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How Does an Air Source Heat Pump Work?
Imagine heating your home with energy pulled straight from the air outside — even on a frosty February morning. That’s exactly what an air-to-water heat pump does. It might sound like magic, but it’s just clever engineering that’s become one of the most popular low-carbon heating options across the UK.
This guide focuses specifically on air-to-water heat pumps, the type that captures warmth from the air outside and transfers it into water for your radiators, underfloor heating, and hot water taps.
In this guide, we’ll walk you through how it works—step by step. Whether you’re just starting your research or seriously considering a switch, this explainer provides a clear picture of what’s happening behind the scenes. By the end, you’ll see exactly how this tech fits into everyday life — and whether it’s the right move for you.
What Exactly Is Heat?
When we say “heat,” we don’t just mean “how warm something feels.” Scientifically, heat is energy moving from something warmer to something cooler.
Everything is made up of tiny particles, atoms and molecules, that are always moving. The faster they move, the more heat energy they contain. When you heat something, these particles jiggle and vibrate faster, so its temperature rises. The faster they move, the warmer they are.
Whenever two things at different temperatures meet, energy flows naturally from the warmer one to the cooler one until they’re both the same temperature — a process called thermal equilibrium.
Think of it like making a cup of tea: pour boiling water into a cold mug and the mug warms up while the tea cools down, until they meet somewhere in the middle.
An air‑to‑water heat pump uses this same principle — but with clever tech that lets it pull heat from chilly winter air and move it inside to warm your radiators, underfloor heating, and hot water.
Think of It as a Fridge in Reverse
If you’ve ever felt warm air blowing from the back of your fridge, you’ve already met the basic concept. A heat pump just runs that process backwards, bringing the heat inside instead of kicking it out.
How an Air‑to‑Water Heat Pump Works (Step‑by‑Step)
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Air Intake
The outdoor unit sits outside your home and has a fan that gently pulls in air from the environment — yes, even cold winter air contains usable heat energy. -
Heat Absorption
Inside the unit, the air passes over pipes containing a special refrigerant fluid. This fluid can absorb heat energy from the air even at very low temperatures. -
Heat Boosting (Compression)
The warmed refrigerant is then squeezed by a compressor. Compressing it raises its temperature dramatically, a bit like how a bike pump warms up when you use it. -
Heat Transfer
The hot refrigerant flows into a heat exchanger inside your home. Here, its heat energy is transferred to water in your central heating system. -
Distribution
This heated water is then sent through your radiators, underfloor heating, and hot water cylinder, ready for you to enjoy. -
Cycle Restarts
The refrigerant cools down, turns back into a liquid, and the whole process starts again.
Where the Warmth Goes in Your Home
Once the refrigerant has transferred its warmth to water, that hot water is pumped through your home’s heating system. The way it’s delivered depends on what setup your home uses:
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Radiators
Heat pumps work best with radiators designed for lower temperatures. Some older radiators may need to be upgraded. -
Underfloor Heating
Heat pumps pair beautifully with underfloor heating because both operate efficiently at lower temperatures. -
Hot Water Tank
A well-insulated cylinder designed for heat pumps stores water heated by the pump, so you have hot showers, taps, and baths when you need them.Most traditional heat pumps warm your hot water using a coil inside the water tank. Water flows through this coil and around the rest of the heating system. A sensor inside the tank checks the water temperature, and when it gets too cold, the heat pump turns on to warm up the coil again.
Each part of your home will benefit differently, and installers will design the system around your layout and lifestyle.
Key System Components
It’s useful to know what parts make up a typical heat pump system, especially what will be installed inside and outside your home:
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Outdoor Unit
This is the visible part that sits outside your house—on the ground or wall-mounted. It contains the fan, evaporator coil, and compressor. -
Refrigerant System
A sealed loop of pipes that carries the refrigerant through its heating and cooling cycle. -
Indoor Unit or Heat Exchanger
Often tucked into a cupboard or utility room, this unit transfers heat from the refrigerant into your home's water system. -
Hot Water Cylinder
If your system is set up for hot water, you’ll have a cylinder to store it, much like a traditional hot water tank. -
Thermostats and Smart Controls
These help you set room temperatures, heating schedules, and even control your system remotely via apps. -
Circulation Pump
Moves the hot water around your radiators or underfloor pipes efficiently and quietly.
How It Works in Cold Weather
One of the most surprising facts about air source heat pumps is how well they perform in low temperatures.
Even when it’s well below zero, the air still contains energy. Modern heat pumps can work efficiently in temperatures as low as -20°C. They also run an automatic defrost cycle to keep the outdoor unit clear of ice.
Modern systems also include:
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Defrost Mode
Automatically melts any frost or ice on the outdoor unit to keep things running smoothly.
You won’t need to do anything; these systems are built to adapt automatically and keep your home warm year-round.
Controls and How You Use A Heat Pump
Operating a heat pump is simple—and often more flexible than traditional heating.
Most systems include:
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Wall-Mounted Thermostats
Just like with boilers, you can set the desired temperature and heating schedules. -
Programmable Timers
Allow you to manage heating zones, boost hot water, or reduce heating overnight. -
Smart Controls
Many heat pumps now come with app-based control, allowing you to monitor or adjust settings from your phone—even when you’re away.
Everything is designed to help you stay in control and optimise energy use.
Ready to make your home warmer, greener, and future-proof?
Now you know how an air source heat pump works—not just the tech, but how it plays out in your home, day to day. From the outdoor unit quietly pulling in heat to the warm water flowing through your pipes, it’s a clean, smart way to heat your home.
Switch Together’s expert installers will handle everything — from system design to professional installation — with minimal disruption to your home. We’ll guide you through every step so you can enjoy reliable, efficient heating without the guesswork.
