Ground-Source vs Air-Source Heat Pumps in Extreme Cold

Same cycle, different source

Nothing inside the two machines disagrees. Both evaporate a refrigerant at low pressure to collect heat, compress it, condense it indoors to release that heat, and expand it back — the cycle set out in how geothermal heat pumps work.

The difference is entirely in what they collect from. One is connected to the air, which in a cold climate can swing 40 °C (72 °F) across a year and 15 °C (27 °F) across a day. The other is connected to soil a few metres down, which moves by a few degrees across a whole season.

The divergence that defines a cold-climate design

Here is the awkward geometry of air-source heating. As the outdoor air gets colder, two things happen at once, and they pull in opposite directions.

The building wants more heat. Heat loss is roughly proportional to the difference between inside and outside, so demand rises in a straight line as the temperature falls.

The machine can deliver less. A colder evaporator means lower refrigerant pressure and density, so the compressor moves less refrigerant mass per revolution and the machine's heating capacity falls. Meanwhile the temperature lift has grown, so each unit of heat costs more electricity.

The temperature at which the two lines cross is the balance point. Above it the heat pump covers the load on its own. Below it there is a shortfall, and something has to fill it.

A ground-source machine does not escape the physics — its lift grows as the delivery temperature rises and its loop cools — but its source line is nearly flat, so its capacity line is nearly flat too. The crossing either happens far below any temperature the site sees, or never.

What the cold does to an air-source machine, step by step

1. Lower suction pressure. Colder air means a colder evaporator and a lower-pressure, less dense refrigerant vapour. The compressor is a volume pump, so less mass flows per revolution.
2. Higher compression ratio. The gap between evaporating and condensing pressure widens, which lowers compressor efficiency and raises discharge temperature.
3. Lower COP. The lift has grown, so more electricity is required per unit of heat delivered — the relationship laid out in the COP table in the geothermal article.
4. Defrost, in the wrong conditions. Between roughly −5 °C and +5 °C (23 °F to 41 °F) with damp air, moisture freezes onto the outdoor coil. Ice insulates it and blocks airflow, so the machine periodically reverses the cycle to melt it off — taking heat *from* the building, and pausing heating while it does.

Defrost is the reason a damp winter at 0 °C can be harder on an air-source machine than a dry one at −10 °C. A ground loop has no equivalent: there is nothing to frost.

Below the balance point: what fills the gap

Three answers, and most cold-climate systems use a combination.

• Electric resistance elements. Simple, cheap to fit, and with a coefficient of performance of exactly 1 by definition. Every kilowatt-hour they consume delivers one kilowatt-hour of heat, against three or four from the heat pump — which is why a system that quietly runs on resistance heat for weeks looks like a heat pump that has failed, even though it is still warm inside.
• A second heat source. A boiler or stove sized for the extremes, with the heat pump doing the bulk of the season's work.
• A bigger heat pump. Moves the balance point lower, at the cost of a machine oversized for the mild majority of the season, which then short-cycles when lightly loaded.

The design question is not "will it need back-up?" but "how many hours a year will back-up run, and how much of the season's energy is that?" A machine that meets 95% of the annual load and leans on an element for a handful of nights is generally a better design than one sized for the worst hour.

What cold-climate machines changed

Air-source performance in deep cold is not what it was, and two developments did most of the work.

Vapour injection. An extra port part-way through compression admits a second, intermediate-pressure stream of refrigerant tapped through an economiser. This cools the compressor internally, allows a higher compression ratio without excessive discharge temperature, and raises the mass flow reaching the condenser — which is precisely the quantity the cold was taking away.

Variable-speed compressors. Rather than one fixed output switched on and off, the compressor modulates. In mild weather it runs slowly and continuously — more efficient than short cycling — and in deep cold it runs hard, so nameplate capacity is available where it is needed rather than averaged away.

The result is machines rated with real capacity at low outdoor temperatures. The gap to ground-source narrows; it does not vanish, because the source is still air.

The questions that actually decide it

Three questions decide most real cases. How cold is the design temperature, and for how many hours a year? What delivery temperature does the building need — because a low-temperature emitter system flatters both machines and rescues neither from a badly chosen balance point? And is there ground to work with, in area, access and geology?

Where the answers point to ground-source, the follow-up is which loop geometry suits the site, which is the subject of closed-loop versus open-loop systems. Where they point to air-source, the thing worth understanding before the first cold snap is what normal defrost behaviour looks like, so that it is not mistaken for a machine freezing up.