Well last night was fairly cold (-2.5C at 2m above ground) and the heat pump started returning an intermittent (self-resetting) error condition. This is because the return temperature of the heating fluid into the heat pump from the mixing 'accumulator tank' is too high. When this cools the HP restarts. Indeed the temperatures are quite high (I have now added monitoring sensors) and the mixing in the accumulator tank means that the GSHP sees higher temperatures than the house heating loop (for this reason I have never been so keen on this tank!).
The system suggests backing off the 'slope' to reduce this, or that the circulation over the heat pump is greater than in the house circuit (which must be true if the GSHP's circulating pump is similar). I have backed off the slope to 5.5 and the sensitivity to house temperature to 8. We shall see - tonight should be as cold.
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