It looks as if the problem with the ground source heat pump has finally been fixed (although it would appear that there was no problem to be fixed). It is going to take a few days for the house to get up to temperature, but at least it is a start.
The heat pump has been throwing LP (Low Pressure) alerts for the past couple of weeks. The ground loops were flushed. But there was no air in the system. All of the strainers on the ground loops were checked. Again, no problem. The circulation pump for the ground loops was replaced. But the same problem persisted. The software on the heat pump was updated. No difference.
The Nibe engineer spent 5 hours on site and finally struck on the idea that the temperature of internal underfloor heating was too low. Apparently, there needs to be a significant temperature difference between the ground loops and the underfloor heating otherwise the low temperature in the underfloor heating will reduce the temperature (and thus the pressure) on the ground loops.
The answer appears to be to use the internal immersion heater to raise the temperature of the underfloor heating. Then the ground source heat pump could be engaged and, voila, it all works as it should.
We restarted the heat pump about 6 hours ago (around 4pm this afternoon) after engaging the immersion heater for a while. The good news is that it is still running (and generating heat). Fingers crossed.
Here’s the graph for the internal temperature from our weather station. You can see where the heat pump was turned on around 4pm. Hopefully, the graph overnight will look just as good. Fingers crossed.