DIY Wood Stove Heat Exchanger to DHWT and Radiator

sinthome

Hi, I have a DIY copper coil heat exchanger wrapped around the flue of my wood stove, which passively circulates to a standard electric domestic hot water tank. I feel it is a safe design, with multiple TP relief valves, pressure gauge, an air bleeder and drain valves. In the coldest weeks of the winter, I often find that I use my wood stove more than I use the hot water. Currently, I have it plumbed so I can dump the extra hot water into a bathtub with the drain plugged, instead of just wasting the heated water. However, it can be a little cumbersome and overly humid solution, so I am looking to incorporate a small hydronic panel radiator in the adjacent bedroom. I can imagine several different ways to do this, but I am trying to determine what makes the best sense for the situation. Code compliance isn't an issue, but I'm still trying to make it safe and efficient. I know I will have to add a pump to circulate water to the radiator, but I'm unsure if sharing and circulating the same hot water makes sense or if there is a good way to set up a separate closed loop for the radiator. If I plumb it to the same water lines as the domestic hot water tank, it seems like I could just add tees with check valves and put the circulating pump specifically in-line to just the radiator, such that when the pump is off it will still effectively thermosiphon from stove to tank and so the pump would only kick in when the DHWT hits a maximum set temp. I guess that is what I'm leaning towards but I'm hoping y'all can help me think through the idea and make sure the details all make sense.

Larry Weingarten

Hello, I think most of the people on this board would advise against using the same water in the heating and domestic hot water systems. Imagine that the radiator isn't used for months in summer, but the water in it is sitting there stagnating. You don't want that water mixing with your hot, domestic water. So some sort of a heat exchanger, whether internal or external, would make it safe.
Another consideration is how you are getting heat from the flue. If your system works a bit too well, you might be getting creosote buildup inside of the flue, which could encourage a chimney fire. Hopefully you brush the flue pipe periodically.
All that said, I think your idea of finding a good use for the heat is sound. Do make sure the relief valve on the tank gets tested periodically also.

Yours, Larry

retiredguy

A few ideas that you could ponder;

You could add a 2nd domestic hot water storage tank to use the excess hot water that is generated.

In the past, I have used domestic hot water to supply some heat in my garage in winter using finned tube radiation. My solution to having the water sit in the piping all summer as @Larry Weingarten cautioned was to have isolation valves as well as flush valves installed on that radiation. I would flush the radiation before using the radiation to get rid of the stale water. This idea would be similar to using that excess heat to heat a room as you had mentioned.

If you are removing too much heat from the flue gasses as Larry mentioned you could reduce the length of piping (tubing) used as the heat exchanger.

You have to be very careful when mixing heating water with domestic water. This is the reason they manufacture double walled heat exchangers. hope this helps

jeepmanblair

The system in my house uses the same water for heat & domestic use and the stagnation issue is taken care of my a timer on the circulating pump. The Lennox air handler runs the pump for around 30 seconds every 6hrs (ish!) all year so stale water would not be an issue. A DIY version of this would be easier, in my opinion, than dealing with isolation & drain valves. Full disclosure, I'm just a home owner. 

pecmsg

Read up on Legionnaires disease............... it cause and effects

hot_rod

Those HX are still fairly common on European wood stoves.
You could either build it as a separate hydronic loop with over heat protection, use a isolation plate HX if it does tie into potable water, or some provision to flush that piping weekly.

Ironman

A panel radiator is made of stamped steel. That plus an oxygenated open loop = rust and sludge and an eventual leak.

Solid_Fuel_Man

Yoyr best bet it using something which is all non ferrous. Copper fin tube would be the best approach I'd say. 

Iron or steel isnt going to last, and will rust up your water. Will eventually leak. 

A bronze or stainless circulator and an aquastat on the tank set to 160 or so would work. 

If you want a separate system, a plate heat exchanger with a bronze/ss circulator on the Domestic side and all standard iron stuff on the other side. You would need an expansion tank, air elimination, and a 30psi relief valve on that side as well. Bring both circulators on at the same time with the aforementioned aquastat on the tank. 

If you go with a separate system (recommended) then you can use whatever you want for radiation and you can have more rooms on it if youd like as well. 

sinthome

Hey, thanks everyone. That is a good point about the rust issue with an open system, I hadn't considered that. Are there any anti-rust additives that are commonly used for hydronic heating systems? A couple more questions-- which is the most suitable HX for my intended use, a flat plate or sidearm style? Should I expect this to take significantly longer to heat the domestic water tank or are the efficiency losses not that noticeable? How large should the holding tank be if I am just circulating through the HX on the domestic hot water tank and one additional radiator loop?

Solid_Fuel_Man

A sidearm will have the benefit of thermo siphoning for your domestic. However it wont be fast. With a big enough tank this isnt an issue. 

A flat plate will need a pump on each side but will generally be much faster recovery. 

Sizing each type of HX obviously makes a lot of difference, but these are general rules of thumb. 

I'd used a side arm directly on the tank. Then you dont need a nonferrous pump for the open side.