How cold it gets under the ground, when ground source heatpump sucks out all the heat? Will the ground freeze?
Probably the answer is ‘depends’. I have heard anectotal evidence from ground source heatpump installers, that they had to dig out too short pipes from underground, during summer, where it was just a big frozen lump. So probably it depends on the pipe length. The temperature underground can be measured, but I do not have any sensors buried there. All I can see is incoming liquid temperature, called brine in and outgoing liquid temperature, called brine out.
For information, I do have 12 kw heatpump, 800 meters of underground pipes, around 120 cm under ground and the ground down there described as moraine (sandy, some clay and small rocks).
I have manually collected some temperatures so here is a table. No permafrost yet. I would say, that if Brine In gets into negative, ground is frozen.
| Date | Brine In Temperature C | Notes |
| 7.9.2023 | 15 | |
| 15.9.2023 | 14.9 | |
| 6.10.2023 | 13.1 | |
| 10.10.2023 | 11.3 | Heatpump did work all night |
| 22.10.2023 | 8.5 | Constant heating, usually gear 1, up to gear 3, sometimes stops. |
| 29.10.2023 | 6.68 | Constant heating with no stops. |
| 10.11.2023 | 7.01 | Warmer outside (+7 C) gear 1 heating or off |
| 18.11.2023 | 5.81 | Outside is below freezing (-6 C). Heating gear 1-5 |
| 21.11.2023 | 4.9 | Gear 21 desired, but heatpump has maximum gear of 10. Electricity price peaked, so comfort wheel was set lower and after electricity price normalized it is heating as mutch as it can. |
| 22.11.2023 | 3.7 | Outside is -7 C, was -9 C at some point. Brine Out is getting close to 0 C |
| 1.12.2023 | 2.98 | Outside is -2 C. A lot of snow. |
| 3.12.2023 | 2.83 | Outside is -5 C, Brine Out is -0.37 C Snow. Constant heating |
An image of how heatpump changes gears and keeps room temperature almost constant. And how brine temperature almost does not move at all. The peak in room temperature is an error, but illustrates nicely, that brine temperature is not usable metric, when the pump is not working.
Another image showing how Brine Out is indeed moving when heatpump switches gears.