Chimney hot water heat exchanger, looking for feedback.

Post by **megamike** » Sat. Sep. 09, 2023 9:47 am

So we have a forced hot air keystoker. Growing up, we had a wood boiler. The old man and I rigged up a ghetto heat exchanger for hot water. The downside of having hot air now is there isn't really an option to supplement hot water.

Like most on here, we don't have access to NG. We have electric everything and it's killing us with huge bills. 1500kw a month, and the rates are too high. The hot water heater heater is responsible for about 1/3 of our usage in the winter, even after replacing our 20 year old one last year.

So, I thought I could divert the water line feeding the hot water tank towards the stove, and wrap a huge coil of copper around the chimney. Then cover the copper coil in rockwool insulation.

I'm not an expert, but this would probably be a bad idea on a wood stove where draft is important. But... since the keystoker has a forced draft system, I don't see this being an issue. Safety wise it would be fine, because the furnace will shut down if there is CO detected, which I can confirm first hand. On the other hand, wrapping a bunch of rockwool around the chimney could over heat it when there isnt any demand for hot water. It would also probably boil the water. I just don't want my furnace to shut down when it's-20F outside because I decided to do something absolutely stupid.

Thoughts? Or has someone else done the work and made a better solution?

Post by **Rob R.** » Sat. Sep. 09, 2023 9:58 am

The stack temperature on your Keystoker is not even close to a 70’s wood stove. I am sure you would recover some heat, but I don’t think it would be worth the effort.

Post by **megamike** » Sat. Sep. 09, 2023 11:01 am

You think? If I remember right, my flue gets between 200F and 300F when it's cold out. I would think wrapping copper in rockwool would be too hot as opposed to not being worth the hassle. I'd be willing to put $100 into copper coil to test this out on a closed loop with a pump before I try it.

Post by **davidmcbeth3** » Sat. Sep. 09, 2023 12:13 pm

I got 1700 kwh usage last month .. no electric HW.

OP may benefit from a hybrid HW heater and adjusting the hot water mixing valve to output a lower temp.

Making a tankless HW system for the Keystoker would be a task..perhaps a tanked system

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EDIT .. realized OP system is not a boiler system .... I'll leave my post. You may point at it and make fun of me. I guess. Calling me a Vikings fan will be too much though, that would hurt my feelies.

Post by **waytomany?s** » Sat. Sep. 09, 2023 12:22 pm

Seems like there was a thread where someone used a radiator attached to the side of the stove to do a water preheat type of thing. I don't remember enough to do a good search. Maybe someone else remembers.

Post by **davidmcbeth3** » Sat. Sep. 09, 2023 12:30 pm

When you want hot water , all you have is cold; when you want cold water, all you have is hot. Thats the 5th law of thermodynamics.

Post by **megamike** » Sat. Sep. 09, 2023 12:36 pm

So when we did the heat exchanger with the wood boiler 25 years ago we ran the pipe for the hot water heater through a heat exchanger and it was an easy DIY. But that was a boiler, and not forced hot air like we have now.

I am looking to do a preheat type of thing witb the chimney, but I'm mostly concerned about the water boiling in the pipe when were not home and using the hot water.