Two furnaces...one flue. Bad idea?

[ejb]

I know its against most building codes, BUT how common is it to have two appliances(oil boiler plus wood boiler) on one chimney flue? Is this one of those really, really bad codes to ignore, or one that is overkill?

I have a FHW oil furnace in my cellar, vented into a brick fireplace...I want to add a wood boiler to supplement heat in the winter, but alas, just one flue.

I could vent them together, pay big $$$$ to add a new flue, or replace the entire furnace with a combo fuel furnace (I really like the specs on the TARM boilers, but they are $$$$).

To addon a wood boiler and ignore the code it will cost about $2000 and probably pay for itslef in less than one year.

To go with topend TARM multi-fuel boiler (with heat storage) will likely cost close to $15,000 and take 7 or so years to break-even. (but can stay with single flue)

Adding a flue...don't know what that would cost, but probably not cheap....

Opinions?

 

[MikePA]

I am not a chimney expert, but I think having two furnaces share a flue is a real bad and dangerous idea. Of course, you'll never know it, but the people who find your body will know that it was CO from one furnace that came back into the house through the other furnace was what killed you in your sleep. /forums/images/graemlins/shocked.gif /forums/images/graemlins/crazy.gif /forums/images/graemlins/confused.gif

 

[Wannafish]

"To go with topend TARM multi-fuel boiler (with heat storage) will likely cost close to $150000 and take 7 or so years to break-even. "

Wow! That's one high heating bill - $150,000 and will break even in 7 (or so) years! /forums/images/graemlins/grin.gif

Glad I live in Michigan.

 

[ByronBob]

When I bought a house it had 2 pipes into 1 flue. Although against code it worked ok. How much rise will the pipes have? If the rise is steep the odds of backdraft are less. If shallow then I wouldn't do it.
If you do try it I would buy several CO2 detectors to put in the basement and other places in the house. They do work well.
I'm not a lover of codes but this is one instance where I understand why they don't want you to do it.

 

[slowrev]

Just one safety note, if you heat with a combustible gas please have at least one Carbon Monoxide detector in the house.
Birds can plug up the vent, or a heat exchanger can start leaking. especially with the thin heat exchangers in the new high efficiency furnaces.
Be safe, stay alive.

Ben

 

[bczoom]

ejb,

Something else I want to throw out for consideration. Since you've added a 2nd fossil fuel furnace, do you need to look at or consider the return air (or source of air) in the basement?

I had a furnace in a previous house that was dumping its exhaust back into the basement because the burning side of things wanted air to burn but without adequate RA, the pressure in the basement was low so it sucked it right back out of the exhaust side of the furnace.

Brian

 

[beenthere]

I read that as $15,000, not $150,000 /forums/images/graemlins/wink.gif

 

[beenthere]

If it were a gas boiler and not oil, I would (myself) possibly consider using the one chimney.
I have a wood boiler on a separate chimney because originally I had an oil boiler (now have gas and the gas boiler and gas hotwater heater both vent out the same chimney).

The reason I wouldn't with oil, is the way the oil is burned with a gun-type burner. This forces air into the chimney, different than the venting of a gas burner on the boiler.

 

[Jagmandave]

In my house, built in 1964, the hot water heater vent taps into the same pipe that the furnace uses, which then goes into the chimney, seems to work just fine

 

[beenthere]

Mine did that too, but the flue size of the hotwater heater was I believe 3 or 4", whereas the wood boiler is a minimum of 6" and maybe 8".
Probably are a variety of options, depending on forced draft vs. non-forced, along with temperatures, etc. that are expected, as well as provisions for a chimney to take on creosote from the wood and soot from the oil burner. I suspect the two don't mix well in the same flue or chimney. /forums/images/graemlins/wink.gif

Bottom line, two furnaces (wood and oil) in one flue is probably a 'bad' idea. I wouldn't do it, even if only one is operating at a time.