How the two differ in kind
Start from the fact the marketing skips: in cooling mode a heat pump IS an air conditioner — same compressor, same coils, same physics, same efficiency class. You are not choosing between two cooling machines; you are choosing whether the machine should also run backwards. The reversing valve that lets it do so turns the outdoor unit into a heat source that moves heat indoors at multiples of the electricity it burns.
Winter is where the decision lives. A heat pump's capacity and efficiency both fall as outdoor temperature drops, exactly when the house needs more — the balance point where its output meets the house's demand decides how much backup you need. Modern cold-climate units have pushed that point impressively low, but the physics never disappears; the sizing calculator's job is to find YOUR balance point, not to assert one.
What already heats the house frames the economics. Replacing electric-resistance heating, a heat pump is close to arithmetic-guaranteed savings — it delivers the same heat for a fraction of the kilowatt-hours. Against cheap gas the answer is genuinely local: tariffs, climate and how long you will own the system decide, and anyone who claims a universal answer is selling one of the two boxes.
The factors that actually differ
Show
| Heat pump | Air conditioner (cooling-only) | |
|---|---|---|
| Cooling performance | Identical machine, identical job. | Identical machine, identical job. |
| Heating | Included: moves heat indoors at 2–4× the efficiency of resistance heat, falling with the temperature. | None — winter belongs entirely to whatever else you have. |
| Cold-climate behaviour | Capacity declines as demand rises; balance point + backup strategy are the design. | Sits idle; irrelevant. |
| Defrost reality | Outdoor coil ices in cold damp weather; defrost cycles cost some output and make steam-cloud theatrics. | None. |
| Runtime and wear | Works year-round — more hours, earn-back in displaced heating fuel. | Seasonal duty, seasonal wear. |
| Upfront cost | Modestly higher for the reversing capability and controls. | The cheaper box for the same cooling tonnage. |
| What the machine actually is | The same vapour-compression machine with a reversing valve, so in cooling mode it IS an air conditioner and performs like one. | The same machine without the valve. It cools and cannot heat. |
| Sizing conflict | Has to satisfy two loads that peak in different seasons and are often different sizes. Sized for the heating peak it is oversized for cooling; sized for cooling it needs supplementary heat. | One load, one peak, one sizing decision. |
| What happens below the balance point | Supplementary heat, sized from the GAP rather than the whole load — and the balance point is a deliberate design choice rather than a property of the machine. | Not applicable. |
| Defrost | In cold damp weather the outdoor coil ices and the machine reverses briefly to clear it, during which it is not heating. Normal, designed for, and the reason a seasonal figure is below a steady-state one. | Never runs in those conditions, so it does not arise. |
| Which efficiency number applies | Two: a cooling seasonal figure and a heating seasonal figure, and they are not interchangeable. A machine can be excellent at one and ordinary at the other. | One cooling figure, which makes comparison between units straightforward. |
| What it replaces | The cooling system AND the heating system, which is where the capital comparison actually lives. | The cooling system only. Heating remains a separate appliance with its own cost, flue and fuel. |
Which one, and when
Choose heat pump when…
- Anything currently heated by electric resistance — the savings are structural.
- Moderate winters, where the balance point sits comfortably below your normal lows.
- One system doing both seasons appeals more than maintaining two.
Choose air conditioner (cooling-only) when…
- Heating is already solved cheaply and you only need summer handled.
- Brutal winters where a heat pump would lean on backup exactly when running costs peak — unless you are specifying a true cold-climate unit deliberately.
- Minimum upfront cost for pure cooling capacity.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Do heat pumps work below freezing?
- Yes — modern cold-climate units deliver useful heat far below 0 °C, some rated to −25 °C and beyond. The honest caveats: capacity at those temperatures is a fraction of nameplate, efficiency falls with it, and defrost cycles nibble further. The design question is never 'does it work' but 'where is the balance point for THIS house, and what carries the load below it'.
- Is sizing the same for both?
- Cooling sizing is identical — same load calculation. The heat pump adds the heating side, and the two pull in different directions: oversizing for winter can leave summer cooling short-cycling. Cold-climate practice sizes near the heating load with variable-speed compressors absorbing the mismatch; the calculators here run both loads so you can see the tension rather than have it hidden.
- What about dual fuel?
- Often the adult answer in gas-heated cold climates: the heat pump handles the shoulder seasons and mild winter days at its efficient best, and the furnace takes over below the economic balance point. The controls do the switching. You keep the cheap deep-winter fuel and displace the easy majority of heating hours with moved heat.
- Does a heat pump cool as well as an air conditioner?
- Yes, because in cooling mode it is one. The refrigeration circuit is identical; a reversing valve swaps which coil is the evaporator and which is the condenser, and in summer that arrangement is exactly an air conditioner's. Cooling performance is therefore judged on the same seasonal efficiency figure and compared on the same basis. The only practical differences are that the heat pump has a slightly more complex outdoor unit and that its sizing may have been driven by the heating load rather than the cooling one — and an oversized cooling machine short-cycles and fails to dehumidify, which is the comfort complaint that follows.
- Can one machine really be right for both seasons?
- Only if the two loads are reasonably similar, and in most climates they are not. Sizing for the heating peak produces a machine oversized for cooling, which cycles and leaves the space cold and clammy because latent cooling needs run time. Sizing for the cooling peak produces a machine that cannot meet the heating load on the coldest days, which is what supplementary heat is for. Variable-capacity equipment narrows the conflict considerably by modulating across a wide range, which is why inverter-driven machines dominate where both seasons matter. Where the mismatch is severe, a dual-fuel arrangement — heat pump for the mild majority of the heating season, a fuel-burning stage for the tail — resolves it without compromising either.
- What is the balance point and why does it matter?
- It is the outdoor temperature at which the heat pump's declining output exactly meets the building's rising demand. Above it the machine carries the house alone; below it something has to cover the difference. Where that point sits is a design choice with consequences at both ends: set it high by undersizing and the backup runs for much of the winter at resistance-heating efficiency, which destroys the economics; set it low by oversizing and the machine cycles inefficiently for the other eleven months. Finding it means looking at how many hours the local climate actually spends at each temperature, not at the coldest night anybody remembers — and that distribution is the real design input.
- Why does the outdoor unit make odd noises in winter?
- Almost certainly defrost, which is a designed cycle rather than a fault. When the outdoor coil ices in cold damp weather the machine reverses briefly, sending hot refrigerant to the outdoor coil to melt the frost, and that transition involves a distinct change in sound, sometimes a whoosh of steam and a period when the indoor air is cooler. It is most frequent near freezing with high humidity — the conditions that put the most moisture on the coil — and much rarer in dry cold. A unit that seems to defrost constantly usually has an airflow problem at the outdoor coil rather than a control fault: drifted snow, leaves, or insufficient clearance around it.