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Wood Burning Pool Heater

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Convert a fire pit or old grill into a wood burning pool heater (water heater).

I bought a 16' x 4' pool, but found that a gas or electric heater would cost more than the price of the pool itself. I also had a lot of limbs and split wood from a fallen tree in my yard. This made for a very easy and effective combination. Now, when I burn wood in my fire pit, I also heat my pool.
 
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Step 1: How it works

Cold pool water is pumped from the pool via the submersible pump into a 50 foot coil of copper pipe mounted above the fire inside the pit/ grill. By the time the water makes it out of the pipe, the fire has heated the water. This is similar to how a standard hot water heater works. The heated water exits the pipe back into the pool.

Step 2: Materials

The parts you will need are all common parts that can be found at most local hardware stores.

  • One old, but useful, fire pit or grill
  • 60 foot coil of 3/8 copper tubing
  • 10 feet 1/2 water hose
  • plastic 1/2 female water hose adapter (recommended) and zip tie
  • 6+ feet black foam pipe insulator
  • 1/2 hose clamp (comes with female hose adapter above)
  • 4 wire copper pipe hangers
  • 1000 gph Submersible sump pump with water hose adapter
  • scrap brick or stone

Step 3: Installing the copper tubing

Picture of Installing the copper tubing
You will need to first place your coil on the inside of the lid to check for size. The tubing is flexible, so you can adjust to fit. **be careful when bending copper tubing as it kinks easily** Drill two holes through the lid for the ends of the pipe.

I would recommend feeding at least 2 feet of tube through one hole for the intake and 10 feet for the outtake. Drill small holes for the u-shaped wire pipe hangers and install, bending to fit the bulk of the coil. The coil cost about $75. A little costly, but it is much more efficient and easy to install than using a series of elbows and straight pipe, as in my original design.
I like it, good job.
onemoroni110 months ago
Very creative and good use of materials. Your projects are like mine. They evolve as the results show areas to redesign for improvement.
joshnort2 years ago
guys, you can just install a hydro therm


get it at nortonhydrotherm.com

save yourself the head ache. it will pay for itself in no time.

josh
tim_n joshnort10 months ago
I always enjoy it when someone comes in spamming a perfectly good instructable. Why not just go out and buy a fully fledged £6,000 hot tub spa! It'll pay for itself in expensive spa weekends.

I'm currently customising a gas bottle furnice and building my own tub from 2x4 and plywood sheets. Total outlay will be about £200 if I'm lucky. Not say $2,290 for the same thing, but square with shiny chrome.

I don't suppose you're the norton of norton hydotherm mr josh 'nort'
No offense, but people join this website to see what they can build cheaply. Maybe not the most effective way or THE cheapest way, but they test themselves on what they can do. It's fun for a lot of people. Yes I agree that the Hydro Therm would pay for it's self over time, but this dude did something original out of things he already had or could cheaply get, that is why this iis featured in an E-book on the front page. I think this instructable is flipping amazing because of the originality of it.
bizydad (author)  Blaaken1 year ago
Thanks for that amazing compliment man. I had no idea this made it to an E-book, that's awesome. I'm going to make some time to add some more recent project to the site now. Thanks again for the post.
peads924701 year ago
I am really impressed with this idea. Absolutely something I have been looking for. Simplicity and results. I wish I had thought of it.
eyesee1 year ago
Beware of the water temperature is too high
bizydad (author)  eyesee1 year ago
Pumping hot water into a cold pool feels good underwater, but that's a good point i overlooked. Don't try to take the outlet out of the water while the water is flowing. Other than having to make very rough adjustments to the temp by adding more/ less logs, this heater worked great for me. But always play nice with fire.
EmmettO4 years ago
 Have you ever had trouble with the garden hose melting with the hot water in it? I would also worry about the copper melting in the fire as a wood fire can melt or at least heat stress copper.

I understand that this is probably pretty serviceable for a summer or two but I would think it would have to be rebuilt after a number of fires. What is your experience with it?

Ideally I would use stainless steel pipe (if I could find any) in the fire and then copper to the pool. That would last for a very long time.
sdello EmmettO4 years ago
Did you ever get your answer regarding the copper melting in the fire.  That was my concern as well
EmmettO sdello4 years ago
 No, nothing yet. . .
bizydad (author)  EmmettO4 years ago
 Sorry.  The garden hose supplies cold water from the pool and pump, so it never gets hot enough to melt.  I have had some pretty intense fires and there has not been any melting or distortion in the copper coil.  The water coming to the pool via the copper pipe can be quite hot (I measure up to about 110F I think).  So I used black pipe insulator to protect the pool and occupants.  The stone in the pit help insulate the heat and focus it on the pipes and they take the most abuse from the fire.  I may have to replace the stones over time.  I plan on setting it back up this summer, so I will let you know any new developments.  Thanks for the question.  

I used the copper coil because it gave me the longest run of pipe to heat, but melting could be a problem in the future.  Because this is not a pressurized system, there should not be any danger of a blow out.  Still, it is a good point you raised.  I'll keep a close eye on it the summer and let you know.
sdello bizydad4 years ago
Where can i buy the copper coil.  All I ever see is copper coil for refridgeration use. 

Steve
Sdello, ACR tubing (ACR=air conditioning & refrigeration) tubing is sized differently that the tubing that you would use in the plumping field. ACR tubing is measured by ID (inside diameter), while tubing used for plumbing uses OD (outside diameter). But if you would buy 1/2 ACR tube it is the same size as standard 5/8 plumbing tubing.Which everyone call as 1/2" tubing It may seem confusing but if you would use both types of tubes during a career spanning 38 years as a journeyman pipefitter, you get to understand it.
Sorry about this, I screwed up ACR is measured by OD. I've been retired for 8 years now & one does mange to forget some things.
bizydad (author)  sdello4 years ago
 I bought mine from the plumbing section at Home Depot.  About $60-70 for 60 foot coil I think.  My original plan was to build a manifold out of copper pipe and fittings, but when I saw the coil I realized I would get far more efficient heating with a lot less work.
sdello bizydad4 years ago
Are there different types of copper coil.  All i see advertsed is for refrigeration purposes.

Steve
bizydad (author)  sdello4 years ago
sdello bizydad4 years ago
Thanks very much.  I think I am going to make the heater using either a propane grill or charcoal grill.  What do  you think?

Steve
bizydad (author)  sdello4 years ago
 It's a judgement call, I think.  I don't know if a propane grill burner would produce enough heat.  However, neither is cheap if you use them regularly.  I tried the charcoal and was pleased with the results.  I got a nice even temperature that remained constant for at least an hour.  I had better luck using the fire logs because they burn very hot and last 2-3 hours.  It was pretty easy to control the temperature by simply adding another log to the fire.  

Which ever way you go, let me know how it turns out.
i have a 27 foot round pool, what i was wondering what size of cooper tubing would i need to heat this pool ? thanks Barry
EmmettO bizydad4 years ago
Understood. I would expect the pipe or outlet hose to fail in time. Even if it does, it wouldn't be a big deal in this set up. Just replace that portion of hose/pipe and it's back up and running. I'm just thinking about how someone could make this kind of set up permanent. 
procco EmmettO3 years ago
I don't think the copper tube will melt as long as you are running water through it.  The water absorbs most of the heat and the tubing never gets hot enough to melt.  Think of the science experiment where you boil water over an open fire in a styrofoam cup (or plastic water bottle).
Whole different story if there is no water running through the tubing though.
EmmettO procco3 years ago
 That's true to an extent but when dealing with a hot fire, steam bubbles will form in the pipe. When that happens the temperature of the copper at that point can rise rapidly and stress the metal.
Demascus2 years ago
We used a system like this to heat water for an out door camping shower, The copper will not melt, however I suggest keep your water running through it at all times when the fire is going. at your exit point perhaps a T fitting with a hot water heater pressure relief valve.(not that expensive) As our shower system did not have a circulation pump, and just used the line water pressure. When not in use the fire being a fire and logs being logs burning, it would build up steam pressure and blow the rubber hose off the end. Hense the pop off valve. However I feel as long as you have water circulating thru it you wont have this problem.
I have just built my heater it is a old 210 litre oil drum, in which cut lengths of 15mm copper pipe just long enough to fit length ways down the drum (25 metres or about 80 feet) joined them with standard "Yorkshire" pre-soldered elbows in a "zig-zag" formation this forms the heat exchanger of about 5/6 staggered bays as this will present the most area to the fire/heat as it perculates through on it's way to the flue. In the "filter box" i have fitted a domestic heating boiler pump BEFORE the incoming to the filter to draw off some of the water and send it to the heater and the water exits back into the filter pipe to return to the pool. I have NO plumbing skills but with Yorkshire fittings done it FIRST time no leaks! Water temp on first 3 hour burn started at 13 degrees Celcius and went up to 18 degrees. Second 2 hour burn next day raised by a further 4 degrees to 22 degrees celcius! Whole outfit cost about £130 and has saved me that to raise the pool temp to 22 degrees by what the electric would have cost me! As for the sceptics and melting the pipe NO NO NO it does not JUST KEEP THE WATER FLOWING and DO-NOT bank the fire up.....It works try it!!!
Your right about the copper tube. As long as you have flow the copper tubing will not burn through. And, as far as that goes on the other end, when it's freezing cold out as long as you have flow the tubing will not freeze. This principal works with pipe also.
phukaduck1 year ago
could this be done in an old BBQ?
So often old bbq's are being given away on kijiji / craigslist
and i'm pretty sure BBQ's are engineered to retain heat.

pull out the burners, put in a low prfile fuel log type fire, and install the copper on the underside of the lid?
or run off propane i suppose, and just put the copper on the grill.
jxk862 years ago
Try finding a local pool company that does service work on lp/natural gas pool heaters. I worked for a company for a few years that replaced several heaters yearly. The reason I say this is because there is already a wonderfully engineered heat exchanger in an old gas pool heater that most pool companies just throw out when they get replaced.

I tried this once, worked great but decided to go solar.

Anyway, I used a Hayward brand natural gas pool heater. When you strip it down above the burn chamber is a six or seven tube heat exchanger, complete with fins, and manifolds at each end. A simple replumb of the fittings to match however you are getting the water from your pool, then stick it on top of you fire in whatever means you can manage. I set up a feed line off the main return from my pump, in and out of the heat-ex, then back to the pool using garden hose. I had to max out the flow rate to stop the water from boiling in the heat exchanger.

It would be better to put the copper tubing on the top of the top and not inside. Inside the copper will eventual burn through and flood out th fire pit. I did this with a wood stove once and ended up with a flooded mess...
svilloch2 years ago
This was a great instructable!! Was able to raise an 18x48 pool about 4 degrees in a day, but I've been going through a lot of firewood and fire starter bricks in order to keep it going and the water coming out really fluctuates, getting cool quickly, unless the fire is insanely large. I am gonna try to put in more bricks to allow for a smaller fire and hopefully that will help heat more efficiently. Thanks again! (Thanks for the comment on the bricks exploding.. didn't realize that could happen)
Good call with the insulation and thermal mass of the masonry. I'm sure your fire burns better now without losing heat to the surrounding environment so fast.

Technically you should probably use fire-rated brick; then again in this application if one of the bricks decided to explode (which they can do), your face would probably be nowhere near it.

Do be careful when refueling though.
barry8823 years ago
i have a 27 foot round pool, what i was wondering what size of cooper tubing would i need to heat this pool? thank you
lukeyj153 years ago
It seems that people are worried about melting the copper coil.
Even if it did melt, the water would put the fire out straight away. A slight problem if you're using propane however.
dkahuna3 years ago
Hi like your Idea I run solar panels on my roof and my pool is inside my house,, I have a wood furnace built into a rock wall and I have found a wood insert airtite that will fit snuggly in the furnace I am going to incorporate your idea with just a little tweak I am going to hook the hose line to one of the water jets on the side of the pool so I don't have to cut or splice or add any pumps I will let you know how it works GREAT idea.
thehorn4 years ago
In regard to protecting the coil from the effects of direct flame:
 What if you "sandwiched" the coil between the top cover and a piece of galvanized sheet metal? In my mind the sheet metal would serve as a shield, thereby protecting the tubing from the direct effects of the flames.

bizydad (author)  thehorn3 years ago
 I'm not sure if that would reduce the heat efficiency for the pipes.  I did a little research on using copper pipes for heating and steam.  Seems copper pipe has a melting point of 1083 C and can be used to transfer steam as long as any welds are secure.  

Since this system is not pressurized and the water temperature in the copper pipe fluctuates between 80-110 F, I don't think there is a need to insulate the pipe from the direct flames.  At worst, if the pipe rupture they would simply act as a sprinkler to extinguish the fire.   I have yet to see any significant damage or distortion in the pipe, so I think it should be o.k.

Thanks for the question.  Hope this helps.
recowger4 years ago
during WWII, I lived in Clarksdale Arizona and we used a coal burning water heater called a "monkey stove".  I realize the monkey stove of today is a two holed wood or coal burner to heat water or cook meals.
Here is a good website for a good piece of metal pipe that would be usable with your idea.   Also think about heating the house using this method.

http://www.hilkoil.com/

I use something like your idea, but I put a small car radiator and fan next to the wall, use a 12 volt battery (deep cycle) and run the water through the radiator and the fan to move the heat into the trailer.  It works wonders and 15 or more surplus pallets cut up will do for a long cold spell and I have good warmth in the room.
bizydad (author)  recowger4 years ago
 Those look really good.  They look like a much larger diameter pipe, which means less pipe.  I used 50 feet of the coiled copper pipe because of the small 3/8 in diameter pipe.  Thanks for the link.

Thats a good idea with the car radiator.  I had thought about looking for a junk radiator coil off a refrigerator or a/c unit before I found the coil.
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