Homeowners are increasingly getting heat pumps installed.

The main benefits are clear:

  • Cost and energy savings
  • Consistent temperatures
  • Lower carbon emissions.

But how exactly do heat pumps work?

Read on for a simple, concise overview.

How heat pumps work

Heat pumps ultimately work by transforming low-grade heat from the environment into high-grade heat.

This transformation is done via gas compression.

Inside a heat pump is a special fluid called a refrigerant. When this is compressed — in much the same way air is compressed in a bike pump — it heats up.

Unlike in a bike pump, where this heat simply dissipates, a heat pump captures it.

A component called a heat exchanger transfers this heat into water by passing the hot refrigerant past a separate water circuit (without the two fluids ever coming directly into contact).

This water then carries heat to radiators, underfloor heating, and hot water.

To repeat the process, an expansion valve releases the pressure on the refrigerant, causing it to cool sharply.

It (the refrigerant) then passes through an outdoor heat exchanger, where it absorbs heat from the outside air — even in cold temperatures — before being compressed again. The cycle then repeats.

This process is one of the most eco-friendly and efficient ways to heat a building.

The fridge analogy

Heat pumps are commonly described as operating in the same way as a fridge but in reverse.

This analogy oversimplifies the similarity, as the technology used in both cases is not identical. However, it can be useful for explaining the technology to non-experts.

Fridges cool their internal space down. But their back outside panels turn warm.

So, fridges cool one space down whilst warming another space up. Heat pumps follow the same thermodynamic principle to create the opposite effect.

The simple physics behind the process

Air, soil and rock all absorb energy from the sun, which gets stored as thermal energy. This applies even at very low temperatures like −10°C or −20°C.

At its smallest scale, thermal energy is simply molecules in constant motion. The faster they move on average, the more heat a substance contains.

When a gas is compressed, those molecules are forced into a smaller space.

As they collide with the walls of that shrinking space, each collision gives them a small speed boost. It’s just like a tennis ball gaining speed when hit by a swinging bat rather than bouncing off a stationary wall.

The molecules speed up and the gas heats up.

This is exactly what the compressor inside a heat pump does to the refrigerant: it forces the molecules closer together, converting mechanical energy into heat.

Types of heat pump

There are three main types of heat pump.

Each works in essentially the same way, but with slightly different means.

Ground source heat pumps

Ground source systems extract heat from the soil or rock just below the earth’s surface. A network of pipes — called a ground loop — is buried in shallow trenches or in deep vertical boreholes (if space is limited).

A water and antifreeze mixture is pumped through these pipes, absorbing heat from the ground as it circulates. It then carries that heat back into the pump, where it’s transferred to the refrigerant to begin the compression cycle.

The big advantage of ground source heat pumps is stability and consistency.

The surface just below the ground generally stays at a fairly stable temperature all year round. This is around 10–12°C in the UK, regardless of what’s happening above it.

Air source heat pumps

Air source systems are the most common type you’ll come across.

A fan draws outside air across a large metal heat exchanger which contains cold refrigerant. Heat moves from the air into the refrigerant, even on a cold day.

This works because even air at −10°C still contains thermal energy. Modern air source heat pumps can operate efficiently down to as low as around −20°C.

Air source heat pumps use weather compensation curves to control for the range in outside temperatures they operate in.

Read here for a detailed breakdown on the differences between ground and air source heat pumps.

Hybrid heat pumps

Hybrid heat pump systems combine an air source heat pump with a traditional gas boiler in a single system. The two work together, with the system automatically switching between them depending on conditions.

For example, in mild weather, the heat pump does most of the work. When temperatures drop sharply and the heat pump has to work harder, the gas boiler steps in to help out.

Hybrids are a practical option for homes that can’t host a full heat pump switch, but want to cut their energy use in the meantime.

Water source heat pumps

Water source systems work on the same basic principle as ground source. However, instead of using pipes buried in the ground, they use pipes submerged in a nearby body of water (lake, river, pond, etc.).

Water is excellent at storing heat. A lake or river holds onto its thermal energy far more consistently than the air above it, making water source systems among the most efficient heat pumps available.

There’s just one catch… You need to be near a suitable water source, which makes them less common.

But for homeowners lucky enough to have such a location, they can be a highly effective choice.

IMS Heat Pumps

We have over 20 years experience of installing air source and ground source heat pumps in domestic and smaller commercial properties all over the UK.

Our engineers can explain exactly how a heat pump works, and what you need to do to maintain its performance for the long-term.

The design and installation of a heat pump can all be handled by us, working with key project stakeholders such as architects, project managers, and builders.

For more information about how we can help you, contact us today.