Solar Hot Water Heater + Wood Cookstove Coil

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sola gratia

Feb 19, 2022
Long time reader, first time poster.

I own a Kitchen Queen Grand Comfort 750, which I will do a review on for Coaly soon, I promise, but first I sure could use some advice before laying down a lot of hard to come by dollars.

My plan is to fit the KQ with the water coil they offer and then use it for back up water heating to a Solar Bank 80 gallon system, with a solar controller, 110 backup element and a recirc valve for helping with overheating or freeze control issues. The SB is a passive system with the tank mounted on the roof, with an inner and outer tank and insulation between the two.

For those that aren't familiar, the KQ is an Amish built wood cookstove with a massive fire box. It is located in a lightly insulated, heavily used sun room, that was converted from a patio or car port at some point. The room has exterior side sliding windows on three walls and what used to be the exterior of the house as the fourth wall. When the KQ is running, there are windows and a door on the house that we open to bring in as much heat as possible to the house, but it is not the main source of heat for the rest of the house. The idea is to keep the sun room toasty in winter, which as I said, we use as much as any room in the house, and then possibly use the Queen in the summer for canning or emergency cooking, without heating up the rest of the house. Not relevant, but we have a Drolet 1800i insert that heats the rest of the house, and it works great!

The Sun Bank would mount on the roof at the opposite end of the sun room facing southwest. We are in the northeast TN mountains, and our winter sun is a few days each week, so I will need some back up, otherwise the little 110 element in the SB is going to work overtime. The summer won't be a problem, but even if we went into a long stretch of rain during the summer, I could fire up the Queen to get the water up to temp, if needed.

I don't have my exact plumbing schematic down on paper yet, but that is my next phase in the plan. The distance will be about a 25 foot pipe run between the KQ and the SB. The solar controller will be mounted in the sun room, where my office is, and I can view the temp in the tank at anytime. The 110 element in the tank is wired to the controller and can be set to kick on when the temps are low. The recirc pump will be wired to the controller and I will have it kick on when the temp gets too high in the tank. The pump will be mounted in the furthest bathroom and can use the house piping to dissipate some hot water. I will of course have the TPV valve on the SB to dump hot water into the gutters on the roof if it gets way out of control, but that should be seldom, not routine. We also homestead, home school, work from home and use hot water all day, so I feel like we can manage the logistics of the system well enough.

Right now, my biggest concern is overheating, but I don't know if I am on track with that. I have never heated water with the Queen and the small coil that goes in the fire box, so maybe I won't be able to heat as much as I think. I have also read some that say a horizontal tank is a bad idea in a thermosiphon system like this, because the stratification of the temps of the water aren't as distinct as they would be in a vertical tank, so maybe that could be a big issue I haven't considered enough.

My questions to you guys are, first, has anyone done something similar and can attest it will actually work? I can't find much on the net about a system like this, but it seems to me to be the best of both worlds, free hot water from the sun in summer and from wood burning in the winter.

If you have not done something similar, what do you think? What should I be on the look out for? Any blind spots pointed out would be greatly appreciated!
I have known folks with heating coils in wood stoves. The trick is you need to have water running through the coil 24/7 that the wood stove is in use. With some creativity you can avoid a pump and use themosyphoning where the heat from the coil goes into a tank located above the unit with a pipe run down to the coil but the problem is at some point when you have heated the water in the loop where does the heat go?. If you do not have way of dumping the heat, at some point the water will boil and over pressurize the system. A system with steam in it is a dangerous thing as when it fails the steam expands to 1600 times the volume of water. Things can blow up throwing fragments and hurting or killing someone. A temperature and pressure relief valve plumbed to the floor is a safety device to keep explosions from happening so that is mandatory but if a coil goes dry after the water has leaked out of the system, it will damage the hot coil in the stove permanently. Typical solar hot water systems do not have this problem as they only heat water to about 80 degrees over the ambient temp.

My guess is you could use the solar hot water panels as a way of getting rid of heat in the summer by forcing the circulator pump to run if the system temp gets too high. Heat would get radiated from the panels. This may work as long as you have power to run the circulator but if the power is out then you would need not be able to use the woodstove unless you can remove the coil. My friends set up has the coil outside and above the wood stove directly against the wood stove body with quick connects so that in the rare chance that he needs to run the wood stove without power, he can remove the coil. In the winter my guess is the water would normally not get to boiling temp but until you could prove it you take the chance of blowing a relief valve and burning out the coil.
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Thanks for the reply, peakbagger and you bring up some good points. Hopefully, by being able to monitor my temp in the tank, I can avoid any boil overs by opening a spigot and letting out hot water, while bringing in cold water to the tank. I don't normally burn a fire overnight or while away, in the Queen, so anytime I have a roaring fire, I would be able to monitor the system for overheating issues. This will of course be paramount in the early implementation of the system, to make sure there are no catastrophic failures, like you mentioned. I also plan to have an outdoor hot water hydrant to use for filling stock tanks on the farm in winter. This hopefully will kill two birds by keeping the animal's water from freezing quickly, while letting heat out of the plumbing system.

I would also be able to remove the coil from the cookstove fairly easily, if needed in a grid down situation. For extended periods without power, I would need a small inverter and battery setup to make sure the controller and pump could stay active, which wouldn't take much.

Thanks again for the wisdom!