best temp. differentail for hot water solar system
 

what is the best temp. differential for a hot water solar system
between the hot water tank and the solar panels to turn on and start to circulate
also what should the temp. differential be to turn off the circulating system

If you are not worried about over-heating your storage tank water,
then you can set the pump to start, when the collector is 5 to 10 degrees warmer than the tank.
Turn off the pump when tank temp = collector temp.

A problem might occur, if you have too much collector gain and your tank temp
gets so hot, the controller shuts the pump off. And the collectors over-heat.
Your coolant or collector might be damaged.?. IIRC, it's called Stagnation.

I think the type of damage depends on system components used.

Hi,
My experience is with drain back systems, but this probably also applies to closed loops.

A lot of people seem to end up around 15F for the turn on differential because if they go below this, then the collector first starts up in the morning the water coming in from the storage tank cools the collector down so much that the collector sensor hits the turn off temperature right away. That is, the controller ends up short cycling the pump.

Its best to mount the collector temperature sensor near the return line so that by the time the water gets to it, it has been warmed as much as its going to be warmed and there is less chance of short cycling.

The best turn off temperature usually ends up being around 2F, but the best way to find out is to take two thermometers and measure the temperature of the water going out to the collector and the water coming back. You want to set the turn off temp so the ACTUAL difference in temp between supply and return is about 2F ish -- its not really worth running much longer than that.

I say ACTUAL because the temperatures you really measure of the supply and return fluid will not exactly match the set turn off differential when the controller turns the pump off. This just has to do with the sensors in the collector and tank not being able to measure the actual temp of the supply and return water -- they may be off by a surprising amount.
My DHW system would actually be cooling the water in the storage tank before the controller shut it down with the turn off differential set at 2F.

One of the things that can fool the collector sensor is mounting it where it gets direct sun -- sometimes just putting a little shade over the sensor can help. The tank sensor should be mounted as close to the actual pump inlet pipe as possible.

One of the best thinks you can do to get a bit of a performance boost out of your system is to check the actual temperatures at turnoff time and make sure its not really cooling the tank and not quitting too early and missing heat. While this might not be a huge improvement, you get the improvement every day for the whole life of the system.

Gary