I have an Espar S3 D5E hydronic heater using an 801 controller. The Eberspacher 801 will turn on and off the hydronic heater, but it does not control the fan and the thermostat in the 801 seems to always call for the water to be heated, even when the thermostat in the 801 is set at the lowest level. Is the 801 compatible with the S3 D5E which means that there is some other issue that I need to trace, or do I need to change to an Easy Start Pro Controller?

An Eberspacher 801 controller would NOT be my first choice or recommendation for an S3 D5E Espar Hydronic heater. To be honest it seems a very strange choice of controller for this heater model.

If you wish to upgrade your controller to an EasyStart PRO timer or Easystart Select you should please first confirm which version of S3 D5E you have installed? is it a CL or CS version. This is important to learn.

However please be aware that all Eberspacher controllers are not designed to switch off/on the heater via referencing the ambient cabin room temperature. There should be additional means to control cabin temperature - for example thermostatic radiator valves on radiators, or perhaps a room themostat temperature controller that switch independently blower motors within heater matrices.

Thanks, let me know