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Wood fired water tanks for thermal storage

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posted 7 months ago

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I am trying to figure out if i can safely use my tiny hobbit stove by salamander with a boiler to heat water in salvaged electric water heater tanks? I have access to all the tanks i need. I am in zone 6a and the greenhouse is 10 x 14. What i have come to for my idea is using the wood stive to heat water that will first run in copper pipe under slate slabs, 2 inch thick also salvaged, shelving then i would like to pipe it into a 40 gallin water tank then into a second tank before running it back into the stove to come reheat and repeat. I have been looking at how to use the wood stove boiler to heat the water . I keep coming to the issue of a closed system possibly being something that would rupture and create a huge crater where my greenhouse uses to be. This is why i am considering running it thru the copper under the slate shelf before entering the water tanks to help lower some of the temperature. I have wondered if a pressure tank would help or if just a pressure valve would do. I will scribble my drawing down later . Any ideas or wisdom from anyone who has built a wood fired water heater system? Thank you in advance

Jay Angler

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posted 7 months ago

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Sean Brown wrote: I keep coming to the issue of a closed system possibly being a bomb.

This is what Paul Wheaton refers to as "boom-squish". We *really* don't like our permies doing that.

I know that they have a wood-fired outdoor boiler at Wheaton Labs, and I know they were very careful about having pressure relief. Hopefully they'll speak up. I think there are some threads about it.

Will the wood stove be vented to the outside?

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Douglas Alpenstock

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posted 7 months ago

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Please clarify: are you planning to use a pump to circulate water through this tubing? Or are you planning to let the heat from the stove create steam to move the heat around?

Steam can create as much explosive force as TNT. Do not go there.

Sean Brown

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posted 7 months ago

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I wasnt planning on using steam at all. No pumps just the thermal siphon if i understand the way the water moves around. Closed system . The salamander stoves water boiler is to make hot water not steam. The consideration of using the copper water pipe under the atone shelf is to pull some of the higher heat out of the water. The stove will be vented thru the roof . Not wanting a dual purpose greenhouse smoke house.
The goal of any thermal mass system for my greenhouse is to be able to maintain temps no lower than 60 ° at 5 ft. During the sunny days of winter i believe that the sun will warm the tanks . On weeka like we have had this month with days on days of cold and grey the wood stove would hopefully be able to increase the water temps in two tanks to help tmy cause of ginger growing without electricity.
I need to get thermometers in the greenhouse so i can see the temps in the morning. I am just finishing the interior drywall . Thanks again for any input.

William Bronson

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posted 7 months ago

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I'm all about storing wood fired heat in water heater tanks.
It's hard to imagine a compelling reason to make it a closed system.
I think thermosiphoning should work on an open system.

Every water heater I've encountered has a dedicated T&P valve coming off a threaded bunge near the top of the tank.
Cut the pipe that comes with the valve to about 2 feet long.
Turn the valve so it points up instead of down.
Open the valve and wire, tape or ziptie it in that position.

You don't mention what form your heat exchanger will take.
An open pot on top of the stove , or a coil in an open pot should be no problem, but a coil around the exhaust flue or/in or on the firebox could be.

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R. Ford

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posted 7 months ago

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I prefer an open system with a pump myself, as a closed system with water needs room for expansion, and water expands exponentially when boiled or heated to near boiling. If you plan to only use this at night, i'd use a small solar panel to charge a small battery and power a small pump. Leaving one storage tank open topped to prohibit that "boom, squish" that we prefer to avoid. I've never set up a large thermosyphon, BUT if you can do it where you have a holding tank that is open topped and the return dumps into the top of it, and the exit is near the bottom, I'd say give it a try.

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Sean Brown

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posted 7 months ago

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William Bronson wrote:I'm all about storing wood fired heat in water heater tanks.
It's hard to imagine a compelling reason to make it a closed system.
I think thermosiphoning should work on an open system.

Every water heater I've encountered has a dedicated T&P valve coming off a threaded bunge near the top of the tank.
Cut the pipe that comes with the valve to about 2 feet long.
Turn the valve so it points up instead of down.
Open the valve and wire, tape or ziptie it in that position.

You don't mention what form your heat exchanger will take.
An open pot on top of the stove , or a coil in an open pot should be no problem, but a coil around the exhaust flue or/in or on the firebox could be.

I have a hobbit stove ( https://salamanderstoves.com/product/the-hobbit-stove/) with the water boiler installed in it. i have no problem having a system that would be able to vent pressure out just not completely open as to let vapor into the greenhouse. Unless of course it is allowing pressure to vent off and with that i would run a pipe out of the greenhouse. Dont want standing water on my floors.
I really would like to gather as much energy storage from my fires. The reason for running the copper pipe under the slate , now thinking a slate top and bottom with the copper sandwiched in between, is to A, remove some of the heat from the water and hold it at in the stone and B, the cooler hot water would still circulate into the tank ot tanks and the remaining heat would be trapped . My next question is horizontal or vertical orientation of the tanks? The stove is only 24 " off the ground so im not completely sure if i need the stove at the height of the top of the system of if can be lower? I can get 3 tanks in 52 inches vertical or 3 stacked on the horizontal same width but would take up more space vertically. The tanks would be positioned at the north wall and my south glazing is oriented so i get the most solar gain at mid winter. Of course the black painted tank will absorb solar heat passively but i am trying to grow ginger and tumeric in zone 6a. The plan is to have the plants growing on shelves above the tanks to take advantage of the radiant heat throughout the night. Ots these very vold grey days that i would beed to fire up the stove. Im trying to find schematics for a build if any kind .

IMG_20250213_081840094.jpg

Sean Brown

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posted 7 months ago

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Rusty Ford wrote:I
Im not sure about having a open source of water. Concerned with the moisture in the winter greenhouse. To that, where would i place the open tank in the system? If im understanding, which i might not really, the open tank would be the last tank in the system? So if i connected the first water tank to the stove then plumbed that tank to the open tank then back to the stove, cold water return, that would work? Im really looking for drawings even if they are napkin sketches. I guess i really dont have a grasp of open and closed systems.

William Bronson

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posted 7 months ago

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I think if you are using an off the shelf side boiler made for the stove in question, you should have no issues with steam flashover.

I would keep the tanks vertical if possible, and feed your hot water into the bunge that the drain valve is screwed into.
The return water can exit via the T&P valve .
I would run the tanks in parallel, incoming hot water split to enter each tank at the same time, cooled water exiting the T&P valves and becoming one stream again.
Seal off the original cold water intake, and run the original hot water outlet to a floor drain or directly outside.

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Tim Siemens

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posted 7 months ago

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Open system is much safer and will still thermosyphon. Just be careful, if you vent to outside, that the vent doesn't freeze shut in cold weather and create a sealed system.

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Sean Brown

Posts: 43

posted 7 months ago

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William Bronson wrote:I think if you are using an off the shelf side boiler made for the stove in question, you should have no issues with steam flashover.

I would keep the tanks vertical if possible, and feed your hot water into the bunge that the drain valve is screwed into.
The return water can exit via the T&P valve .
I would run the tanks in parallel, incoming hot water split to enter each tank at the same time, cooled water exiting the T&P valves and becoming one stream again.
Seal off the original cold water intake, and run the original hot water outlet to a floor drain or directly outside.

It is a factory installed small boiler tank inside the stove proper. The hot water out should be at the same height as the inlet on the tanks? Then run the cold lines out the top? What about any air that would be trapped in the tank? Ideas on purging the air out? What about my idea of using the water to geat a stone shelf before running into the tanks?

Sean Brown

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posted 7 months ago

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William Bronson wrote:
I would keep the tanks vertical if possible, and feed your hot water into the bunge that the drain valve is screwed into.
The return water can exit via the T&P valve .
I would run the tanks in parallel, incoming hot water split to enter each tank at the same time, cooled water exiting the T&P valves and becoming one stream again.
Seal off the original cold water intake, and run the original hot water outlet to a floor drain or directly outside.

Asking questions and seeking out response leads to more questions and answers. With that, I started looking more into it. From what i found people use the t&p valve as a pressure relief valve and the hot water inlet.The pages i looked at used the drain valve to return the cold water back into the boiler. If i was to run the tanks in parallel would it be prudent to have a vertical pipe a over each t&p inlet and have a valve attached incase of pressure? Seeing that i would be dumping hot water into both tanks. It does seem at this point of the thought experiment i could pipe the hot water thru the coiled pipe in between the slate and then dump the heated water into the top of the tanks and return the cold water back to the boiler.
The picture and set up is not mine but credit is due to
http://inthewilderness.net/2017/01/27/homestead-hot-water-thermosiphon-loop/

Screenshot_20250213-142250.png

Sean Brown

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posted 7 months ago

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Looking at my wall space , tank and stove dimensions i think i would have room for 7 40 gallon tanks, 5 for solar only and 2 for heated water . I need to figure out my set up . Probably two passive tanks the stove the two active tanks then the remainder passive tanks. Im hoping that the passive tank nearest the stove will draw some heat from it while its burning. I may have access to bricks that i could stack behind the tanks to add some thermal mass? I have under 2ft from the wall i wish to allocate for the thermal battery. I want to use the space above the tanks to grow those long season tropicals, ginger tumeric, lemon grass, during the start of the season . Im hoping to have this greenhouse produce enough long season crops to at least "pay" for itself. I am also using of course to start plants and keep two cold tolerant citrus trees thru the winter.

I agree. Here's the link: http://stoves2.com