Controller

The system is governed by a RESOL differental temperature controller designed for solar applications. My system uses the DeltaSol AL controller.

From My Solar Hot Water System

The controller measures the temperature of the panel on the roof and compares it to the temperature at the bottom of the tank. If the roof is 12F hotter than the tank, the pump will circulate warm water from the roof to the tank. When the difference shrinks to 5F, the pump stops. This cycle continues until the top of the tank reaches 160F, after which the system enters a stagnation mode where no pumping occurs.

The system will exit stagnation if tank temp drops below 156F, or if the roof temperature exceeds 240F. The latter is to protect the system from boiling or the glycol from getting heat damage. If the roof is too hot, the system will periodically pump short periods to try and keep it cool...if the storage tank gets to 190F the whole system shuts down. The system is sized so that the tank can hold all the BTUs generated by the collector in a day if the starting temp of the tank is 160F. So, it should never shut down unless there is a problem.

At night, if the tank is still above 160F, the system will run to let the excess heat radiate to the night sky until the tank is at 160F. This ensures there is enough capacity in the system to hold the next days heat.