Our pool was at the end of our deck. Half of the deck has a roof over it. (You know how hot an asphalt roof can get!) We had our passive solar heat panels on that roof and they worked great for more than twenty years.
Sometimes the pool water became so warm that we would run the system overnight to reverse the process and cool the water.
We've had our solar pool heater for 20 years but ours is mounted on our roof. There isn't enough room in our yard to place it there. Works great! We run it year round so that the water doesn't freeze in the Winter. The electrical cost for us is minimal.
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Yes you can mount the solar heater on your roof. It would take a small pump that has a little higher output pressure to push the water up to the roof, and then it will gravity feed back into the pool.
Do you have a south facing roof? Perferrable if it is the back of the house, in other words if your front door is on the north side, then the pool heater will not be visable from the street. Something that some neighbors might consider a eyesore.
There was a guy who made a solar panel to heat a 300 gallon tank in his basement, and used that water to both pre-heat his water going to the water heater, and to pump through a hot air coil to warm the house in the spring and fall, so he can delay running the boiler longer into the fall.
His solar panels where ground mounted, and I watched them on a you-tube video. He used cheap plastic tubing, and used silicon to mount it to a aluminum back sheet metal (tip silicon is a insulator, not a heat conductor, use something that conducts heat to mount the PVC pipe to the aluminum) then has 1/2" PVC pipes seperated by about 4" and 10' long, with tee's to a 1/2" line at the top and bottom. This made the panel about 10' 4" tall, and he covered it with some sort of plastic sheeting. This was leaned back about 45 - 50 degrees, so it collects summer heat pretty well, as well as winter heat. Laying it a little more flat will collect more heat in the summer, and 45 - 55 degrees will give the best heat at your lattitude.
He was pumping in 85F water and getting out 130F water. Not sure how wide his panels where, but it looked about 20 - 25' wide, and not sure how many gallons per minute his pump was rated at.
On your metal roof, it might be possible to just put down the tubing, then cover it with some glasing of some sort, and that will protect it from snow in the winter. You would need a "Vacuum Breaker" and "Air Vent" to allow all the water to drain out when the pump is off, and to let out the air when the pump comes on. In this case, if the pump is left off except for very sunny days in teh winter, it will be able to leave it on the roof all year long.
For glasing, if you happen across a bunch of used sliding glass doors, or find some plastic that is about 4' X 8x. you might be able to build something on your roof, enclose it with wood or metal, then cover it with the plastic or glass, in such a way that it can be screwed down and be removeable to fix any leaks ect. If at the top of the ridgeline, it would be ideal. The metal roof would not be penetrated, so any water that leaks from a PVC pipe or gets under the glasing would still run out under the panel encloseure area, and onto the un-covered roof, and onto the ground.
After making the enclosure, paint it black with a sprayer. This would be after installing a series of 1/2" PVC pipes to run the water through it, and back to the pool. After painting, and let it dry a day or two, then cover with the glasing, so you will not be loosing to much heat to the winds.
Idealy, the tubing will slope downhill about 1/8" per foot, so 10' would be 1-1/4" higher on one end than the other. This will allow more than 90% of the water to drain out of the collector, so it will dry out when freezing weather is near.
30 runs of 10' pipe separated by about 4" would give a 100 square foot panel, and you can decide if this is enough heat or not, then expand it by another 100 square feet or not.
Parts list is a pump rated at about 2 GPM at 15' of head pressure.
30 each 10' X 1/2" PVC pipe
60 each 1/2 X 1/2 X 1/2 PVC tee
various couplings, a few tees, and fitting to get into your pump line and have 1/2" output to your pump, then another fitting to get the warmed water back into the return line towards the pool.
You will also need a air relief valve and vent to let air back into the line, this would be installed on the high point of the inlet to the water heater.
You can save a couple dollars on tees by making it 20' wide, and say 5 - 8' tall, and using a coupling in between each 20' section, not as many tees, and you might cover the same roof area, with less fittings overall.
Build it to suit your glasing. If you find something 6'8" tall (slidging glass doors) or 8' plastic glasing, make it fit under your glasing.
Because the glasing will hold heat between your roof and the glasing, it does not rely completely on the heat going into each tube, but atcually will heat the metal roof, air between the roof and glasing, and then heat the tubing from all sides, with about 15% of the heat going into the black painted PVC pipe.
This guy built his 3" apart, but I am suggesting 4", just to save a couple bucks in PVC pipe. He is recommending CPVC piping, rated at 200F and 10 - 20 PSI. PVC is a little less pressure rating and less than 150F.
I have 6 panels (4'x12' each) mounted on our pool shed roof. The pool pump has enough power to pump the water from the pool inlet through the filter and then up onto the roof through the panels and then back down to the pool. The pump is 3/4 hp. I have had this set up for 21 years. I use 2" pvc white pipe to carry the water up to and back down from the roof. The solar system is made by Helicoil. I've had a couple of leaks over the years but nothing that wasn't easily fixed. I believe the reason for the wear and tear is the snow pack that lays on the panels over the winter. Sometimes we get a foot of snow at a time and I try to carefully scrape the snow off the panels with a roof rake when there is that much snow laying on them. Overall for the length of time we've had this system and the few issues we've had, I can't complain at all. Given two or three sunny days in a row, it will heat my 17000 gallon inground pool 15 degrees. I have to shut the system down as it will get the water up to 90 degrees if I don't keep track of the water temperature. These companies also make an automatic diverter valve system that when the water temp. gets to a predetermined level, turns a valve plumbed into the system which sends the water directly from the filter and back to the pool bypassing the solar panels. Our's died out a few years ago and a new one is like $300 or more. I find it more economical to check the pool thermometer and unplug the pump myself.