As outdoor air gets colder, the evaporator has less heat to pick up. Capacity falls and COP falls. Below about 5 °C with humid air, ice forms on the coil. The unit must defrost, usually by briefly reversing the cycle.

Design rules for cold sites:

• Size at the local design low, not at the rated 7 °C point.
• Choose units built for low ambient (for example, vapour injection or CO₂ cycles).
• Protect water circuits with glycol (antifreeze) or drain-down. About 30% ethylene glycol protects to roughly −15 °C; about 40% to roughly −24 °C (typical).
• Glycol lowers heat transfer and raises pumping power; allow 5–10% derating (typical).
• Keep snow and ice away from the coil and condensate drains.

At altitude the air is thinner. At about 3,500 m air density is roughly 70% of sea level. A fan moves the same volume but less mass, so coil capacity falls. Water boils at about 88–89 °C at that height, so keep vent and relief settings in mind.

Worked example — derating at Leh

A heat pump gives 20 kW at 7 °C ambient at sea level. At Leh, design ambient is −15 °C. Use capacity factor 0.62 (from the chart), altitude factor 0.90 and glycol factor 0.95 (typical assumptions; check maker data).

• P = 20 × 0.62 × 0.90 × 0.95 = 10.6 kW
• A 20 kW-rated unit gives only about 10.6 kW at Leh. Size accordingly.

Remember

• Capacity and COP both fall as ambient falls.
• Defrost cycles take a share of every cold hour.
• Altitude reduces air mass flow and boiling point.
• Always protect water circuits from freezing.