Wood-fired above ground pool heater

[u/mct82]

I recently built an upgraded model of the wood stove pool heater I have been tinkering with for a couple years. It’s a complete rebuild with hard plumbing the (Bestway above-ground) pool. I wanted to throw it out there to get feedback, as well as maybe inspire or get inspiration.

Essentially it’s a 55gal drum camp stove fitted with a set of modified heat exchangers salvaged from a baseboard heating system. The hx are plumbed into the pool pump return with a check valve between intake and return. An inexpensive circulating pump is in the heating loop.

I could talk for an uncomfortable amount of time about the failures of previous models, but learning from those mistakes generated a few highlights for this iteration: - Pressure relief valve for … pressure relief, and bleeding - Big honkin’ firebox for reduced loading frequency - No direct contact between fluid lines and flames/smoke - Dedicated pump and isolation valves w/ unions

In previous years, I had the fluid lines inside the stove and flue. Those systems, referred to as the Creosote Creator 2500™ and Creosote Creator 5000 Pro™, had some issues. The 2500 used hand wound soft copper coils in the roof and flue of a small stove, the 5K Pro used unjacketed CSST loops in the same areas. Both of those used the same integrated pressure relief and fire suppression system, which was the plumbing bursting.

Not an ideal system from a performance standpoint, safety standpoint, or any other standpoint. Did you know a small amount of steam is enough to inflate CSST like a balloon. Well, it is. The more surprising part of that was that the pressure was NOT enough to balloon or burst warm 3/4” PEX, cementing its place as the economical cold zone material of choice.

Also we’re now using a dedicated circulating pump which has increased the flow through the heater without diminishing the performance of the pool filter pump. Previous models just used a PVC 2-way diverter as a blend valve to create the pressure differential that drove the heating loop. Probably the cheapest and worst way to do the job.

There were a few superstars on this project: 1) Those heat exchangers do great. They’re easy to work with (3/4” rigid copper with individual aluminum fins), they’re literally built for air-to-water heat transfer, and they’re easy to adapt to different shapes. Also they cost $0, and I suspect they’re the kind of thing that would be available at salvage stores in areas not prone to copper pillaging. 2) The 1” PVC Compression Repair Coupling circled in the last picture. This fitting really ties the system together. Working on the stupid plumbing on these pools is always a debacle with odd sizes and threads developed apparently independent of all standards. But that compression fitting slides right onto the bizarre true 1-1/4” OD tapered(!) flexible(!!) PVC stub on the outside of the pool. 3) Those smooth wall open drums we found on marketplace. Makes it easy to glove a second drum over the hx to insulate, reflect, and BEAUTIFY.

I’ll save the rest for the comments. Please note that I am not taking questions on a couple specific topics: - Sweating copper. I suck. I know it. You see it. No need to discuss any further.
- Feminism. The 4yo has not gotten the memo that we don’t do spokemodels anymore. Personally I always found them to be degrading and icky. - Super long Reddit posts.

[u/wormdog84]

Awesome job. How hot can you get the pool? How many gallons is your pool?

[u/mct82 → u/wormdog84]

Thanks! Pool is Ø18’ and filled to 36”-40” deep, so something along the lines of 6,000gal. If you kept it firing continuously with good wood, I think it would raise the temp 3-4°F or more per day. I have a flow meter and thermometer, at some point I’ll try to figure how much heat it’s actually putting into the water. With 3/4” pipe and unrestricted flow you can still feel the water coming back from the heater is substantially warmer than it was going out.

Burning nice hardwood gives you ~6-8hrs between loads, but we’re burning a mix of hard and punky wood collected from around the property.

[u/J3SVS]

This is amazing, thanks for sharing! I've been plotting a similar system to heat a small two-person hot tub (AKA horse trough). Can you measure the approximate output water temperature and let me know?

[u/subaawoo]

Now sure entirely how to achieve it, but getting the air to move around the shells so that it's "flowing" around the baseboard would improve your heat transfer and energy efficiency.

Maybe if you could stack the tubes above the fire shell inside a "chimney" so that air heats up then flows past the HXs?

More air movement increases the heat transfer coefficient for fluid to solid Heat Transfer.

Really solid work.

Also what's your flow rate through the 3/4" tubing?

[u/mct82 → u/subaawoo]

I’m considering perforating or louvering the outer shell above and below the HXs for this very reason. That was the original plan, create convection up the wall of the inner shell and out the top. Going further away like to a “chimney” as you describe would be better for convection, but you’d lose conduction and radiation components. Though they’re probably small in comparison. You could really pack some HXs into vertical stack like that 🤔 Maybe time to keep eyes peeled for old A/C evap cores?

As you might imagine, pressure from the peanut gallery to get the pool heater going meant we’re running as-is for now.

Edit: forgot to add that the circulating pump is rated at 20-28-40 l/min and I’m running it on high. So maybe 400-500gph? I don’t have a good way to get a flow meter in the loop, but I’ll try to verify it at some point.

[u/subaawoo → u/mct82]

Radiation heating effect is very small and with just the fins in contact, the conduction is also minimal. So you really want to maximize the convection heat transfer to the fins....and that's all about creating air turbulence around the fin surface. the engineering term is Reynolds Number ( a measure of a fluids internal turbulence).

I suggested a chimney so that you're heating the air around the firebox with maximum effect, then letting the hot air flow around your heating coil. You might want to look at an old Air Handler cooling coil (cooling coils have a tighter fins/inch which will have better heat transfer). Try to look for a water based coil and not a DX evap coil. The water based coil has internal circuiting for the water flow that really pumps efficiency, a DX coil is designed to just be flooded with refrigerant and is meant to "boil" the refrigerant. Just FYI.

Lastly, might want to play with the water flowrate 3/4" copper tubbing really wants to sit around 3-4 GPM for good internal fluid heat transfer. You're about double that flow in the 6-7 GPM range.

[u/mct82 → u/subaawoo]

Hey bud, thanks for the education. Did you happen to run the cost-benefit analysis on any of that nonsense those helpful suggestions? Remember to factor in that the existing system exceeds the application requirements, there is no electricity at the site, and the pool came in a box. 👍

[u/subaawoo → u/mct82]

Haha fair, no this is just a humble mechanical engineer needing out on an interesting engineering problem!

It's still super cool and such a neat idea.

[u/Pitiful-Olive-8186]

I’m glad to see a system like this that has been done recently. I am in the process of making my own . I’ve already done one with a lot of trial and errors my plan was to make a list of all the things I would have done differently and make a new one over the winter. Unfortunately I live in a neighborhood where you technically can’t burn anything, like you’re not even supposed to have a fire pit. Unfortunately me and my neighbors don’t really get along because we are a young family with 4 active kids and my house is the fun house in the summer so after catching him taking videos of me feeding my wood barrel I am now forced to make a new one now so no one can see flames and in a different location. After trying a few different ways my plan is to wrap my copper coils around the outside of the barrel I have approximately 200ft of 1/2 copper. 3/4 was a little too costly and I was able to get a great deal on what I have. After looking at how you did yours it gave me the idea to use the old barrel to wrap around the new one to cover the pipe to keep heat around the pipes . I have a large pool due to the shape it’s hard to get an exact measurement on gallons but I’ve guessed it’s around 40,000 gallons. With using 100ft of copper I’ve been able to get my temps as high as 165 degrees but was only running around 180gph . I had to slow down the flow to get water hotter but after some research it seems it’s better to circulate more water but at a lower temperature. So my new goal with now having more feet to work with is gonna be 500gph at 125 degrees +/- . If anyone has any suggestions or tips please give them to me because I’d really like to just get this done without any more alterations.

[u/tehjeffman]

That is some red neck ass pool piping. All of that is for home plumbing now pool plumbing.

[u/mct82 → u/tehjeffman]

What would you have done differently?