Wood Burning Pool Heater

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About: I'm a Special Education Teacher with 7 kids. I use donated and salvaged tech to teach STEM with my students and kids. Someday I'll get all of my projects posted up. For now, it's nonstop DIY, teaching and do...

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.

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

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.

Step 4: Installing Pump and Tubing

Cut the 25' water hose in half and cut off the metal female fitting. Replace with the plastic fitting and a plastic zip tie. Screw the hose into pump, the shut-off valve is optional (not really needed). Place the pump into the pool. **Do NOT plug the pump in yet**

Slip the other end of the water hose over the shorter piece of copper pipe and secure it with the clamp that came with the female hose fitting.

Carefully bend the 10 feet of pipe to shape until it reaches the pool. Place the black foam insulator, used for hot water pipes, over the pipe. Be sure that the foam covers the outlet into the pool to protect anyone from hitting their head on the pipe.

Step 5: Improving Efficiency

I tested both the pool temperature and the temperature of the heated water regularly to asses the efficiency of the system. I found that the water heated from 60F in the pool to 75-95F from the outlet of the heater. The temperature fluctuated in the range of 75-95F directly with the size of the fire. Keeping the fire at full flame quickly used up wood. Most of the heat was lost through the grate sides and not transferred to the pipes.

Modifications:

I added brick and stone to carefully insulated the three walls and leave the door open for easy access to add more wood and clean out of ashes. I also moved the grate up 6-8 inches to allow a smaller fire to heat the pipes more efficiently.

Step 6: Stats and Tips

I checked the temperature of the water with a temperature sensor that came with my multimeter. I bought and filled my pool in May. In my climate (outside Washington D.C.) the days were usually in the 70's, unusually cool this year. My tap water filled the pool in about 8 hours to about 5000 gallons of the approximate 6000 gallon capacity. The water temperature before I began was 58-60F, much too cold for kids.

In the original design, with the open pit, the fire heated the water coming through the pipe from 60F to 75-95F, depending on the size of the fire. The problem was that this required constant addition of fuel to keep the temperature constant above 80F. At this temperature it took approximately 1 hour to heat the 5000 gallons 1 degree Fahrenheit. It took me 3 days to get the pool from 60F to 72F, only to have 6 inches of cold rain cool it back down to 64F a few days later.

After letting the fire die out and cool, I added the brick and stone to insulate and direct the heat. I also raised the grate for the fire to bring it closer to the pipes. This was much more efficient and allowed me to maintain a smaller fire to keep the water output in the range of 95-104F. When I added charcoal to the top rack the temperature remained in the upper range longer and did not fluctuate as much.

At long last I found the out-of-season fire logs and starter bricks. I built a wood fire on the top rack and lit and Enviro-log on the bottom. This gave me a nice roaring fire with only the occasional need to add wood to the top rack. These logs last for about 3 hours and are a very efficient way to fuel the heater. They also dramatically boost the water temperature coming out of the heater.

I noticed the water was hot to the touch under the pool surface. When I pulled the pipe up, I noticed a little steam and got my meter. The pool water was 68F and the heated output was now 137F!!! I added the second half of the water hose to the outlet. With the end still weighted by the metal fitting it came with. It sank to the bottom and heated the water from the bottom up. This water was much hotter, so you will need to be careful if people are in the pool.

Best of luck if you try this out for yourself. The project was relatively easy and worked very efficiently, although all projects have room for improvement. Please leave your comments and suggestions. Thanks for reading.

2 People Made This Project!

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83 Discussions

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dee99999

Question 7 months ago

I have an 18 x 9 Intex above-ground pool, with a Hawyard Perflex pump/filter. I would like to do this, but make the fire box part of the existing water circulation (with Y connections and diverter valves), WITHOUT using a separate submersible pump. Any ideas on what types of hoses I would need to do this, and connect a heated copper pipe to something that returns the water back to the pool?

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ultipost

4 years ago on Introduction

I am thinking along the same lines but with a (to be custom made) BBQ and small hot tub. If no need for BBQ'ing we can just burn wood.

What do you guys think. Could that work?

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single_dad_hacksultipost

Reply 8 months ago

I’m planning to do that w an old smoker style bbq. I want to use wood and not sure if I should somehow make it into a rocket stove type heater or just put coil on top of wood in the bbq. Also thinking of putting coil in a metal container of water to retain more heat when fire is done. Any comments? How did yours turn out?

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single_dad_hacks

Question 8 months ago

Great job! Wondering if u put the coil in a container of water would that help retain the heat requiring less need to keep fire burning?

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mackthefatcat.

1 year ago

Excellent ideas! I would like to add my thanks for several important items that popped up within the comments here. My background: Retired Marine living in Japan. I am building a outdoor hot tub at my cabin on the flanks of Mount Fuji and was studying and investigating several methods of heating it during the fall & winter months I was looking up melting issues with copper pipe as my plan was to use a wood stove and to drill holes in the rear to thread copper pipe through at the top to form a multi S loop system in the top of the wood stove. I would like to thank the author for his time in this as it gave me a couple of good ideas, and for several of the commentators for links they provided.

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joshnort

7 years ago on Introduction

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

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jpharringtonjoshnort

Reply 1 year ago

Must not have been a product people wanted. The website/domain are not longer associated with the product.

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tim_njoshnort

Reply 5 years ago on Introduction

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'

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Blaakenjoshnort

Reply 6 years ago on Introduction

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.

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bizydadBlaaken

Reply 6 years ago on Introduction

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.

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mad_b

1 year ago

HI! this is a cool work! I would recommend search a bit on the net about "rocket stoves", if you want to have a more efficient energy conversion. You will find that the rocket stove has more than 90% of efficiency, thus allowing to consume less wood (or have more heat). An "open" fire like yours generates lots of smoke and wastes most of the wood stored energy.

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radcock

3 years ago

Do you know of any related ideas? Im trying to heat water for a radiator in a converted stable/office.

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GabG

3 years ago

In process 4x66 feet of 3/4 of flex cup 260 feet total 4 1/2 feet hight by 6 long

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matarkin

4 years ago

I want to try something similar with my aquaponics setup during the cold months. I have to be able to regulate the temperature though or I will kill the fish!

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onemoroni1

5 years ago on Step 6

Very creative and good use of materials. Your projects are like mine. They evolve as the results show areas to redesign for improvement.

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peads92470

5 years ago on Step 6

I am really impressed with this idea. Absolutely something I have been looking for. Simplicity and results. I wish I had thought of it.

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bizydadeyesee

Reply 5 years ago on Introduction

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.

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EmmettO

8 years ago on Step 3

 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.