If the designed output of the system matches the energy required at a particular temperature, then the system will run flat out if it is told do do so. Another issue with a system designed that closely to match supply and load is that the house will not get back up to temperature if it is allowed to cool.
If the energy requirement exceeds the supply the house will cool down. However, long periods of very low temperatures are not that common in the UK. Unless they are very poorly insulated, buildings don’t cool down that quickly and most people can cope with internal temperatures a few degrees below normal for a few days. If it’s really going to be a problem you massively over size the heat pump.
Most systems are designed with an amount of extra capacity to allow for contingencies. This is exactly the same as any other heating system, but it’s generally not manifest as boilers are often massively over sized and the output doesn’t drop with temperature.
Many heat pumps list the CoP at various outside temperatures/flow temperatures in their literature. For instance these are Vaillant Arotherm plus heat pump CoPs at 35C flow temperature (one of the better ASHPs at low temperatures).
Vaillant Arotherm+
| Outside temp | 5kw | 7kw | 10kw | 12kw |
|---|---|---|---|---|
| -3 | 3.7 | 3.8 | 4 | 4.2 |
| 2 | 4 | 4.3 | 4.6 | 4.7 |
| 7 | 4.6 | 4.9 | 5.3 | 5.5 |
| 10 | 5.3 | 5.6 | 5.7 | 5.9 |
A CoP of 5 at 3C OAT sounds remarkably optimistic.
Another issue is that of frosting. The humidity here has been approaching 100% for the last 2 months so defrosts have been taking a big toll on performance.