A heat pump is one of those machines that sounds like it should not work: it heats a house in January by pulling heat out of the freezing air outside. It does, and the principle is the same one your refrigerator uses every day. This guide explains how it works in plain language, and what that means for a house around Hamilton.
Moving heat instead of making it
A furnace makes heat by burning fuel. An electric baseboard makes heat by running current through a resistance wire. A heat pump does neither. It moves heat that already exists from one place to another, using a refrigerant that absorbs heat when it evaporates and releases heat when it condenses. Moving heat takes much less energy than making it, which is why a heat pump can deliver several units of heat for every unit of electricity it uses.
Your fridge is a heat pump. It pulls heat out of the food compartment and dumps it into the kitchen through the coils at the back. A central air conditioner does the same with your whole house in summer. A heat pump simply adds a valve that lets it run in either direction.
The cycle, step by step
- Evaporation: cold, low-pressure liquid refrigerant flows through a coil. Even if the air around the coil is −15 °C, the refrigerant is colder still, so heat flows from the air into it and it boils into a gas.
- Compression: the compressor squeezes that gas, which raises its pressure and its temperature sharply, to well above room temperature.
- Condensation: the hot gas flows through a second coil. Air blown across the coil picks up the heat, and the refrigerant cools back into a liquid.
- Expansion: the liquid passes through an expansion valve, which drops its pressure and temperature, and the loop starts again.
In winter, the outdoor coil is the evaporator and the indoor coil is the condenser, so heat moves into the house. In summer a reversing valve swaps their roles and heat moves out, exactly as an air conditioner does.
How there is heat in cold air
Cold air still holds heat energy. There is heat in air at −20 °C, just less than at 10 °C. As long as the refrigerant in the outdoor coil is colder than the outdoor air, heat flows into it. Cold-climate heat pumps use refrigerants and variable-speed compressors designed to keep that process working at low outdoor temperatures. They produce less heat and use more electricity per unit of heat as it gets colder, but they keep working.
COP: how efficient it is
Efficiency is measured as the coefficient of performance (COP): the heat delivered divided by the electricity used. A COP of 3 means three units of heat for each unit of electricity. Electric baseboard has a COP of 1. A cold-climate heat pump might run at a COP of 3 to 4 on a mild 5 °C day, around 2 at −15 °C, and closer to 1.5 at −25 °C. Over a whole Hamilton heating season, a seasonal average around 2.5 to 3 is realistic for a good cold-climate unit.
That falling COP is why heat pumps in Ontario are paired with backup heat. On the very coldest hours, a gas furnace in a hybrid system or an electric heater in the air handler tops up the heat pump.
Defrost cycles
When it is around freezing and humid, frost forms on the outdoor coil, the same way it forms on a cold drink. To clear it, the heat pump briefly reverses into cooling mode, sending warm refrigerant to the outdoor coil to melt the frost. You may see steam rising from the unit and hear a whoosh. That is normal. During defrost, the backup heat may come on briefly so the house does not get a blast of cool air. Good installers raise the outdoor unit off the ground so the meltwater can drain away and does not freeze into ice under it.
Types of heat pump
- Ducted air-source: the outdoor unit connects to a coil on the furnace or an air handler and uses the existing ducts. The most common type in Hamilton-area homes.
- Ductless mini-split: indoor heads on walls or ceilings, for homes without ducts. See ductless mini-splits.
- Ground-source (geothermal): loops buried in the ground or a well supply the heat. Very efficient and steady, but costly to install. Better suited to rural lots than city ones.
- Air-to-water: feeds a hydronic system such as radiators or in-floor heat. Less common in Ontario so far.
Variable-speed compressors
Older heat pumps had single-speed compressors that switched fully on and off. Modern cold-climate units use inverter-driven variable-speed compressors that slow down on mild days and ramp up when it is cold. That is more efficient, quieter and gives steadier temperatures. It is also why heat pumps prefer a steady thermostat setting to deep setbacks.
What a heat pump feels like
The air from a heat pump’s registers is warm, often 30 to 40 °C, rather than the hotter blast from a gas furnace. It runs longer, gentler cycles. Most people find the even temperature comfortable; a few miss the hot air. In summer, it cools exactly like central air.
Is it right for your house?
That depends mostly on what you heat with now and whether your AC needs replacing. Read heat pump vs furnace for the comparison and heat pump rebates for what is available. When you are ready, send one request for a heat pump quote.
Questions people ask
How does a heat pump heat when it is freezing outside?
Its refrigerant is colder than the outdoor air, so heat still flows into it. The compressor then raises that heat to a useful temperature and releases it indoors.
Why is steam coming from my heat pump in winter?
It is in a defrost cycle, melting frost off the outdoor coil. This is normal and usually lasts a few minutes.
Does a heat pump need backup heat in Ontario?
Yes. On the coldest hours, output and efficiency drop, so systems are designed with a gas furnace or an electric heater as backup.
Is a heat pump the same as an air conditioner?
In summer, yes: it cools the same way. The difference is a reversing valve that lets it also heat in winter.