Arduino Uno & Crydom D1202 SSR Relay are in the white box.
(This relay is rated at 2.5A 120vac so a much larger pump could be used)
When Panel sensor is 20*F or more hotter than Tank sensor,
the Arduino energizes Relay.
The Relay powers the 30VDC 1A Power Supply,
which powers the 24-30VDC El-Sid 10W pump.
When Tank sensor rises to within 2*F of the Panel sensor,
the Arduino de-energizes the Relay.
This setup is for a small system in my 8'x16' trailer.
1 3'x8' panel & 40 gallon tank, heats radiant floor.
A cheap mercury thermostat controls a seperate 12VDC El_Sid pump for floor loop.
