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How efficient are air source heat pumps?

Air source heat pumps are one of the most efficient heating systems ever invented, but how efficient are they and how can they be so efficient?

If you’re looking to learn exactly how heat pumps work, head to our dedicated knowledge hub page, but read on to find out about air source heat pump efficiency (as well as how to get the most out of your system!).

Heat pumps don’t create energy

Let’s look at how most homes are currently heated.

The overwhelming majority of UK homes have water-based central heating systems with radiators, most commonly powered by gas or (for more rural homes) oil boilers. Regardless of the fuel used, a fuel-based boiler gets its energy by burning that fuel. Without trying to get too technical: a volume of gas is ignited and a certain amount of energy is released.

Crucially, you can’t ever release more energy than is stored in that volume of gas. In other words, if you ignite some wood in a fire, you can’t get more energy than you have with that wood without adding more wood.

You burn the fuel, you get heat, the fuel is gone, no more energy!

So a gas boiler, oil boiler, or other fuel-based system running at a theoretical (but actually impossible) maximum efficiency can only be, at most, 100% efficient – more than 100% efficiency here violates the law of the conservation of energy. Remember, you can only get energy out that is already stored in the fuel.

While modern gas boilers can sometimes operate really efficiently – sometimes as high as 95% efficiency – that still means that super-efficient boilers still waste around 5% of your energy every time you turn on the heating or hot water. In practice, most gas boilers in UK homes operate at much below 95% efficiency.

Air source heat pumps can be more than 300% efficient

Yet, as you may have seen elsewhere on our site, air source heat pumps are usually expected to run at over 300% efficiency – with many (installed by people who really know what they’re doing) reaching upwards of 400% efficiency!

What this means is that for every 1kW of energy you use, you can get between 3-4kW of heat.

At this point, we’re usually met with a lot of raised eyebrows because it sounds impossible, it sounds too good to be true.

But this isn’t a pitch for snake oil or perpetual motion machines – the key difference is that heat pumps don’t create energy, they simply move energy.

Heat pumps move energy from one place to another

Rather than burning a material in order to extract the energy, heat pumps tap into sources of heat in order to move it. This source of heat is either the ground, underground aquifers, or the air. In short, they work in a clever way that benefits from a difference in temperature rather than a consistent temperature.

So ground source (which isn’t to be confused with geothermal heating) seems like a great, consistent option, but actually has a lot of drawbacks due to the relatively stable temperature of the ground. This is because the law of the conservation of energy still applies here: if you’re drawing energy from a source, you need to make sure that the source can replenish that energy if you want to keep drawing from it.

Think about it this way, covering up a cold turkey doesn’t heat it up, but blowing air over a hot turkey cools it down. Energy is being moved by the movement of air or insulated by the sealing-off from air (and a few other bits of physics).

This is actually a key problem with ground source heating, because the ground insulates – that is, it doesn’t transmit heat very well – it’s great for keeping things at a consistent temperature but doesn’t regain lost heat very quickly. So ground source heat pumps can actually cause the soil around the borehole to freeze in certain climates, which renders the heat pump useless until the ground thaws.

Air source heat pumps, on the other hand, are able to work consistently all year round because the sun replenishes energy in the air much faster than it replenishes energy in the soil – especially when the soil’s covered in snow!

But how can they be more than 100% efficient?

So, the important question here: how can heat pumps be more than 100% efficient?

Well, if they’re not the source of energy – if they don’t have to burn a fuel – then their efficiency depends on how well they’re able to move energy from one place to another. If some clever people come up with better designs, better components, and better materials, there’s no theoretical maximum to how many times more efficiently you can move energy.

In other words, getting to be four, five, or ten times more effective at moving energy is simply an engineering problem – not a problem that breaks the laws of physics.

What energy gets used?

In order to move that energy from one place, however, you have to use a certain amount of energy to power components like pumps, fans, heating elements (for backup defrosting), and compressors.

So when we talk about the energy used by an air source heat pump system, we’re talking about the energy required to operate it and actually extract the required heat from the air. Let’s say that the fan, the compressor, sensors, and pumps use 1kW of electricity (in total) for every hour they’re turned on, if they’re able to extract 3-4kW of heat in that same hour, then they’ve worked at between 300-400% efficiency.

Importantly, the higher the efficiency of the system, the lower the relative cost of the system. We explore the cost of air source heat pumps here – if you can get more energy out than you need to use, then you don’t have to pay for as much electricity.

What’s more, powering heat pumps from solar PV panels and domestic batteries practically lets you get as much heat as you want without having to pay any more on electricity bills. If you’re still on a grid-tied tariff, you can also make use of modern heat pump controllers to smartly run your system during times when electricity is cheap or, as is increasingly becoming the case – you can opt for tariffs specifically created for homes with heat pumps. These use smart meters to offer significantly better rates for the electricity powering your heat pump.

Let’s talk about getting you sustainable heating

We’ve been installing air source heat pumps for over 20 years now and firmly believe that they’re the future for most heating requirements in UK homes. But, if you’re just starting to look into the options, it can be overwhelming to sift through the information.

So save time & stress by getting in touch – we can help size your system appropriately and can advise on everything from installation timeframes to maintenance schedules. For heating you can rely on installed by dependable heating engineers, opt for CB Heating.

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