Wood heat - existing fireplace, options? Stove, heat exchanger, etc

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FlyFish'n

New Member
Oct 23, 2021
62
OH
Hi All,

1st post here.

I'm trying to iron out an option for getting wood heat going here as a backup heat source.

For some background - we have a double-sided fireplace. It is a Lenox CST-38 with an 8" flue pipe and approximately 14-15ft of chimney height.

Without significant modification (IE - taking out the "fireplace" fish tank assembly and rebuilding a new hearth entirely) - an insert-style stove is out of the question.

I have thought of doing a stand-alone stove. To do this the right way it appears that the flue pipe needs to be pulled and replaced with a proper certified stove pipe system. This would require opening up the chase to install a through-the-wall thimble. The good thing is we have plenty of open height so as to taper the connecting pipe from a stove to the thimble with both rise above the stove and angle. It is a vaulted ceiling so we could go pretty high, but if we went this way (stand alone stove) I would want to tap the chase with the thimble at about the 6-8ft height above the floor. The stand-alone stove option would likely render the fireplace unusable, that is unless there is a type of adapter we could get that would allow the use of either the stove or fireplace. Otherwise the fireplace can be decommissioned while the stove is in service, though the thought would be to go back to the fireplace down the road.

The other thought I have is to fabricate a heat exchanger that sits at the top of the "fish tank" fireplace box and allows air to be blown through so hot air comes out in to the room. I don't particularly like this idea. I would have to come up with a way to control the air intake to control the burn, otherwise it is a freely burning fire and no way to regulate other than how much wood is in there.

My primary concern with wood heat is mitigating the risk of chimney fires. To that point - my intent is to get a chimney sweep brush. That could probably be its own entire discussion, but for the discussion in this thread my point here is I am aware of the risk and would want to understand how to properly maintain flue pipe myself. I've done radio tower work and have a lot of fall protection gear - working on a roof isn't a big deal. If I was sweeping the chimney I would be tied off up there for sure.

As to the options - stove vs heat exchanger - I have heard that it is possible to extract enough heat out of the gasses going up the flue to accelerate the accumulation of creosote build up. The reference I got was that keeping the flue temp above 400deg F is preferable.

Wouldn't a stove be more efficient (and by that I mean more BTU's in to the room and less BTU's up the chimney for what ever given available BTU's from the wood being burned) as opposed to any "heat exchanger"? Or is it actually possible to get more heat out of a "heat exchanger" than a proper stove?
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
26,867
central pa
Hi All,

1st post here.

I'm trying to iron out an option for getting wood heat going here as a backup heat source.

For some background - we have a double-sided fireplace. It is a Lenox CST-38 with an 8" flue pipe and approximately 14-15ft of chimney height.

Without significant modification (IE - taking out the "fireplace" fish tank assembly and rebuilding a new hearth entirely) - an insert-style stove is out of the question.

I have thought of doing a stand-alone stove. To do this the right way it appears that the flue pipe needs to be pulled and replaced with a proper certified stove pipe system. This would require opening up the chase to install a through-the-wall thimble. The good thing is we have plenty of open height so as to taper the connecting pipe from a stove to the thimble with both rise above the stove and angle. It is a vaulted ceiling so we could go pretty high, but if we went this way (stand alone stove) I would want to tap the chase with the thimble at about the 6-8ft height above the floor. The stand-alone stove option would likely render the fireplace unusable, that is unless there is a type of adapter we could get that would allow the use of either the stove or fireplace. Otherwise the fireplace can be decommissioned while the stove is in service, though the thought would be to go back to the fireplace down the road.

The other thought I have is to fabricate a heat exchanger that sits at the top of the "fish tank" fireplace box and allows air to be blown through so hot air comes out in to the room. I don't particularly like this idea. I would have to come up with a way to control the air intake to control the burn, otherwise it is a freely burning fire and no way to regulate other than how much wood is in there.

My primary concern with wood heat is mitigating the risk of chimney fires. To that point - my intent is to get a chimney sweep brush. That could probably be its own entire discussion, but for the discussion in this thread my point here is I am aware of the risk and would want to understand how to properly maintain flue pipe myself. I've done radio tower work and have a lot of fall protection gear - working on a roof isn't a big deal. If I was sweeping the chimney I would be tied off up there for sure.

As to the options - stove vs heat exchanger - I have heard that it is possible to extract enough heat out of the gasses going up the flue to accelerate the accumulation of creosote build up. The reference I got was that keeping the flue temp above 400deg F is preferable.

Wouldn't a stove be more efficient (and by that I mean more BTU's in to the room and less BTU's up the chimney for what ever given available BTU's from the wood being burned) to use a proper stove as opposed to any "heat exchanger"? Or is it actually possible to get more heat out of a "heat exchanger" than a proper stove?
You will never get anywhere near the efficiency from a fireplace that you will from a stove. And you aren't allowed to modify that fireplace in any way regardless
 

FlyFish'n

New Member
Oct 23, 2021
62
OH
You will never get anywhere near the efficiency from a fireplace that you will from a stove. And you aren't allowed to modify that fireplace in any way regardless
Can you elaborate please - on all? That is a rather short reply.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
26,867
central pa
Can you elaborate please - on all? That is a rather short reply.
Ok there is nothing you can do to your existing fireplace that would make it anywhere near as efficent as a modern wood stove. It just isn't designed in a way that makes it possible. And regardless you are dealing with a ul listed appliance. Doing any modifications to it voids the listing and violates code.
 

FlyFish'n

New Member
Oct 23, 2021
62
OH
Ok there is nothing you can do to your existing fireplace that would make it anywhere near as efficent as a modern wood stove. It just isn't designed in a way that makes it possible. And regardless you are dealing with a ul listed appliance. Doing any modifications to it voids the listing and violates code.
If the logs are burning already they are putting out a set amount of BTU's. If the BTU's aren't going in to the house they are all going up the chimney, right?

So what would putting more BTU's in to the room, instead of up the chimney, do to that system that would damage something?

That gets back to the cooling of flue gasses I mentioned - the number that was mentioned to me was 400deg. Below that creosote/soot may accumulate quicker. So wouldn't the metric be the temperature of the flue gasses? I would certainly imagine if it was even possible to regulate the flue gas temp to keep it no lower than 400deg then there are plenty of available BTU's that can be harvested and put in the room.

There are heat reclaimers you can put in stove pipes, for example, to pull out more BTU's that, otherwise, would be lost up the flue. Thats the same theory - decreasing the energy lost up the flue.

No matter where logs are burned they output heat - even a camp fire in the woods. Containing it brings in to question draft to feed fresh air and vent exhaust. And if I am understanding the systems correctly - that is the importance of the flue gas temp - so that there is enough draw up the flue at a high enough temp to keep the creosote from accumulating.

It would seem the more energy up the flue (any flue) the more damage that could occur, not the other way around so long as creosote doesn't accumulate and catch on fire.

Is my theory here sound? If not - where and why?
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
26,867
central pa
If the logs are burning already they are putting out a set amount of BTU's. If the BTU's aren't going in to the house they are all going up the chimney, right?

So what would putting more BTU's in to the room, instead of up the chimney, do to that system that would damage something?

That gets back to the cooling of flue gasses I mentioned - the number that was mentioned to me was 400deg. Below that creosote/soot may accumulate quicker. So wouldn't the metric be the temperature of the flue gasses? I would certainly imagine if it was even possible to regulate the flue gas temp to keep it no lower than 400deg then there are plenty of available BTU's that can be harvested and put in the room.

There are heat reclaimers you can put in stove pipes, for example, to pull out more BTU's that, otherwise, would be lost up the flue. Thats the same theory - decreasing the energy lost up the flue.

No matter where logs are burned they output heat - even a camp fire in the woods. Containing it brings in to question draft to feed fresh air and vent exhaust. And if I am understanding the systems correctly - that is the importance of the flue gas temp - so that there is enough draw up the flue at a high enough temp to keep the creosote from accumulating.

It would seem the more energy up the flue (any flue) the more damage that could occur, not the other way around so long as creosote doesn't accumulate and catch on fire.

Is my theory here sound? If not - where and why?
Your theory is right the problem is that there is no way to apply that theory to your fireplace legally at all. And even if you could the fire in your fireplace is burning so incompletely that taking much heat out of the exhaust will creat large amounts of creosote. This has been tried many times many different ways and simply doesn't work well

And yes there are heat relaimers you put in the pipe. And they are an absolutely horrible product that leads to lots of problems.

You are missing a key component of efficiency when heating with wood. That is how completely the wood is burned. An open fireplace does this very poorly.

Modern woodstoves on the other hand burn more completely to start with and then burn most of the material left out of the smoke
 
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FlyFish'n

New Member
Oct 23, 2021
62
OH
And yes there are heat relaimers you put in the pipe. And they are an absolutely horrible product that leads to lots of problems.
Can you elaborate on the problems you've seen? And can you put a root cause to the problems as being the drop in flue temp? Or is there something else more common?
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
26,867
central pa
Can you elaborate on the problems you've seen? And can you put a root cause to the problems as being the drop in flue temp? Or is there something else more common?
In some cases they didn't extract enough heat to do anything. In others the heat exchanger reduced draft enough the fireplace didn't work. And if you do end up extracting enough heat you will have creosote issues.

And don't forget you can't modify your fireplace
 

bigealta

Feeling the Heat
May 22, 2010
333
Utah, NJ
Go to a guys house with a woodstove heating it.
Then go to a guys house with a fireplace heating it.
Which house do you think will be warm, and which house is cold everywhere inside except directly in front of the flames.
And which one uses a ton less wood?
 

bigealta

Feeling the Heat
May 22, 2010
333
Utah, NJ
Fish Tank fireplace (my brother has one) is like a Lambo with a blown engine. Looks good but it is not taking you anywhere.
 

stoveliker

Minister of Fire
Nov 17, 2019
2,860
Long Island NY
I think you need to realize that fireplaces ar for viewing fire. The big flue (diameter) and uncontrolled burning dumps all the heat, except for IR radiation, into the flue.

Wood stoves are for heat. And you get the fire view as a bonus.
You need to decide what you want.

Note that when bholler mentions legal, it means that if chit hits the fan, your insurance my not pay out. Meaning you might loose the largest investment most people have, without a saving grace of insurance. Do you want to run that risk?
 
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Woodsplitter67

Minister of Fire
Jan 19, 2017
2,081
Woolwich nj
I agree with all of the above, solid advice is being given and the OP should take note
A fireplace is mostly for viewing the fire, all heat goes up the chimney and the fire had nothing to transfer the BTUs to

A wood stove is freestanding and the fire transfers its energy to the steel or cast iron before exiting the stove and up the chimney.. the heated steel or cast iron then transfers the BTUs to the room as usable heat
The op is concerned about creosote buts want to install a heat reclamer which will cool the flue gasses and create the creosote that he wants to avoid..