Wood burning pool heater

High_Water · November 05, 2020, 10:04:42 AM

I plan on attempting a wood fired pool heater and I have a few questions:

1. For copper coils, what is the "best" kind to use, is basic soft copper from home depot or something like that good enough or do I need something a little more robust?
2. I suppose depending on the answer to the first question, how close of contact to the fire can the copper handle?

My idea at this stage is something simple, like a drum with coil either inside or outside. It won't get a whole lot of use and its only for a pool so I'm not too worried about efficiency I just don't want to burn up a bunch of copper for nothing.

Ljohnsaw · November 05, 2020, 10:47:32 AM

I remember YEARS ago reading in Mother Earth News about such a heater. They made the "stove" look like a dinosaur (brontosaurs?) and the neck/head was the chimney. They used it to heat a hot tub. You don't want the copper in the fire - if you run it dry or maybe flash boil the water off, I think you would have a big problem. Also, you probably don't want the water getting really hot as it would not be good for the chlorine. I would wrap the metal flue pipe with the pipe. However, you probably want either a large diameter or multiple pipe to handle the water flow. That can get expensive. How big of a pool (gallons and surface area)? Will you keep it covered to conserve the heat? I have 28,000 gallons and can loose 5° (F) overnight (summer) on a calm night. I've seen a drop over 10° when it gets windy (no cover).

High_Water · November 05, 2020, 11:02:40 AM

Its an indoor pool, but its fairly large maybe 24,000 gallons. I haven't had it long enough to know what kind of temperature swings I get from the ambient, all I know is that its just slightly too cold even in the summer because is stays in the shade (I assume), even if its in the 80's or 90's outside. There's an old propane heater out there that doesn't work, but the pool guy told me that it would probably cost $100/day to run that thing. I plan on using it maybe once a month tops and mainly in the warmer months, which where I'm at in TX is March - Oct lol.

mike_belben · November 05, 2020, 11:21:54 AM

Im made a bunch of these type of things tho never for a pool. A friend of mine did a waste oil fired pool heater from a 20lb grill bottle, looked like a drip stove.

its pretty simple.. Honestly hard to get wrong.. Just some ways work better than others. Google side arm heat exchanger. Also look at thermal siphon and MEN heater.. Thats mother earth news. Youll get 50 ideas from those 3. And pretty much any will work. Just make sure you orient the coil so it is not a ferris wheel, but is a roulette wheel. Youll have trapped gas pockets in the ferris wheel type and much better thermal siphon in the roulette wheel.

Id put the coil in the flame and not use any sort of pump. If the coil is below the pool water level it will stay full of water and wont melt the solder joints. As long as you have a gate valve with the proper flow and a slope layout it will auto circulate as temp rises, like it should. If you want to err on the side of caution so as not to drain the pool if it fails, a lobster pot on a firebox with a coil inside will work. It will limit the temp and evaporate the pot often but it works. I did it as a preheater off my woodstove for years.

A direct flame thermal siphon is how i would do it though.. Much much more energy than a hotpot. Thats pretty limited.

doc henderson · November 05, 2020, 12:06:12 PM

just for a reference, our pool is 68,000 gallons, and a 400,000 BTU NG heater raises the temp 1° f per hour. I wish I had got the wood heater, as we rarely use the heater, and it was expensive.

Nathan4104 · November 05, 2020, 01:59:01 PM

I don't suspect this is a practical idea being an indoor pool, and a pool, but..... a buddy of mine has a hot tub and it has a wood stove 'in' the tub. The top is above the water so U can reload, (pain to light) there's a snorkel intake and chimney of course......
it's a stainless stove, and heats the hot tub up from Cold lake water temp to boiling a frog in a few hours.

doc henderson · November 05, 2020, 02:03:31 PM

I have seen them and I think they are called a snorkel stove. kind of a hairy leger thing. no offense to hairy legers anywhere. 8)

High_Water · November 05, 2020, 02:45:27 PM

I've got plenty of room for elevations changes if needed, but for the siphon one thing is not immediately clear, flow direction. My assumption would be cold goes in at the bottom and hot comes out at the top. I know it seems obvious but I have to ask.

mike_belben · November 05, 2020, 03:49:18 PM

You are correct. Hot water goes up. To work correctly the heater loop must be below the height of the water and air must naturally stay out once purged. You also use a gate valve aka check valve to prevent the expanding water from pushing out both ends.. The gate valve will prevent a reverse current from getting started. This way the hot is always pushing out the top port and that evacuation is always pulling cold into the bottom port so that theres a continuous loop of flow.

For an inground pool a pump would be required on a flat ground layout unless you somehow put the firebox underground or something. This is more of an above ground setup.

In my old house i had the cold groundwater line coming into a coil in a hotpot on the stove, then into the gas water heater as normal. I also had a thermal siphon loop from bottom to top of the water heater through another coil next to the stove. It was zero effort. Stove on meant crazy hot water.

47sawdust · November 05, 2020, 04:46:22 PM

Offsite photos removed by Admin

Chofu Heater for Metal Stock Tank
This might give you some ideas

High_Water · November 05, 2020, 05:40:24 PM

The pool house was built on the side of a hill, so I can easily get the fire below the surface of the pool, it would take a little effort to prime the line but I think once the siphon started working it would keep itself going - the key being if I have enough heat transfer I think. Do the inlet and outlet need to be at different elevations or the same. My thoughts are that if you kept them the same then the flow would be generated exclusively by the heat, I can't think what would happen if one were higher than the other - maybe you want the outlet higher, not sure? Am I being clear or just rambling lol.

sprucebunny · November 05, 2020, 06:19:17 PM

Outlet higher.

Those Snorkle stoves work great. I had one in a hot tub for a few years. Needed to have a pump push the water around the tub ( an old cider vat made of cypress).

Ljohnsaw · November 05, 2020, 06:57:50 PM

Quote from: High_Water on November 05, 2020, 05:40:24 PMMy thoughts are that if you kept them the same then the flow would be generated exclusively by the heat, I can't think what would happen if one were higher than the other - maybe you want the outlet higher, not sure? Am I being clear or just rambling lol.

Water seeks its own level. So all the line will be full of water. If you have the intake from the bottom, or near-bottom of the pool, it will be getting the coldest water from the pool. The outlet could be anywhere (the stove is providing the movement) and up high seems obvious but I don't think it really maters much as long as you have a check valve somewhere, preferably on the intake. I like the in-pool stove, never heard of that before!

doc henderson · November 05, 2020, 07:12:14 PM

unless off grid, I would add a small circ. pump to keep things moving. the heat gradient is better for transfer, if you keep it large i.e. keep cooler water moving through the system.

mike_belben · November 05, 2020, 07:14:22 PM

Suck from down low and push it out up top. You need separation to prevent sucking in the water that was just blown out the heater.

Jeff · November 05, 2020, 07:26:43 PM

Ive got a water to water heat exchanger here from when we heated our 13,000 gallon pool with a bio-mizer I have no use for....

Iwawoodwork · November 05, 2020, 08:20:50 PM

In the early 1990's I added wood heat to the 400 gallon hot tub. I used part 1/2" galvanized pipe coils as in a wood cook stove then added a full coil of soft copper tubing on to that inside the large steel wood heating stove, my stove was about 20 feet from the hot tub circulation system. I used old steel braid hyd hose to transfer the water because it would handle the heat and was kinda flexible also go it for free from work. What I fount is that once I had the fire going good the hot tub water would get warm in about 30-45 min, but no way to regulate the heat and at times would have to add cold water to the hot tub. also should have molded the coils into soft fire brick mud that comes in bags. was a very cheap way to heat the tub.

47sawdust · November 05, 2020, 09:02:21 PM

As Doc mentioned having a circulator in the loop is good idea.
Thermosyphons do work but a circulated system will work better.
Good luck.

LeeB · November 06, 2020, 01:04:54 AM

Why not use a solar heater? Southeast Texas, you out to be able to get plenty of heat from a solar circuit. Would most likely need a circ pump of some kind. I have one that have not used yet. It's a bunch of approx 1/2" plastic tubes side by side in a black mat that lays out on the ground or on an elevated platform.

Ljohnsaw · November 06, 2020, 01:10:48 AM

Quote from: LeeB on November 06, 2020, 01:04:54 AMWhy not use a solar heater?

That's what I have. I can get my pool up to 90°+ when we have our 110° days for a couple weeks. However, some other heat source would be nice to start the swim season in May rather than June and continue through the end of September.

High_Water · November 06, 2020, 09:22:34 AM

I've thought about solar, its a good option I think, but I don't want to have to leave the pump running continuously and I was hoping for something that I could run overnight (of course fire may not be good for that either).

doc henderson · November 06, 2020, 09:28:43 AM

If you had a wood heater outside on a slab, I would not be afraid (after a test/learning period) to run it overnight.

mike_belben · November 06, 2020, 09:40:49 AM

Me either.

High_Water · November 06, 2020, 10:13:58 AM

Its not that I'm afraid of leaving the fire burning, its just that I don't know if it will stay burning hot/long enough to heat the water from midnight till 8am. I guess that would come down to how good you are at building and maintaining a fire in a fire box - my fire experience is mostly just bonfires or brush piles.

I appreciate all the input, I've got a little bit better idea of what I want to do. I think my first attempt will be fairly simple, steel drum with copper coil, and I'll just have experiment from there. Like I said I'm thinking once a month tops so it may take years for me to figure out what I like lol.

Iwawoodwork · November 06, 2020, 10:40:56 AM

My experience with the hot tub, I would suggest a if a barrel it should be at least a big as a 55 gallon drum or larger in diameter, length about right. then put the coils in a circle against the inside of the barrel, then use the flexible boiler mud/fire brick and mould the tubbing in the mud . probably have to use a metal plate on the top 1/3 to cover and hold the tubbing in place. you can build/have built a fire box tight enough that it will hold a fire for 6+ hrs using wood like oak or madrone.