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

[morphosis]

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

 

[Geez]

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:

 

[Dalmatian90]

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.

 

[Rookie1]

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.

 

[AOD]

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

 

[wistattman]

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

 

[Gooserider]

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

 

[Rookie1]

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.

 

[Crofter]

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.

 

[CowboyAndy]

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'.

 

[Crofter]

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

 

[KsWoodsMan]

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.

 

[mtfallsmikey]

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

 

[A. Stanton]

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.

 

[Zackman1801]

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?

 

[Crofter]

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.

 

[Gooserider]

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

 

[Wood Doctor]

"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.

 

[TallElf]

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.

 

[turnkey4099]

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