It is not recommended that the Eberspacher Hydronic D5WSC, or any other Eberspacher water heater is controlled via a thermostat, this may cause issues with short-cycling.
If the heater is starting when the temperature gets to around 5 degrees celcius, it is possible that you have a frost protection stat fitted to the wiring harness, it may need adjusting if your heater is firing up when the temperature is not near to what you would expect in terms of 'frost protection'. If you have a thermostat like the Webasto Eberspacher Espar heater Frost stat thermostat | 4111263A | Butler Technik, you can adjust the temperature it switches 'on' by removing the front 'tamper proof' cover.
There are other frost protection devices that could be installed, to find this you should locate the Yellow S+ switching wire from the main heater unit and trace it back to the control switch.
If you require further support on this, please contact our technical team at Butler Technik
