Ambient heat: Fluctuation in time
Coldest air exactly when heat is needed most
What this tab covers How availability changes over seconds, days, seasons, years and longer, how predictable it is, and what that means for the plants that use it.The ground and large water bodies are steady sources; outdoor air swings daily and seasonally and is coldest exactly when heat is most needed, which lowers performance on the coldest days.
Variation by time scale
| Time scale | What happens | How large |
|---|---|---|
| Daily and weather | Air temperature changes through the day and with cold spells; heat pump efficiency falls as the air cools.7 | UK median air-source efficiency 2.44 on days with mean temperature as low as -6 °C, vs 2.80 over the year |
| Seasonal | Heat demand peaks in winter when air is coldest, so the source and the need are out of phase.8 | Field data from seven studies: typically two to three times as efficient as fossil boilers well below 0 °C, more than twice at -10 °C; efficiency (COP) still above 1.5 near -30 °C |
| Seasonal (ground) | Shallow ground stays close to its annual mean temperature, so ground-source units hold their efficiency.2 | 10-16 °C in the upper 3 m |
| Decades | Equipment and ground loops are long-lived.3 | Heat pump 20+ years; underground loops 25-50 years |
How predictable it is
High: performance follows outdoor temperature, which is forecast days ahead; the challenge is the peak electricity demand that cold snaps create, not unpredictability.
Capacity factors and utilisation
| Plant or measure | Value |
|---|---|
| Seasonal performance factor, air-source heat pumps (UK field trial, median, 742 heat pumps) | 2.807 |
| Ground-source heat pump efficiency (US DOE) | 300-600%3 |
Ambient heat never runs out, but its quality changes with the weather. For air-source heat pumps, which are about 85% of heat pumps in buildings (REN21), every degree of outdoor cooling widens the temperature lift and lowers the heat delivered per unit of electricity, just as demand rises. Real-world data show the effect is gradual: a large UK field trial measured a median of 2.80 units of heat per unit of electricity over the year and 2.44 on days averaging -6 °C.
Ground and water sources avoid most of this swing because a few metres down the temperature barely changes through the year, which is why ground-source systems can use 25-50% less energy than air-source ones (US DOE).
Sources
- Geothermal Heat Pump Basics NREL
- Guide to Geothermal Heat Pumps US Department of Energy
- Heat pumps shown to be three times more efficient than gas boilers (Electrification of Heat demonstration) Energy Systems Catapult, 2023
- Coming in from the cold: Heat pump efficiency at low temperatures (Gibb, Rosenow, Lowes and Hewitt, Joule 7) Cell Press, 2023