How does an air source heat pump work? A beginner’s guide
Most people walk past them every day without noticing them. It’s just a simple box on the side of a house, quietly humming away. That’s an air source heat pump, and right now, it’s one of the most sensible heating upgrades a UK homeowner can make. Gas prices haven’t stabilised, the 2035 boiler phase-out is approaching, and the government grant offering up to £7,500 off installation won’t last forever. So, taking a few minutes to understand how this technology works is well worth your time. In this guide, we’ll cover the basics: what a heat pump does, how it performs in cold weather, what the efficiency numbers really mean, and what the installation process looks like in practice. What is an air source heat pump? At its core, an air source heat pump is a device that moves heat rather than creates it. It pulls warmth from the outside air and transfers it into your home to run your radiators and heat your water. The energy source is electricity, not gas or oil, which is part of why running costs tend to be lower. The comparison most engineers reach for is a refrigerator in reverse. A fridge extracts heat from inside the cabinet and dumps it into your kitchen. A heat pump does the opposite: it pulls heat from the air outside your house and brings it in. Even on a cold January morning, there is usable heat energy in the air. The pump finds it and concentrates it. From a homeowner’s point of view, not much changes day to day. The system connects to existing radiators and a hot water cylinder. You set a temperature. The house heats up. The difference is what is happening behind the scenes. How it works, step by step The process runs in a loop. Once it starts, it keeps going without much input. Step 1: Air is drawn in A fan inside the outdoor unit pulls air across metal fins, similar in principle to the grille at the front of a car. Cold air still holds heat energy. Not much, but enough. The unit is built to extract it. Step 2: The heat moves into a refrigerant The extracted warmth transfers into a refrigerant fluid circulating through the system. This refrigerant has a very low boiling point, so even a modest rise in temperature turns it from liquid into gas. That change of state is what the system relies on. Step 3: The compressor does the heavy lifting The gas gets compressed. This is where the real temperature increase happens. Compressing a gas concentrates its energy and raises its temperature sharply. If you have ever noticed a bicycle pump getting warm during use, the principle is identical. At this stage the refrigerant is hot enough to do useful work. Step 4: The heat transfers into your home The hot gas passes through a heat exchanger, which moves its warmth into the water circulating through your radiators and stored in your hot water cylinder. The heat ends up exactly where you need it. Step 5: Reset and repeat Once the refrigerant has released its heat, it cools back into a liquid and the cycle starts again. The system runs quietly, continuously, with no combustion taking place anywhere in the process. What about cold weather? This is where most people’s scepticism sits, and it is understandable. The idea of extracting useful heat from freezing air sounds counterintuitive. The Samsung units that Wunergy installs are rated to operate at temperatures down to -25 degrees Celsius. The UK very rarely drops below -10. So British winters, even the bad ones, are well inside the working range of these systems. Scandinavia is the practical proof. Norway, Sweden and Finland have colder winters than anything we see here, and they lead Europe in heat pump adoption. This technology was not designed around mild British weather. It was designed for places where winter is genuinely difficult. Performance does ease off a little in extreme cold. At -10 degrees, a good ASHP produces roughly 2.5 units of heat per unit of electricity. A modern gas boiler at peak efficiency converts about 90 to 95 percent of its fuel into heat. The heat pump is still considerably more efficient even at its winter low. COP: the number that explains the savings COP stands for Coefficient of Performance. It is a ratio: heat output divided by electricity input. A gas boiler with 90 percent efficiency has a COP of 0.9. It uses one unit of energy and produces 0.9 units of heat. An air source heat pump with a COP of 3.5 uses one unit of electricity and produces 3.5 units of heat. That gap is why running costs are lower, even accounting for electricity being more expensive per unit than gas. The units Wunergy installs carry an ErP A+++ efficiency rating. That is the highest band available, and it is the rating that confirms eligibility for the Boiler Upgrade Scheme grant. What does an installation involve? A full air source heat pump installation from Wunergy covers everything the system needs to work properly: Most jobs run over two to three days. The Wunergy team handles the whole process from the initial site visit through to final testing and handover. Why the switch makes sense right now Is your home a good candidate? Most UK properties work fine. There are three things worth checking before anything else. If you are not sure whether your property is suitable, Wunergy carries out free surveys across Birmingham, Worcester, Bromsgrove, Redditch, Stratford-upon-Avon and the surrounding areas. You get an honest assessment before any decisions are made. Questions we get asked regularly Can it replace a boiler completely? Yes, in full. The system handles space heating and hot water together, so the boiler comes out and the gas supply can be capped. Nothing needs to stay. My electricity bill, will it go up? It will rise. But the heat output per unit of electricity is three to