Wood Fired Pool Heater

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Summary

I like to swim in our 18 foot round above-ground pool when the water is nice and warm, in other words over 80 degrees Fahrenheit.  A solar cover certainly keeps the water warmer but a house addition and tree growth  blocks some of the good afternoon sun that provides much of the heat.  We also have a considerable amount of firewood from trees that have been felled so why not marry the supply of wood to the need of pool heat?

There are few details readily available for building one's own wood-fired pool heater.  I gleaned what little I could find and cobbled together the rest over a period of time.  Nothing here is fancy, pretty, or optimal but I learn so much from the Internet on various topics that I'm taking this rare opportunity to contribute something back (to the pool of knowledge!).  

My original mental designs including use the pool's filter pump to circulate the water through the heater.  After further researching the required parts and complications, I decided to make this a self contained unit with its own pump.  In that manner I could allow someone else to use it as well.  Besides, the submersible pump cost $10 and the various fittings and valves for attaching to the filter line would be well over that.

Feel free to email with any questions, to the address on the Home Page.


Disclaimer:  All contents here are strictly provided for information only.  Use at your own risk, I'm not responsible.  Remember, you are playing with fire.


Construction

Parts List

  • 16 gal steel barrel, formerly held lacquer paint thinner.  Top removed.

  • 10 feet of 1/2 inch thin wall copper tubing, some elbows and miscellaneous hardware

  • Two garden hose bibs/valves with solder-on ends

  • Small outdoor submersible water fountain pump from Harbor Freight

  • Approximately 10 feet of garden hose and 2 female fittings for same

  • Approximately 3 feet of aluminum downspout or similar material for a smokestack

  • Recycled grate so air can get beneath the fire.  I cut down an old gas grill grate

  • Plastic spring clamps for securing the input and output hoses to the pool wall edge


Assembly

All the construction was simple, novice, amateur, and barely functional.  I make no claims to be a metalworker - I just cobble stuff together and certainly won't quit my day job in computers!

After much measuring I cut the copper tubing into 6 equal lengths of approximately 20 inches.  This was soldered together in a zig-zag or Z fashion.  See Figures 3 - 4.  Not readily apparent is an attempt to curve the joints slightly to match the curvature of the barrel.  Let's be fancy and call this the heat exchanger.

Meanwhile, back at the barrel, I cut a rectangular smoke hole on top of the barrel, just a few inches from the back of the barrel.  I made it slightly larger than a piece of downspout.  Think ahead; plan on not having to cut the seam of the barrel for either the smoke hole nor the valve holes.

I positioned the heat exchanger at the top of the inside of the barrel.  See Figure 5.  I mounted the inner end of the heat exchanger (toward the closed end of the barrel) with copper tie down straps.  The front was a bit more problematic because of the valves so a piece of hardware strapping was cut and bolted to the barrel.

With the heat exchanger loosely in place, I marked and cut the holes for the hose bibs.  Solder these in place with the hose exits pointing horizontally away from the barrel.  These are the water entry and exit points.  It goes without saying that I also mounted the ears of the hose bibs to the barrel with small machine screws.

Stick a piece of downspout or better smokestack in the smoke hole, and place the fire grate inside.  I'm not sure the smokestack is really required but it seemed like a good idea.  I doubt it generates noticeable draft for the fire.

Pressure test the heat exchanger.  Because of the handy garden hose connections, you can put your house garden hose on one end and some sort of valve, like a garden hose, on the opposite end to close it off.

Attach the input hose.  Garden hose end goes on the heat exchanger, the other end may go nicely on the pump nipple with a hose clamp.  Attach the output hose, hose fitting to the heat exchanger and placing the other end lobbed in the pool.  I used plastic spring clamps to hold the hoses in place.  Hint: if one hose, like the output hose, drops out of the pool even without the rig running it will siphon through the pump and drain water out of the pool.  Ask me how I know that?! 

You are ready for fire!


Operation

You just built a wood fired swimming pool heater, so I would guess you are talented enough to build a fire.

Be sure to turn the pump on before firing up.  I believe the water in the copper helps keep the solder from melting.  The water in the copper gets very hot with even the smallest fire if there is no flow.  Likewise, don't turn off the pump until the fire is really out.  Red coals emit considerable heat.  While the water at the output hose may not feel very warm at that point, turning off the pump will allow the water in the coils to boil.  Hearing the water boil in the tubes makes me want to hum "The Wreck of Ol' '97" but I assure you it isn't good for the plumbing.

Since the pump you use may not be appropriately safety rated, do not allow swimming in the pool while power is applied to the pump.


Revision 1

I noticed the fire smoked quite a bit, even when burning decent wood.  It burned but with little fervor.  I owed this to lack of air despite the entire front of the drum being open.  I placed a small muffin fan in front of the fire and it burns much better and a bit cleaner.  See Results below for proof.


Results

Very impressive.

Input water temperature was 80 degrees F for my evaluation.  Without the muffin fan ("naturally aspirated") the output water temperature was 92 degrees F.  With the fan running ("turbo"), the water temperature was 96 degrees F, 97 degrees F when the fire is going good.  I've not run it consistently to determine the total rise of the 18 foot pool. 

Fire Type Input Temp Degrees F Output Temp Degrees F
Naturally Aspirated 80 92
Turbo 80 97

What Could Be Better

  • I opted for hard copper versus soft copper tubing, principally because hard was cheaper and I didn't have a tubing bender handy.  Results may be better if the turns could be closer or perhaps coiled. 

  • More copper.  My 10 foot is probably minimum.  Perhaps with soft copper it would be possible to place 20 feet or so in the top of the barrel.

  • Think ahead to your installation and how the hose bibs should be situated.  The garden hose tends to kink more easily with hotter water.

  • If I were going to move the heat exchanger between barrels, then mount the hose bibs just outside the barrel's mouth instead of drilling holes and soldering them into place.


Disclaimer:  All contents here are strictly provided for information only.  Use at your own risk, I'm not responsible.  Remember, you are playing with fire.


PHOTOS



FIGURE  1


FIGURE  2

    
FIGURES 3 - 4


FIGURE  5


FIGURE  6

  
FIGURE  7


Click on the photo above or here Short Windows WMV format video clip of the pool heater in operation.  At the time of filming I was burning pine which smokes a bit more than the wood I typically burn, plus it was in naturally aspirated mode.  The rake handle is used to support the hose - afterwards I have arranged the hose such that it is not needed.  Repositioning the hose bibs would eliminate this challenge.


Me and My Pool Heater

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Revised July 8, 2007

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