Altherma frequent compressor cycling

[afternine]

So the Altherma is trying to achieve set point temperature by modulating the compressor speed whilst the set point is being modulated itself by the weather compensation software in the system.

Exactly.

Also, if the pump stops with the compressor off mode then whenever the pump starts, in a temperature sampling, then is it monitoring the temperature of the water returning from the underfloor system or is it checking the temperature from the hydrobox?

It has a termistor before and after the heat excanger in the hydrobox. While the compressor is off, the water passing through heat excanger is not heated, so both thermistors read the same value. Altherma displays on the control panel the temperature that is on the heat exchanger outlet, so when compressor starts you can see water temperature raising, but you can not see the water temperature coming from the manifold (return flow) any more.

It seems to me that the underfloor loops are not maintaining a stable temperature (maybe that's the way it is designed?) which goes against the design of underfloor heating; correct me if I'm wrong here.

The underfloor loops keep rather stable temperature, since there is 15cm of insulation beneath the loops, and they are covered with 6cm of concrete, on which ceramic tyles are laid. The water from radiators is joined (mixed) with water from underfloor loops before going to heatpump. As far as I could observe it looses (the mixed water) about 1°C in 2 hours. This is not an exact measurement.

I think that you should keep the pump running to provide a more steady load on the controls.

That's why I set it to "alwasy on".