Digital differential thermostat

DDT2 is a digital differential temperature controller for solar hot water systems and other applications with one relay output. It covers the most common hydravlic configuration with one heat source (thermal collector) and one heat sink (water tank).

The meaning of differential thermostat comes from a temperature difference between two measured temperatures: the temperature of the heat source and the temperature of the heat sink. On these observations and pre-set values the controller makes a decision: output relay on or off — transfer heat or not.

In mode one; if source temperature rises above the sink temperature the controller will switch relay output on and will switch it off when the sink temperature rises above the set target temperature or the sink temperature approaches source temperature. In mode two; it operates as normal thermostat: switch relay output on only if the temperature is below target (desired) temperature. When temperature rises above the set point the controller will switch relay output off.



Current EU version of a controller works on 230 V mains and has Celsius readout. Other customizations on request.

All controllers are supplied with a pair of waterproof temperature sensors soldered to 2 m long leads. These are DS18B20, NTC10k or a combination.


Digital differential thermostat case

Technical data

More information

Usage (hydravlic configurations)

Differential thermostat could be used as a solar controller, as a domestic hot water tank controller or as a fan controller in a greenhouse.

Hydravlic configuration for solar water heating Alternative hydravlic configuration


Technical support is available on mail support@[this domain].