How do I add a wood furnace to my existing oil furnace

[JA600L]

Hi guys,
I am buying a house with an oil furnace. The oil furnace is attached to the only flue in the basement.

Is there any way possible to add a wood furnace on to the oil furnace?

Is there any way that they could share the same flue. (Assuming the two are connected and working with each other. )

I plan to upgrade to a heat pump in the future but for right now this is what i have to work with. The PSG Caddy Is what I've been looking at. I'm a newbie with furnaces. I've been using a wood stove for 12 years. Thanks!

 

[peakbagger]

There are devices called power flues that vent oil boiler out through vents in the wall (with lots of limitations on locations of vents) That would free up a flue. Much as I dislike and outdoor wood boiler you could install one outside.

 

[JA600L]

The outdoor wood boiler could be an option at some point down the road, but I'm really just looking for a way to incorporate a wood furnace at this point

 

[brenndatomu]

What size is the chimney flue? 8"?
Is it lined...or just masonry?

 

[JA600L]

8" masonry from what I can tell.

 

[3fordasho]

In the picture with the blue pressure tank, there is an opening in the foundation wall with plywood over it - is that an old window opening?
If so I would consider running 6" Class A Supervent through that opening and go straight up the side of the house for a new flue for the wood furnace.

 

[gthomas785]

Wood sharing a flue with any other appliance is almost always forbidden, and even if it's legal in your area it's a bad idea. If you want to have both I'd second peakbagger's suggestion of converting the chimney to wood and using a power vent for the oil. In that case you'd benefit a great deal from dropping an insulated liner down the chimney when making the switch.

 

[brenndatomu]

I'd avoid a power vent, except for a very last resort...if you talk to people that have them, they will usually tell you they'd not install another.
I like the idea of putting a new class A stainless steel chimney out through the wall, either through that old window(?) or a newly created hole...that said, you can do oil and wood on the same chimney, heck, it used to be super common!
You'd have to make sure that you don't need to have an inspection/anyone sign off on it, as most won't anymore, but if done correctly, wood and oil can play together just fine...wood and gas is the combo that I would avoid on the same chimney, too many potential issues there.
You'd have to find out what size burner/nozzle is in the oil furnace, as if its too big, then it would really need to be on its own flue.

 

[gthomas785]

you can do oil and wood on the same chimney, heck, it used to be super common!

Used to be is key. Just cause it was done a lot in the past doesn't mean it works well; technology and standards have improved.

wood and gas is the combo that I would avoid on the same chimney,

Now I'm really curious, what's so different that makes oil okay but not gas? I can think of plenty of reasons to avoid sharing a gas flue with wood but those reasons would apply to an oil burner also.

 

[brenndatomu]

Just cause it was done a lot in the past doesn't mean it works well

Understood, and agreed, sorta.
If things are done correctly, it can work well, and there is absolutely nothing wrong with it...I have researched this at length, and there is no national law or code against it...there are however some codes against it locally, in places, but it seems like that was done as more of an easy "out", as opposed to going to the work of making sure that it is just done correctly. It's just been discouraged so long now that everybody (including some HVAC techs) thinks that it is "illegal", nope.

Used to be is key. Just cause it was done a lot in the past doesn't mean it works well; technology and standards have improved.


Now I'm really curious, what's so different that makes oil okay but not gas? I can think of plenty of reasons to avoid sharing a gas flue with wood but those reasons would apply to an oil burner also.

Well, one of the big ones is standing pilot heaters...it allows an easy point of backdraft into the basement, especially if the pilot goes out.
Another, at least in my mind, is if something failed allowing the gas appliance to dump unburnt gas into the flue (I know, unlikely, but not impossible) another source of flame tied into the flue would not be a good idea...but not an issue with oil. I'm sure a standing pilot flue tied into a wood burning flue could be another easy source of air if there was a chimney fire too...but with a modern wood heater, dry wood, and proper burning/cleaning habits, that risk should be zero anyways...but the code police will still always try to save "the stupid" from themselves.

But the biggest thing is just having things sized right...small enough to draft well with a modern efficient wood heater, and large enough to handle both furnaces running at the same time...which may not be possible depending on the size/heat load of the house (a large oil furnace)
As an example, an 8"chimney, SS lined (316 Ti preferred) and insulated, will handle a wood furnace like the OP is interested in, with an 80k btu oil furnace no problem...maybe even a little larger, depending on the burner and furnace design.

 

[bholler]

Understood, and agreed, sorta.
If things are done correctly, it can work well, and there is absolutely nothing wrong with it...I have researched this at length, and there is no national law or code against it...there are however some codes against it locally, in places, but it seems like that was done as more of an easy "out", as opposed to going to the work of making sure that it is just done correctly. It's just been discouraged so long now that everybody (including some HVAC techs) thinks that it is "illegal", nope.

Well, one of the big ones is standing pilot heaters...it allows an easy point of backdraft into the basement, especially if the pilot goes out.
Another, at least in my mind, is if something failed allowing the gas appliance to dump unburnt gas into the flue (I know, unlikely, but not impossible) another source of flame tied into the flue would not be a good idea...but not an issue with oil. I'm sure a standing pilot flue tied into a wood burning flue could be another easy source of air if there was a chimney fire too...but with a modern wood heater, dry wood, and proper burning/cleaning habits, that risk should be zero anyways...but the code police will still always try to save "the stupid" from themselves.

But the biggest thing is just having things sized right...small enough to draft well with a modern efficient wood heater, and large enough to handle both furnaces running at the same time...which may not be possible depending on the size/heat load of the house (a large oil furnace)
As an example, an 8"chimney, SS lined (316 Ti preferred) and insulated, will handle a wood furnace like the OP is interested in, with an 80k btu oil furnace no problem...maybe even a little larger, depending on the burner and furnace design.

There absolutely is national mechanical and fire codes forbidding it. Although there is one state NH I believe that hasn't adopted that specific code.

And yes it can work but absolutely never works well. Every one we work on is a glazed mess or creosote. To much dilution air is introduced into the flue through the oil or gas furnace and drops the flue temp. It's a very bad idea and is a code violation in almost every state in the country

 

[gthomas785]

there is no national law or code against it...

Check out 9.8.2 in NFPA 211

9.8.2 Unless listed for such connection, solid fuel–burning appliances shall not be connected to a chimney flue serving another appliance.

 

[peakbagger]

Maine reportedly has a limited exception for existing homes. It was passed against the strong objections of the Maine Fuel Board.

 

[bholler]

Maine reportedly has a limited exception for existing homes. It was passed against the strong objections of the Maine Fuel Board.

Ok maybe it was Maine. I knew there was one

 

[gthomas785]

Taking the safety and code issues out of it,

But the biggest thing is just having things sized right...

YES - and it's simply not possible to have a single chimney that is always correct size for an oil burner OR a wood furnace OR both together. In one case or another the performance will suffer. My wood stoves are picky enough about draft as it is that I would never even contemplate adding a variable source of (hot/cold//damp/dry//high flow/low flow) exhaust into the flue.

I'll admit power venters do have their drawbacks; they can be noisy especially if not maintained, and it's another potential point of failure in the mechanical system which is not ideal when the oil heat is being relied on as a backup. So, get a second chimney if possible. But sometimes economic, aesthetic, or physical placement issues can dictate that a chimney is not practical so a power venter would be the next best option.

 

[salecker]

I have an 8" chimney in my boiler building
Wood boiler goes straight out 3 ft then 90 up into the 8"
Oil boiler sits 4 ft away from the wood boiler. It has 6" that goes up 4 ft then 90 into the 8" shared by the wood boiler.
Zero issues since it was installed 13 years ago. Both units have been in operation at the same time with zero issues sharing the 8"
This building is not attached to my home.

 

[brenndatomu]

There absolutely is national mechanical and fire codes forbidding it

Well, apparently all 3 people industry people I talked to were full of crap...I never found proof positive online either...although you almost have to be a lawyer to read through code stuff, plus some of it has to be paid for, so no access for Joe Shmoe without being willing to pay up.

yes it can work but absolutely never works well. Every one we work on is a glazed mess or creosote

Guess I caught a unicorn, because I have one with none of these issues at all...since 2017-2018...I'd have to look it up again.
When I clean I get a little dry powdery soot, and maybe a bit of dry flakey "crispys" at the top...but mostly all powder. The last time I seen any real creosote was with the Yukon Husky wood/oil combo furnace...now that thing could make a slobbery mess!
Obviously a modern clean burn furnace and good dry fuel is key to all this for me.

Check out 9.8.2 in NFPA 211

Thanks.

YES - and it's simply not possible to have a single chimney that is always correct size for an oil burner OR a wood furnace OR both together. In one case or another the performance will suffer. My wood stoves are picky enough about draft as it is that I would never even contemplate adding a variable source of (hot/cold//damp/dry//high flow/low flow) exhaust into the flue.

See my above reply...no issues. I have more trouble (still very rare) with the other stove that is hooked up a "textbook ideal" flue...6" lined/insulated 17' height...this is on a Drolet 1400i, which is not a picky stove for needing strong draft.
I can do cold starts in warm weather, no problem...no smoke rollout on reloading...even running the two furnaces together works flawlessly, although that is very rarely done.

 

[bholler]

Well, apparently all 3 people industry people I talked to were full of crap...I never found proof positive online either...although you almost have to be a lawyer to read through code stuff, plus some of it has to be paid for, so no access for Joe Shmoe without being willing to pay up.

Guess I caught a unicorn, because I have one with none of these issues at all...since 2017-2018...I'd have to look it up again.
When I clean I get a little dry powdery soot, and maybe a bit of dry flakey "crispys" at the top...but mostly all powder. The last time I seen any real creosote was with the Yukon Husky wood/oil combo furnace...now that thing could make a slobbery mess!
Obviously a modern clean burn furnace and good dry fuel is key to all this for me.

Thanks.

See my above reply...no issues. I have more trouble (still very rare) with the other stove that is hooked up a "textbook ideal" flue...6" lined/insulated 17' height...this is on a Drolet 1400i, which is not a picky stove for needing strong draft.
I can do cold starts in warm weather, no problem...no smoke rollout on reloading...even running the two furnaces together works flawlessly, although that is very rarely done.

So I shouldn't correct I accurate info given about codes now? Why not?

 

[brenndatomu]

So I shouldn't correct I accurate info given about codes now? Why not?

I never said that...I just said apparently the people I talked to were full of it, and that what you said about how it will work, has not been my experience...would I suggest for someone to do an install like I have after having your experience? Probably not.
Not changing mine though either...no reason to.

 

[bholler]

I never said that...I just said apparently the people I talked to were full of it, and that what you said about how it will work, has not been my experience...would I suggest for someone to do an install like I have after having your experience? Probably not.
Not changing mine though either...no reason to.

And I work on lots that people aren't changing either. I tell them it's not right but have no power and don't want the power to say they cant.

 

[bholler]

Well no let me correct that not lots anymore. Only a few now.

 

[brenndatomu]

And I work on lots that people aren't changing either. I tell them it's not right but have no power and don't want the power to say they cant.

Are their chimneys clean? Mine is.
Is their draft good? Mine is...verified by my Dwyer manometer, which is connected 100% of the time.
Dunno what I'd be fixing...

 

[bholler]

Are their chimneys clean? Mine is.
Is their draft good? Mine is...verified by my Dwyer manometer, which is connected 100% of the time.
Dunno what I'd be fixing...

No their chimneys are horrible. No idea if their draft is right and I am not doing anything but cleaning them with a note on the invoice calling out the code violation or violations

 

[brenndatomu]

I am not doing anything but cleaning them with a note on the invoice calling out the code violation or violations

Gotcha...yeah you gotta CYA

 

[bholler]

Gotcha...yeah you gotta CYA

Exactly