Reclaim Flue Heat from top of chimney?

[adam7]

Hey all, I recently installed a Lopi Answer wood stove in our fireplace that previously had gas logs. I used a SS flex liner.

My idea is to make and install a heat exchanger that would attach to the top of the flue and run water through it. The flue gases would travel horizontally while being cooled so as not to stop the flow. I think I should be able to lower the gases from ~300 to 150-200 or lower.

I would run the water lines to a storage tank and a radiator with fan in the house.

The heat exchanger will be easy to disconnect in case of power outage, etc.

Has anyone tried this or any other method of reclaiming heat at the flue exit point?

Can you see any down side to this?

 

[bholler]

The downside is that you would have a nasty mess of creosote clogging it in a matter of days.

 

[Ludlow]

Sometimes good ideas are best left unrealized lest they become bad ideas.

 

[mellow]

It's been done for a long time, you will be more effective with the coils ON or IN the stove itself if you're going to do it correctly.

http://solarhomestead.com/using-your-wood-stove-to-heat-water/

 

[maple1]

Not a good idea.

 

[bholler]

It's been done for a long time, you will be more effective with the coils ON or IN the stove itself if you're going to do it correctly.

http://solarhomestead.com/using-your-wood-stove-to-heat-water/

Even done like that it can say to much heat causing creosote buildup

 

[electrathon]

Your general concept may be good, but the application of grabbing the heat above the stove is poor. You will have terrible creosote issues. The article above is good. I have an old thread on here ( https://www.hearth.com/talk/threads/hot-water-from-your-stove.146898/#post-1977739 ) about my install. If you are going to do it be aware of expansion and circulation issues. You pump must be drinking water rated if you do your domestic hot water. My only big disagreement with the article posted above is that I don't feel that copper, especially soft copper, should ever be located inside a firebox. The temperature is far to hot and it is far to corrosive of an environment. Use stainless, or if you are doing hack work use galvanized inside the box. Transfer to hard copper once outside the box.

 

[maple1]

Your general concept may be good, but the application of grabbing the heat above the stove is poor. You will have terrible creosote issues. The article above is good. I have an old thread on here ( https://www.hearth.com/talk/threads/hot-water-from-your-stove.146898/#post-1977739 ) about my install. If you are going to do it be aware of expansion and circulation issues. You pump must be drinking water rated if you do your domestic hot water. My only big disagreement with the article posted above is that I don't feel that copper, especially soft copper, should ever be located inside a firebox. The temperature is far to hot and it is far to corrosive of an environment. Use stainless, or if you are doing hack work use galvanized inside the box. Transfer to hard copper once outside the box.

The concept is not good either. OP intends to heat water to then use for space heating via rad and fan. Would be a wasted effort IMO.

 

[electrathon]

The concept is not good either. OP intends to heat water to then use for space heating via rad and fan. Would be a wasted effort IMO.

I totally agree, he will get far less heat than he thinks he will. But heat is heat, any movement of BTUs into a cold area is an improvement. Yes, likely will not be able to heat a cold room to toasty warm, but a 60 degree room being raised to 63 degrees is still warmer. It is similar to people turning on their furnace fans and seeing little difference in the back room temperature. If you blow 73 degree air into a 60 degree room there is a very slight change in the room temperature. Poor by comparison of turning on the furnace and blowing in 120 degree air.

 

[maple1]

I dont think it would make enough difference to be noticeable. Or even raise room temp measurably.

 

[jetsam]

Overcooling the flue costs you draft, and creosote condenses on surfaces below 250°F. The more heat you strip off, the worse your problems will be. Google for "Magic Heat heat reclaimer" for the most famous example of bad-idea flue products. In its defense, it was a less bad idea in the era of low efficiency stoves, when more of the heat was going up the flue.

There have already been a couple fireplace explosions in the news this year. The last one I saw was a hot water heating system that the new owners had been told was disconnected. Make sure any hot water system has expansion tanks and, more importantly, relief valves.

Not telling you that you can't do it, but a box fan blowing on the stove would be more efficient and far safer.

I have heard of stoves with DHW coils built in, but I don't know how common they are.

(Including this just to see if I can hear bholler hit the roof from here....)

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[Jan Pijpelink]

Overcooling the flue costs you draft, and creosote condenses on surfaces below 250°F. The more heat you strip off, the worse your problems will be. Google for "Magic Heat heat reclaimer" for the most famous example of bad-idea flue products.

There have already been a couple fireplace explosions in the news this year. The last one I saw was a hot water heating system that the new owners had been told was disconnected. Make sure any hot water system has expansion tanks and, more importantly, relief valves.

Not telling you that you can't do it, but a box fan blowing on the stove would be more efficient and far safer.

I have heard of stoves with DHW coils built in, but I don't know how common they are.

(Including this just to see if I can hear bholler hit the roof from here....)

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Red neck science.

 

[jetsam]

Red neck science.

Now he just needs to invent a multipath flue brush..... and invest in a CO alarm for when those 24 gauge hvac boots rot out...

 

[electrathon]

OK, the two directions for the exhaust to raise through was a wacko idea in the above video. If he was going to try to add more inside the house pipe why did he not run the elbows in a "S" manor? I see no reasoning for splitting the draft in two directions. I am not advocating it, just seems like if you are going to do it there are better ways.

 

[BethelStrong]

I’d use thinner metal, for ambiance.


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[wooduser]

People so often imagine they can improve on the engineering of heating equipment.

They see that heat going up the chimney and imagine they can get more of it that the engineers who designed the stove sloppily allowed to be wasted.

But it just 'taint so, in most cases, and that includes the gas furnaces and such I used to repair.

Actually, it would be easy for engineers to take more heat out of combustion gasses. EASY! But that heat serves useful and vital purposes, and messing with it is a bad idea.

I think of condensing gas furnaces, which now have exhaust gas temperatures of about 120 degrees or so. But to be able to do that, they had to change furnace design substantially to allow water vapor produced by burning gas to condense in the furnace, and to dispose of that water effectively. Before those changes in furnace design, cooling off the combustion gasses could be easily done, but it would create real problems and hazards as that water vapor condensed in chimney and flues and combustion gasses came out in the home instead of going up the chimney.

So the short answer is that there is good reason why YOUR STOVE has that heat going up the chimney. Perhaps in the future alternative stove designs will allow cooler flue temperatures, but not yet ----not with the stove design you have.

 

[Easy Livin’ 3000]

I think the current best ideas are secondary burn tubes and catalytic combusters.

Maybe one day there will be robotic chimney cleaners that combine a rotary brush, spot application of chemicals, and maybe even a torch to burn off stubborn spots. Don't think it's far off, regarding technology capabilities, but would the market be large enough to support it?

Then perhaps much more heat could be captured, and it would just be an ongoing automated maintenance issue.

All this is for far off day when fossil fuels become precious.

 

[wooduser]

I think the current best ideas are secondary burn tubes and catalytic combusters.

That's certainly a big improvement, but still that's just burning up the fuel rather than letting it go up the chimney.

Is there a way to extract more useful heat from the combustion process, similar to what condensing gas furnaces do that results in an output temperature of 120 degrees? Not easy to do, certainly, but few worthwhile things are.

A big step for gas furnace efficiency improvement was installing a combustion air motor to force combustion air and combustion products through the equipment, rather than keeping them hot enough so they would circulate by gravity,

Suppose that combustion air motor was also used to form a blast furnace like effect with extra air to maximize combustion of gasses beyond what current stoves will do You might take the current combustion products and subject them to a blast of additional air for further combustion, followed by cooling them off and condensing combustion products like creosote out of them, which would then drop back into the fire.

Perhaps such a jet of air could substitute and improve upon the current methods of burning those wood gas smoke components. One issue I could see is that for oil or gas, an interruption in power easily shuts off combustion. That would likely be a lot harder in a wood stove.

As soon as you have forced air combustion you start having a lot more options on how to do things than if you rely on gravity.

Not that any of that would be easy. The current stoves are really an amazing improvement over what went on before, but that was also true of that great development of wood combustion of the 12th century ----the chimney.

 

[mellow]

Maybe one day there will be robotic chimney cleaners that combine a rotary brush, spot application of chemicals, and maybe even a torch to burn off stubborn spots. Don't think it's far off, regarding technology capabilities, but would the market be large enough to support it?

Pretty cool new device was at the wood stove design challenge. Cleans up the exhaust so you won't have to scrub your chimney as often.

https://oekosolve.ch/en/products/oekotube/

 

[electrathon]

Pretty cool new device was at the wood stove design challenge. Cleans up the exhaust so you won't have to scrub your chimney as often.

https://oekosolve.ch/en/products/oekotube/

I personally am all over the idea of cleaner chimneys. This device looks very much like an electrostatic air cleaner like we use in some furnaces. Its purpose is to make the airborne particulates stick to the inside of the chimney rather than letting them move out to the atmosphere. It is basically the fastest creosote chimney blocking device ever.