Solar differential controller

Solar thermal systems generally use a special relay called a Differential Controller. This relay activates a pump when the temperature in the solar collector is hotter than the temperature at the bottom of the solar storage tank.

Two sensors (storage tank and solar array) are required for a proper "differential". One sensor is attached to a pipe near the bottom of the solar storage tank. The second sensor reads the water temperature as it leaves the solar collectors. Both sensors must be insulated (with fiberglass or foam) to prevent ambient temperature from influencing the reading. It should be noted that a sensor clamped to a hot pipe will NOT accurately read the actual water temperature. In fact, the water will generally be 15 to 20 degrees warmer than the sensor indicates.

Fortunately, for the purposes of a well functioning solar hot water system, actual water temperature is not important (unless, of course, it's too tepid for a hot shower). What matters is the difference between the water temperatures at the two locations. After all, if the water is actually hotter than what the sensor indicates, so much the better.

The DeltaSol® BS controller is available in 4 versions, depending on the demands.

Technical data
Size: 172 x 110 x 46 mm
Housing: plastic, PC-ABS and PMMA
Protection type: IP 20 / DIN 40050
Ambient temp.: 0 ... 40 °C
Mounting: wall mounting, mounting into patch-panels is possible
Operation: by 3 push-buttons at the
front of the housing
Functions: Differential temperature
controller with optional add-on system
functions. Function control, operating
hours counter for solar pump, tube
collector function, pump speed control,
thermostat function and heat quantity
measurement.
Inputs: for 4 temperature sensors Pt1000
Outputs: 2 semiconductor relays

Sources: Radiant floor heating company, Reso-Elektronische Regelungen GmbH