How Does a Heat Pump Work, and What Does It Need From Your Heating?

An air source heat pump works like a fridge running in reverse. It uses a refrigerant and a compressor to take heat from the outside air, concentrate it, and pass it into the water that runs through your radiators or underfloor heating. It does not burn fuel, so there is no flame and no flue. For each unit of electricity it uses, a well-designed system delivers several units of heat, and it does that best when the water in the heating runs cooler than a gas boiler would make it. That last point is why a heat pump is a whole-system decision, not a like-for-like swap.

The Refrigerant Cycle, Step by Step

A heat pump moves heat rather than making it, using a fluid that boils at a very low temperature. The cycle has four stages and repeats continuously while the heating is on.

1. Evaporator. A fan draws outside air across a coil containing cold refrigerant. Even cold winter air holds heat, and because the refrigerant is colder still, it absorbs that heat and turns from liquid to gas.
2. Compressor. An electric compressor squeezes the gas. Compressing a gas raises its temperature, so the refrigerant leaves the compressor much hotter than it arrived.
3. Condenser. The hot refrigerant passes through a heat exchanger where it gives its heat to your heating water. As it cools it turns back into a liquid.
4. Expansion valve. The liquid passes through a valve that drops its pressure and temperature, ready to pick up heat from the air again.

The electricity runs the compressor, the fan and the pumps. The heat itself comes from the outside air, which is why the output can be several times the electrical input.

What Efficiency Figures Actually Mean

A heat pump's efficiency is expressed as how many units of heat it delivers for each unit of electricity, and that number changes with the weather and the water temperature. At any moment it is called the coefficient of performance, or COP. Averaged over a year it is the seasonal figure, SCOP, and that is the one that matters for running costs.

Two things push efficiency down:

• Colder outside air. There is less heat to collect, so the compressor works harder. Heat pumps keep working well below freezing, but they are less efficient on the coldest days.
• Hotter heating water. The bigger the gap between the outside air and the water temperature you ask for, the more work the compressor does. A heat pump sending water out at around 35°C to 45°C is far more efficient than one pushed to 55°C or more.

The first you cannot control. The second is decided largely by your radiators, your pipework and how much heat the house loses, which is where most of the design work sits.

Why Radiator Size Matters More Than With a Boiler

Because a heat pump runs cooler water, each radiator gives out less heat than it did on a gas boiler. A room that was comfortable on a boiler at high temperature can fall short once the water runs at 45°C.

The answer is not to turn the heat pump up, because that erodes the efficiency you paid for. The answer is to make sure each room has enough emitter area to be heated at a low flow temperature. That comes from a room-by-room heat loss calculation, compared against what each radiator can deliver. In most homes it shows a handful of rooms that need a bigger or double-panel radiator, not a whole-house replacement. Our guide to whether your radiators are big enough for a heat pump explains how that check works.

Underfloor heating suits heat pumps well because it covers a large area and naturally runs at low temperatures, but correctly sized radiators work too.

What the Rest of the System Needs

A heat pump relies on steady, even flow through clean pipes and a properly balanced system. Problems a boiler tolerates can cost a heat pump a lot of efficiency.

• Pipework. Heat pumps circulate water at a lower temperature difference between flow and return, which usually means a higher flow rate. Narrow microbore pipe, common in 1970s and 1980s London homes, can restrict that and may need upgrading on some runs.
• Clean water. Sludge in radiators and pipes blocks flow and fouls heat exchangers. A system with years of magnetite in it should be cleaned before a heat pump goes on it. See our power flushing service for how that is done.
• Balance. An unbalanced system leaves far rooms cool, and the usual reaction is to raise the flow temperature. Balancing properly lets every room reach temperature at the lowest setting.
• Controls. Heat pumps work best running steadily for long periods rather than blasting on and off, often with weather compensation that adjusts the water temperature to the outside temperature. Controls set up for a boiler may need changing.

Hot Water With a Heat Pump

A heat pump heats a stored hot water cylinder rather than providing instant hot water like a combi boiler. If you have a combi now, you will need space for a cylinder, usually in an airing cupboard or loft.

The cylinder has to be designed for a heat pump, with a large internal coil so it can take heat from lower-temperature water. Most setups also include a periodic higher-temperature cycle, often using an immersion heater, to control legionella bacteria. That is a normal part of the design, not a fault.

Air Source, Ground Source, Monobloc and Split

Most London homes that get a heat pump get an air source unit, because a ground source system needs space for trenches or boreholes. The two work on the same principle; ground source collects heat from the ground, which stays at a steadier temperature through the winter.

Air source units come in two main layouts. A monobloc keeps all the refrigerant inside the outdoor unit and sends heated water into the house. A split system runs refrigerant pipes to an indoor unit. Which is used depends on the property and the installer's design. Either way, the outdoor unit needs a spot with good airflow, and its position has to respect noise and planning rules; flats, listed buildings and conservation areas often need more checks.

Who Does What When You Switch

Fitting the heat pump and preparing the heating system are separate jobs. The heat pump itself should be designed and installed by an MCS-certified installer, which is also a condition of the government's Boiler Upgrade Scheme grant in England and Wales.

We work on the wet side of the system: surveying radiators and pipework, upsizing radiators where the heat loss calculation says a room will fall short, cleaning and treating the system water and rebalancing. If a heat pump is not a good fit for your home, because of space, heat loss or budget, we will tell you that plainly. Book a free survey and we will show you what your system would need before any heat pump goes on it.