off on a chimney fire. In my case, they would cancel the insurance if they even found out about it. The gave my sstove installation a _very_ thorough inspection before continueing coverage.

3. You can't sell the house without fixing it right.

Harry K


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## CowboyAndy (Nov 28, 2008)

turnkey4099 said:


> Pretty well covered by all posters.
> 
> 1. It does not meet code.
> 
> ...



One common misconception is that an insurance company wont cover something that is not code/legal/etc. They will infact cover you for something, no matter how stupid. They will most likely drop you and other carriers will charge you an arm and a leg if at all...

homeowners ins is designed to cover acts of nature and acts of stupidity.


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## Rookie1 (Nov 28, 2008)

TallElf said:


> I agree. The only reason i have a Naggy.. (Nat gas) is for the hot water baseboard that backs me up when I run away for more than 10 hours.
> 
> Still thinkin of doin an OWB eventually, but love the feel of the wood stove inside. besides, can't persuade the wifey to go to a cook stove. NOT YET.



Thats the same reson I have oil furnace, If we go away for a while I dont want house getting too cold. All my other appliances are elect. Im not doing away with that.


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## Crofter (Nov 28, 2008)

CowboyAndy said:


> One common misconception is that an insurance company wont cover something that is not code/legal/etc. They will infact cover you for something, no matter how stupid. They will most likely drop you and other carriers will charge you an arm and a leg if at all...
> 
> homeowners ins is designed to cover acts of nature and acts of stupidity.



If you make an installation that is forbidden in the contract you signed, after you have signed it, and that installation is found to be the cause of the damage, lots of luck! Hire a lawyer and hope you live longer than the insurance company can stonewall!


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## jburlingham (Nov 28, 2008)

CowboyAndy said:


> *One common misconception is that an insurance company wont cover something that is not code/legal/etc. They will infact cover you for something, no matter how stupid.* They will most likely drop you and other carriers will charge you an arm and a leg if at all...
> 
> homeowners ins is designed to cover acts of nature and acts of stupidity.



They may or may not chose to cover a variety of items. It is true that depending what you want to spend, there is an insurance carrier that will offer an overpriced policy to cover things, however they are not required to cover things they deem a risk, and they can deny coverage, cancel policies, and force repairs to maintain coverage. It is more common to see them force changes or cancel policies now do to economy.


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## panhead9 (Nov 30, 2008)

One main problem in this is creosote dropping down to chimney base and plugging off boiler as quite a few chimneys are filled to where the smoke pipe enters it thus creating a CO factory for all occupants. This is a problem as is draft with 2 appliances in the same flue on different levels.


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## woodfarmer (Nov 30, 2008)

Wood Doctor said:


> "It is ESPECIALLY bad to share a flue between appliances on different levels, as it's a good recipe for getting smoke backpuffing from the lower one."
> -------------------
> So, if your are going to do this and starving to death, I would recommend that the wood stove enters the chimney above the oil furnace entry. However I do not recommend flue sharing.
> 
> Remember that downdrafts are real occurrences. Totally separate but parallel chimney flues will often downdraft into each other. In the right conditions, the active fire can belch smoke all the way down the dormant flue and back into the house.



actually it is the opposite, oil above wood if they share the same flue. 
also, the reason an add-on wood furnace may share the same flue pipe as an oil furnace is because the manufacturer has design them to operate together, with the understanding that only black pipe be used and not galvanized flue pipe


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## Crofter (Nov 30, 2008)

Will the combination wood/oil units actually fire together. I looked at one a friend had maybe thirty years ago and there seemed to be an interlock (that kept them from operating simultaneously) that you had to manually switch.


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## Melvin Caldwell (Nov 9, 2018)

I had an oil furnace that had to be disabled last year due to a cracked fire box. Can I vent a wood furnace into that flue?


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## Marshy (Nov 9, 2018)

While I wouldn't advise anyone to use that type of connection, it can work without adverse consequences if the chimney drafts well. Have I done it? Yes. Did I have issues? No. Did I know it was against code? Yes. Did I have puff back? No. Excess creosote? No but, likely because it was an old stove with no baffle so flue temps were hot and wood seasoned down to 20% or less. I eventually changed, got rid of the stove and put in a wood boiler and ran it's own class A manufactured stove pipe. If anything happens to the house/chimney, I want insurance to cover it. I'm not sure they would if they found it to not comply with code. Besides, the code is there for our own good really.


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## NSMaple1 (Nov 10, 2018)

Melvin Caldwell said:


> I had an oil furnace that had to be disabled last year due to a cracked fire box. Can I vent a wood furnace into that flue?



Maybe. Depends on if it is sound and if it meets the wood furnace requirement specs.


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