Thermo-siphoning in Active Solar Hot Water System

[flyaway]

I have a serious thermo-siphoning issue - 25 to 30 degree overnight temperature losses from solar hot water tank. Replaced a Watts 1/2 inch check valve which was most logical culprit. Installed a new Kitz 3/4 inch check valve. Still have same problem. When I close off the glycol ball valve, my overnight heat loss is about 3 degrees; that is, no thermo-siphoning.

I do have a Metlund on demand recirculation pump installed above the solar hot water tank. I do not believe this is an issue.

System pump is a Taco. Thinking about adding a Grundfos pump with built in check valve. The entire system is essentially SunEarth components: collector, tank and controller.

Any good thoughts will be greatly appreciated. I'm stumped but determined.

[hj]

Without knowing more about your system and its controls, we cannot help you. BUT, normally that would indicate a control problem, since the system should ONLY operate when the solar panel temperature is in excess of the water storage tank, and there was a need to raise the tank's temperature. With the average system, it loses temperature when the pump runs at the wrong time and then the solar panel becomes a "radiator" rather than a "collector".