Pardon me if this isn't the right folder, but as the questions I have are related to the plumbing for the home-made heater box that I put together, so this seemed the appropriate place.
The heater I made uses about 200' of black poly (sprinkler) piping in a coil. I use 1-1/2" PVC for all of the existing plumbing, though I sometimes wished I had taken the time to find a local supplier for flexible PVC, that's beside the point. To get the water from the pool to the heater, without having to purchase a separate pump, I added a "wye" connector coming down from the output side of the filter. I used a bushing female thread to attach a brass hose bibb. From there it's a simple matter of garden hose (wanted the flexibility and not having to worry as much about kinks) to travel ~30' to the box. Then the poly coil, then back out of the box in another garden hose which attaches to a bibb, in another bushing, in another "wye" just before the ball valve that controls flow to the pool's return jet.
Having used this setup for a few days now, and having done some experimentation, I have come to one conclusion, and devised one theory that I'd like some checking on.
It occurs to me that the heated water may not be returning to the pool with the plumbing assembly I have cooked up, or at least not regularly. With both bibbs turned full on, sometimes I can feel the outer rings of the coil (the supply side) are noticeably cooler than the middle rings (the return side), which tells me that cool water is flowing in and out of the assembly... some days it seems like there's very little difference between the two. If I disconnect the hose from the return side of the piping and drop it straight in to the pool (I don't worry much about siphoning since the box sites above the water line) it seems to me as if it will heat the pool significantly better. As I haven't had 3-4 back to back days with the same weather, this last observation could be subjective, but the one about the different temperature of the coil portions I believe is not. Is it possible that there isn't enough pressure returning to the second "wye" adapter to allow the hot water to rejoin the flow from the filter? Do I have any clue what I'm talking about?
The second question I have is this: It occurs to me that the effect of the heat from the surface of the pipe being transferred to the water flowing through it during the day when the sun is shining also means that at night when the air is cooler than the pool that some of the heat is being lost through this system in the evening. Even on an overcast day it's possible that I will shed more heat than I would gain. This means that I have to pay attention and go turn a valve when I want the water pushed through the system. What I envision, and I believe is possible if I wanted to get in to designing something myself (which I don't), would be a two-thermostat valve system that checks the temperature of the water on the supply (pool) side and if the temperature is below a certain point (I don't really want the pool warmer than 84 degrees, though my wife would disagree) opens, the other side of this valve, or maybe a second valve connected to the first, whichever, would check the temperature of the water from the heater, and if it's above a certain point (we'll just say 90 degrees for argument's sake, there would obviously be some fine tuning necessary), opens. In this manner if the pool is colder than I want it to be, and the water from the heater is warmer than the water in the pool, then the water gets pushed through the heater. If either of these conditions aren't true, then the water doesn't go. A quick search for thermostatic valves though only showed me items in the $300 and up range which I'm guessing are for commercial/industrial applications. Is there a "cheap" thermostatic valve available that would serve for what I'm describing?
Thanks in advance for responses, and I have no problem being told I'm stupid as long as there's some helpful commentary to go along with it!