Looking to turn waste heat from wood stove into hydronic heat.

BlackCatRacing

Hello, I am new here and am looking for advice to turn a very small wood burner setup into a safe hydronic heat setup. The wood burner and Hydronic heat are already installed. I have pex running through the concrete (Left over from a hot water system I had installed years ago and a small wood burner that I run when I am in the shop. I am looking to wrap the wood burner in copper with a closed loop system and run the heated water to send excess heat into the concrete slab. I was wondering what components I would need to make the system safe and efficient. I would be in the shop when the wood burner is running and looking to have the slab keep it warm overnight.

Any help and advice is appreciated.

Patrick

hot_rod

the challenge is to not pull too much heat that the flue temperature drops and causes condensation, creosote formation, which can destroy the the flue pipe or cause draft issues

jesmed1

You may just have to do some Googling and find some people who have done this successfully. My non-professional guess as a mechanical engineer is that you'd want some kind of stainless steel coil in the firebox or the flue pipe for heat transfer to the water, connected to a large buffer tank that will moderate the water temperature. You probably wouldn't be able to connect the heat exchanger coil directly to the floor PEX because the water temp would be too high.

So a heat exchanger coil to a buffer tank (maybe an old water heater that doesn't leak), and then to the floor PEX. You'd still want some kind of temperature control so that the water temp flowing into the floor PEX is neither too high nor too low.

DCContrarian

There is a website called Hearth.com devoted to woodburning, a lot of people there have done what you're describing, with varying results. I'd ask in the forums there.

BlackCatRacing

Thank you for the advice. I was going to run a copper coil outside the flue and around the firebox. I have an expansion tank and multi speed pump. I also have plenty of valves. I was wondering what else I needed for controls. This might not be the most efficient but I am only trying to get what heat I can into the slab seeing as I already have the pex installed in it. I will check out that other website. Thank you….

psb75

Do NOT put a COPPER water coil IN THE FLUE or in the fire box. The "coil" or "loop" should be stainless steel and it really needs to be in the firebox to be effective. The heat transfer from stove to water will not be good enough if the coil is outside of the stove. The inside of the coil needs to be kept "clean" from too much mineralization (needs occasional "acid flushing" depending on your water chemistry). The concrete slab with pex in it will take a LONG TIME to heat up. Once the slab is up to temperature (65-84°) it should be kept heated so that it doesn't lose a lot of heat. Raising the slab temperature 1 or 2° takes quite a lot of heat.

psb75

Oh, and it is super important to ALWAYS HAVE an appropriate (DHW), working "temperature/pressure relief valve" (TPRV) on the system—near the stove and on a (water tank) if included. Many people have inadvertently made a hot water/steam "bomb."

Bob Harper

You need to investigate this a lot further. Believe me, if it was such a great idea ALL the mfrs. would be making such units. Re-read the advice and warnings above. You would be creating a chimney fire machine. If you have waste heat, you have a piece of junk polluting stove. There is a 30% federal tax credit going on with a $2K cap for biomass (wood and pellet) stoves with a Higher Heating Value of 75% or higher. Replace the stove with a proper one and heat that space. If the stove is located in a space separate from the room needing heat, look into other options.

Outdoor wood-fired boilers are becoming heavily regulated and banned because they fog out the neighborhood with their massive incomplete combustion and pollution.

psb75

All of the efficient, biomass, indoor boilers that qualify for tax credits and incentives are European designed and manufactured. The U.S. is not interested in developing this equipment—it seems. Fuel acquisition, management and handling is a very important part of the process too. This is evolving and the Europeans are ahead in this area also. Solid fuel masonry stoves have been around "forever" (a Eurasian tradition) and are an excellent way to produce home heat.

Kaos

https://forum.heatinghelp.com/discussion/comment/1823358#Comment_1823358

This.

There has been stories of stoves exploding when TPRV is in the wrong location and someone forgot to open the isolation valves before firing the stove.

LRCCBJ

You have one additional issue:

Presuming that you are going to use some type of buffer tank to retain the water at the approx. temp that you need for the floor (say 130F or so)……………at some point the tank will reach that temperature. Normally, the circulator would shut down and stop the flow between the tank and the wood stove. However, you can't stop the flow to the wood stove or water in the tubes in the stove will boil. You won't be pleased with the result when the relief valve blows.

It is not easy to manage the output from the wood stove as you really cannot modulate it well.

IF you had a zone(s) where you could dump all the output from the stove without concern of overheating, this would be preferable.