Fluctuating temperatures

An interesting result for the lounge (fan convector) temperature. The top pairs of curves here are for the out and return heating circuit (5 degreeC per division vertical (zero on the base), and a 2 hour window horizontally). The yellow is the lounge temperature and the red just an on/off trace showing whether the GSHP is working. The outside temperature is very close to zero Centigrade, and has been for a day or two.

Note:-
- The GSHP runs on a fairly long cycle of some 35 minutes (this varies with conditions)
- The fluctuation in lounge temperature is in sync with, and has amplitude about proportional to, the heat circuit temperature (that is about 2 degrees p-p for the room at 20, and 5 p-p for the water at ~50).
- other rooms also fluctuate like this but a good deal less; but the radiators in these rooms have a large heat [hot water] capacity whereas the fan convector (even this big one!) holds very little water.
- the radiators are all balanced to give a substantial temperature drop (~10 degrees) in normal conditions
- the heating circuit is continuously pumped

So one might deduce that the room heating was in dynamic equilibrium with its heat losses with a time constant of less than 30 minutes. But in fact the house temperature varies rather slowly compared with outside conditions with time constants of days. One might wonder about placement of the sensors but mobile thermometers do much the same. So perhaps there are several time constants involved. Hard to know without shutting off the system and watching the temperature fall!

Certainly the rather long cycle time of the GSHP may be fine for ordinary radiators, and even more for underfloor, but it would be good if it were much shorter for fan convectors. I do not see how to achieve this! As a possible fudge one might re-arrange the circuit to put a conventional radiator or some kind of insulated tank in series with the fan convector as a reservoir - but this looks rather complex. As already noted the 'accumulator tank' does not mix significantly and does not provide any significant reservoir function.