Pipe Diameter for Solar Pool Heater

[DaveO]

What are the pros / cons of using 1/2 inch compared to 3/4 inch poly pipe ?

I want to add 4 coils, each 300ft of pipe.

My feed from the pump / filter is a 2 inch pipe ( and so is the return pipe back to the pool ), and I plan to add 4 T-pieces to that line ( 1 T for each coil )

I just can't figure out which diameter pipe to use.

My logic tells me that :
1. using a set outer diameter for the total coil (4ft), will result in the same area of pipe in the coil being exposed to sunlight.
2. the larger diameter pipe will put less back pressure on the pump.
3. the larger diameter pipe ( x 4 coils ) will have a better flow rate than the thinner pipe, thereby giving better thermal transfer to the water.

[russ]

With the set outer diameter of the coil you have set the amount of solar radiation you can collect - it is the same in both cases. You would have less total feet of the 3/4'' pipe in each coil.

It then becomes a function of total surface area available for heat transfer plus boundary flow rate. Believe you would find the 1/2'' pipe to be better but with significant additional pressure drop.

I believe you are better off with the 3/4'' due to pressure drop.

You would want a hand valve (cheap plastic) at the inlet to each coil and a location to check the outlet temperatures of each coil for balancing purposes. You definitely was pipe that is rated for sunlight use - I don't know if all black plastic pipes are or not.

[WildcatSolar]

1/2" black poly pipe works great. I have 2 - 1000ft runs on a nearby roof. I have a valve to restrict a little pressure from the pool pump and return the HOT water on the other side of the valve. This setup raises my pool temp by 8-10 degrees a day when the sun is out. Doubled my swim season.

[russ]

1/2" black poly pipe works great. I have 2 - 1000ft runs on a nearby roof. I have a valve to restrict a little pressure from the pool pump and return the HOT water on the other side of the valve. This setup raises my pool temp by 8-10 degrees a day when the sun is out. Doubled my swim season.

Must be one very small pool! 1000' and 1/2'' - pressure drop would be quite something!

[DaveO]

After much deliberation, I decided to go with 20mm ID pipe and, due to the limitation of the width of the flat roof next to the pool, I added 4 coils, each about 1500mm diameter. Each coil used about 70m of pipe. The remaining off-cuts may very well be used later to add another 1 or 2 coils.

I added 4 ball valves ( for isolation in the event of a failure ) to the 50mm feed pipe, and connected each of the 4 coils to a ball valve.

The return from the 4 coils all feed back into a single 50mm which drains into the second inlet in the pool.

Murphys law, Saturday to Tuesday were all overcast days, so didn't seem to get much heat from the coils. In fact, the weather was so cool on 2 of the days, that I simply closed the master ball valve and by-passed the coils completely.

Wednesday was a great day. The pool started off at 22 celcius (c) in the morning. By mid-day, 23c and afternoon it got to 24c.

Overnight to Thursday morning it lost 1c (back to 23c), but I think that a solar cover on the pool would reduce that. Just not sure if I want the extra cost and hassle for that just yet.

Again sunny Friday, and the water has gone from 23 to 25.5c. A bit more windy today, so will have to see how much of that is lost tonight.

I am hoping that the overnight loss is kept to about 1c a day, so the water has a constant daily increase of 1c.

Water coming back from the coils to the pool seems to be a constant 2c higher than the water going into the coils. Not certain what the flow rate is, but from the research I did, it appears that if you have a huge in / out difference, you're probably losing heat off the coils to the surrounding air. Based on this, I think the flow rate is about right.

With a warm sunny day predicted for tomorrow, I think there's a reasonable probability that we'll get to 26c after mid-day, so our first dip of the season should be a great event tomorrow.

Total cost of the setup was ZAR 2330 ( about $290 ) and took 2 full days to assemble. Monthly running cost : Zero.

With the 2c per day increase, it breaks down to about 0,5c per coil. Adding a second row of 3 coils would therefore add an extra 1.5c per sunny day, and with a 1c drop off at night, I am looking at a real increase of 2 - 2,5c per day - naturally, that will only be up to an optimum point. When we hit mid-summer, I will reduce the flow rate, or just isolate the coils completely, opening them once every few days just to keep the pipes clean.

Thanks to everyone who ever shared info on the forum, and especially to those who replied to my questions. At the end of the day, it was one of 'those things' that I just had to get down and do ( after 3 months of on/off research and thought -- 2 days up and running ).

[russ]

Well done Dave and thanks for the update!

[DaveO]

Update :

After running the system for over a week, we are really enjoying the warmer water. Warm enough to swim, while other locals are still sitting and admiring their sparkling blue pools - too cool to get into.

However, 2 small problems arose which I think should be mentioned, as I didn't know these before starting the project ( so hoping they will help someone in the future ).

The pipe choice that I made (20mm ID) seems to have been the correct choice. I added a pressure gauge to the multi-port valve, and there is very little difference in pump pressure between having the coils on or off.

KINK
I did end up with a slight kink in the inner-most curve of 2 of the coils. I don't think this restriction is serious. IF I had gone with the thinner diameter pipe, a kink would have had a greater restriction on the flow, affecting flow rate, heat transfer, and pump pressure.

Second problem was the choice of sealants. After all the research I did, I can't honestly say that I saw this area covered. I took the advice of the local hardware store and used a gasket sealant ( black tar like substance ) to seal the threaded joins between the 50mm PVC pipe and the nylon fittings, and between the nylon fittings and the threaded ball valves. I think this was the wrong choice.

I found that at full pressure, the joints leaked. At the normal running pressure, they sucked air which resulted in a constant stream of bubbles in the return pipe inlet. Not a problem during running, but when the pump turns off in the afternoon, the return pipe drains ( loses it's syphon ability ) and I have to use the full pressure the following morning to force the air out.

After consulting a few other 'experts', I decided to strip and clean the threaded fittings and replace the black gasket sealant with PTFE plumbers tape and Marine Silicone. Have done all the ball valve sections on the feed pipe-to-coil side and left it 24 hours to cure. Turned the system back on this morning and even at full pressure, no leaks whatsoever. Also, on the return inlet, the stream of bubbles is greatly reduced. Next weekend I will replace the sealant in the fittings where the coils each connect to the return pipe, and believe this will see an end to the problem.

Bottom Line : when looking at sealant options, discuss the usage and compositions of the fittings with more than 1 person. Never assume that an employee in the hardware store actually knows what they are talking about. Apart from the cleaning off of the really messy black sealant, the re-assembly of the fittings (with the tape and silicone) was considerably faster, easier and cleaner (less messy).

On a separate note, I recall reading that the ideal system should have coils sized to 50% of the water surface area. I did some calculations ( coil area -minus- area of the coil's center hole ) and my 4 coils cover a total of 6.7 sq. meters -- pool is 28.6 sq.m so coils are 23.4% of the pool surface.

If I add the other 3 coils that I want to, the coil area will be 11.72 sq.m -- 41% of the pool area. I think this way, as mentioned before, I will have more daily gain, with about the same nightly loss, so the cumulative effect should be greater. So yes, it looks like the ideal 50% theory 'holds water', and as myth-busters would say : Confirmed.

[DaveO]

Additional Update :

After enjoying the benefit of the initial 4 coils, I eventually added the 3 extra coils I wanted on New Years eve. This has increased the coil area to 41% of the pool surface.

Temperature increases since then have been great - on sunny days, depending on the amount of cloud about, water temp increases between 3,5 and 5,5 degrees celcius.

The pool is uncovered, and we live on a sea facing windy hill, so I think that if I get any higher than the current max of 30,5 celcius, then the heat loss ( to air and ground ) will probably start to cap the limit that the coils can heat to.

Yes, I know our new average of 28 - 29 celcius seems a bit warm, but the swim school that my kids go to is 31, and until this last week, my 3 year old daughter refused to get in our 'cold' water. Now I have to argue with her to get her out the water. That alone has made it all worthwhile.

My rough calculations show the energy being added to the pool is around 237,000 BTU per day.

[Naptown]

Nice Job well done. Enjoy the extended swim season.