Air Source Heat Pumps: How Are They More Than 100% Efficient?

What does 400% efficiency actually mean?

It means the heat delivered is four times the electricity used over the same period. A heat pump using 1kWh of electricity at a COP of 4 delivers 4kWh of heat. We are comparing useful heat with purchased electricity, not claiming to create four times as much energy from nowhere.

A direct electric heater converts electricity into heat. A heat pump uses electricity to operate a compressor, fan, circulation pump and controls, transferring heat that is already in the environment.

That is why the output can be larger than the electrical input. The outside air is part of the process. Nothing is being created from nothing.

These heat-pump rows explain the ratios, not a guaranteed result from a particular installation. A kWh is a unit of energy, not a temperature or the size of the heat pump.

The Energy Saving Trust's explanation of heat-pump efficiency uses the same comparison between electricity consumed and heat delivered.

Your freezer is already doing this

People often describe a heat pump to me as a fridge in reverse. I prefer the freezer example, and I do not call it “in reverse”. It is the same process of transferring heat.

Your freezer takes heat from its cold interior and releases it into the room. That is why the outside of a freezer can feel warm while the food inside is frozen. The refrigerant circuit moves the heat; the electricity runs the equipment.

For home heating, we want the warm side of that process. An air source heat pump collects heat outdoors and transfers it into the heating system. It does not need to blow the outside air into your rooms.

But where is the heat when it is freezing outside?

Zero degrees is important to water. It is not a cut-off point for heat.

Refrigerant can boil at very low temperatures, and its boiling temperature changes with pressure. Inside the outdoor coil, it can be colder than the air around it. Heat moves from that air into the refrigerant, even when the air feels freezing to us.

The compressor then raises the refrigerant's pressure and temperature so it can give up heat to the heating water. My guide to how a heat pump works follows the full cycle and explains the components.

Why the same heat pump is not always 400% efficient

Flow temperature makes a big difference. This is the temperature of the water leaving the heat pump for the heating system. Lower flow temperature improves efficiency, provided the radiators or underfloor heating can still deliver the heat each room needs.

Asking for hotter water means a bigger temperature lift. Outdoor temperature matters too: the lift is harder when the source is colder. So the same equipment can perform differently on a mild afternoon and a cold winter morning.

For a simple illustration, delivering 12kWh of heat at COP 4 takes 3kWh of electricity. At COP 2 it takes 6kWh. That is twice the electricity for the same heat, but it is not a claim that a particular temperature adjustment will halve your efficiency.

Hot water is a different job from heating the rooms

Heating a cylinder commonly requires hotter flow water than warming the house. That can mean lower efficiency during a hot-water cycle. How much this affects the annual result depends on household hot-water use and how much heat the building needs.

I would not judge the whole installation from one cylinder reheat or a screenshot taken on a particularly good afternoon. Ask whether the quoted figure covers heating only or heating and hot water, and whether it is an estimate, a measured result or a product test.

The COP, SCOP and seasonal efficiency guide explains those differences. For this article, the main point is simple: 400% is a useful way to explain performance, not a promise attached to every hour of operation.

Does using a quarter of the energy mean a quarter of the bill?

Not when the fuels have different prices. A heat pump operating at 400% uses a quarter as much purchased energy as direct electric heating for the same heat. Against gas, we also need to allow for the boiler's efficiency and the price difference between electricity and gas.

I have used 25p per kWh for electricity and 8p per kWh for gas in this example. These are illustrative prices, not today's tariff quotes.

The heat pump uses much less purchased energy in this example. Its cost is a quarter of the direct-electric cost, but not a quarter of the gas cost.

Change the tariff or the system's efficiency and the costs change. Standing charges are separate, as are installation and maintenance costs. My heat-pump running costs comparison gives the fuller workings and dated price examples.

Good efficiency starts with the installation design

For me, heat pumps are the best way to heat homes because they can deliver the warmth we need using much less purchased energy. But choosing a heat pump is not the same as getting a well-designed heating system.

I look at heat loss, radiator sizes and the proposed flow temperature. Then I want to know what changes will make a worthwhile difference.

That does not mean chasing the lowest possible flow temperature regardless of cost. If three carefully chosen radiator replacements give a good result, there needs to be a worthwhile benefit to spending thousands replacing the rest.

This is what interests me about Heat Geek Zero Disrupt: finding a good balance between efficiency, installation cost and the work needed in the home. We still have to deliver the agreed comfort. A cold bedroom is not a saving.

Why I recommend the Heat Geek installer network

My recommendation is to start with Heat Geek. Its approach puts heating design and system performance at the centre of the installation. That is more useful to me than choosing equipment from a headline efficiency number.

For installations covered by the scheme, the Heat Geek Guarantee backs the agreed overall heating and hot-water efficiency, subject to the installation terms. It is not a blanket promise that every system will achieve 400%, or that it will achieve its seasonal figure every minute.

Ask to see the guarantee for your proposal. My explanation of what a Heat Geek is covers the training and how we work through the network.

Want to know what an ASHP could achieve in your home?

At Vector Heat, we survey, design and install heat-pump systems across Lincolnshire. I can explain the proposed water temperature, which radiators need attention and the expected performance before you decide.

Our heat-pump installation service