After some thought and some great help from the guys over at www.varmepumpsforum.com I’ve come to the conclusion that my sensor GT1 (radiator return) was misplaced!

The handyman who installed my heat pump apparently didn’t know what he was doing. He had placed it on the return pipe from the by-pass tank to which the heat pump is connected — this works only until someone (i.e me) decides to configure the heat pump so that the internal circulation pump (P2) only is active when the compressor is active to save another 46 Watts.

The problem is that when P2 stops, the circulation between the heat pump and the by-pass tank stops which in turn results in that GT1 doesn’t notice that the radiator water temperature drops (P1 is still active so the water is still flowing in the radiators). After a while the real temperature is down to room temperature, but GT1 gets a reading of 30C or more….

So where should GT1 be placed? Inside the drop-tube (unsure about the translation it’s “dykrör ” in Swedish) on the by-pass tank of course!