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How Does an Air Source Heat Pump Work? A Comprehensive Guide

Heat pumps, in general, are revolutionising the way we heat homes and workplaces – making for reliable, sustainable, and efficient alternatives to wood fire or gas & oil boilers.

But how exactly do they work? People are often surprised that it’s possible to heat your home, quite literally, out of thin air.

Here is everything we discuss:

How do heat pumps work?

So let’s start with looking at how heat pumps in general work, as ‘air source’ heat pumps are just one kind out of the few varieties.

You may already know that energy is never created or lost, but simply moved. When heating your home using something like a wood burner, fireplace, or a gas/oil boiler, you’re relying on the energy stored in those materials for the heat.

Put simply, a tree grows by capturing energy from the sun and absorbing carbon from its environment. That energy gets stored in the wood until it is cut down and used for firewood.

If that tree grew millions (or billions) of years ago and fell down in the right conditions, that stored energy could have condensed even further into coal or crude oil. So these forms of heating work by releasing that stored energy through combustion. 

But, as we’re seeing with the accelerating rate of climate change, releasing so much of that energy stored in wood, coal, and oil is creating real problems for environments around the world.

Heat pumps, on the other hand, don’t need to combust anything – they’re not a source of energy in themselves but simply move energy from one place to another.

But heating your home is clearly not as simple as pulling in air from outside in the depths of winter – you’d then just have a cold house! The question, then, is where do heat pumps move energy from?

The simplest description of how heat pumps work is this: 

  1. A refrigerant is pumped through a pipe that’s looped in a medium (such as the air or ground)
  2. It absorbs heat from that medium
  3. This heat is exchanged to something else (like air or water) in a heat exchanger inside of the heat pump
  4. That air or water is then circulated around your house to warm it up

Fundamentally, heat always goes from hot to cold so, as long as the temperature of the medium is higher than the temperature of the liquid, heat will always pass into the liquid and then into the heat exchanger.

So what about when, say, the temperature is below zero? Surely the liquid freezes at 0°C?

Here’s where heat pump designers use some clever physics to squeeze out as much efficiency as possible. This happens with two key innovations:

Running antifreeze, glycol, or refrigerant through the pipes

Exactly what happens at this stage depends on the type of heat pump, but ground and water source heat pumps run a mixture of water and antifreeze (glycol) through the pipe snaked in the ground or water. The glycol won’t freeze until temperatures well below 0°C, so they can get quite cold! 

Remember, heat goes from hot to cold – so if you imagine that the outside temperature is 0°C but the liquid temperature within the heat pump line is -15°C – you have a temperature difference of 15°C that you can extract from the medium.

With ground & water source heat pumps (more on this later), the extraction lines use glycol, where the heat is then exchanged to a smaller, higher-pressure line of refrigerant gas.

But air source heat pumps extract heat straight from the refrigerant line, so are able to operate efficiently in temperatures down to -28°C!

Using compression & expansion

The reason for transferring heat to a refrigerant gas line (or only using a refrigerant gas line) is that gas can be compressed, whereas glycol (being a liquid) can’t – and this is another key innovation.

When you compress a gas, it makes it hotter. And when you decompress it, it becomes significantly colder. So, rather than just having a straight connection between the medium and the heat exchanger, the heat pump lines are designed with a compression stage and an expansion stage.

On the way out of the medium, when the refrigerant has taken on as much energy as possible, it is compressed to generate heat! This heat then also passes through the heat exchanger (usually called a “condenser”) and into your home’s systems to provide extra warmth.

Now, after the condenser, there’s a thermal expansion valve that lets the liquid suddenly decompress (in a fridge system, this is sometimes called the “evaporator”). And this cools it dramatically which let’s the process start again.

They’re mostly closed-loop

While systems do exist (primarily for water source) that make use of an open loop within a medium, most heat pump systems are designed with closed loops in order to maximise efficiency. Air Source Heat Pumps, like the ones we specialise in installing, are all closed-loop systems because a closed loop is much more straightforward and needs a lot less maintenance!

Different types of heat pump: Pros & Cons

So heat pumps are very clever tech, but what can they draw heat from?

As mentioned, the three main mediums are air, water, and ground – each with their own pros and cons. The different methods of tapping into them essentially makes them better-suited to different geologies and climates. Let’s look at each one.

Ground source heat pumps

Ground source heat pumps work by burying a pipe in the ground – either through a number of really deep boreholes (sometimes up to 150m deep!) or through a “slinky” pipe that’s coiled around a relatively shallow ditch and in-filled.

Crucially, ground source heating isn’t the same as a geothermal source like hot springs or volcanic ground. Geothermal sources are able to create a lot of consistent energy because they’re drawing from the Earth’s internal heat (from natural hot springs or volcanoes, for example) – but areas where this is a suitable method of energy extraction are relatively rare. Ground source heat pumps, on the other hand, are actually drawing from the solar energy absorbed by the soil they’re buried in.

Ground temperature tends to fluctuate less than air but that doesn’t necessarily make it a better source for heat pumps because, as we’ve seen, heat pumps work best by tapping into the difference in temperature – not necessarily by drawing from a consistent temperature.

Now, another key point here is that when you extract energy from one place and release it elsewhere, the energy at the source needs to be replaced. In other words, if you’re pulling energy out of the soil, you need to rely on the soil absorbing more energy. In order to do so, it has to remain conductive (which means, primarily, wet) and there needs to be enough sunlight to replace the extracted energy.

So, in the depths of winter when the ground is covered in snow (which reflects sunlight), the ground source energy isn’t very quickly replaced.

What’s more is that, at ground temperatures of around or below 0°C, drawing heat from the soil can actually freeze the soil around the pipe loop – which can potentially damage the pipe loop at worst (and require it to be re-bored) or render it temporarily unusable.

Ground Source Pros:

  • Great for climates where ground temperatures don’t often get below 0°C
  • Minimal maintenance required

Ground Source Cons:

  • High installation costs for boring and/or excavation
  • Can be unreliable in certain climates
  • Unsuitable for certain terrain

Water source heat pumps

Rather than running a pipe loop through soil, water source heat pumps drill down into an aquifer to access underground water sources (though sometimes, rarely, a nearby river is used). But the method of extracting energy is the same: run a cooled water/antifreeze mixture through the pipe loop, warm it up from the water source, extract that heat through a heat exchanger.

It’s water source heat pumps that sometimes feature an open loop – where water is pulled directly from the aquifer, has the heat extracted, and then is released further downstream. This creates a lot less chance for the loop to break and need replacing, but also then requires a lot more filtering to make sure debris from the aquifer doesn’t clog up the heat pump components.

A core benefit of water source heat pumps is that the temperature of aquifers tends to stay around 10°C and the energy is regularly replaced by flowing water.

Water Source Pros:

  • More consistent energy extraction than ground source
  • Not as affected by seasonal changes in temperature

Water Source Cons:

  • High installation costs through having to bore deeply into an aquifer
  • Higher maintenance costs through filter replacements of open-loop systems
  • Limited suitable locations for installation
  • Unknown knock-on effect of over-utilising water sources, potentially freezing aquifers & creating more environmental problems

Air source heat pumps

Air source heat pumps, at first glance, feel like they shouldn’t be as effective and consistent as ground or water source pumps – but they’re actually the most consistent, reliable, and cost-effective solutions (in certain climates).

Rather than running a long loop of glycol (water-antifreeze) through the ground, a loop filled with refrigerant is snaked around the front of the heat pump unit like a car radiator – this is the “evaporator”. A fan then pulls air over that evaporator loop so the slightly supercooled refrigerant can extract the air’s energy.

In the same way as the other methods, this is then compressed to heat it up further before exchanging the heat through a heat exchanger.

Importantly, this makes air source heat pumps effective year-round and able to continue operating in really extreme climates. So long as the ambient air temperature is above the -28°C temperature of the heat pump’s refrigerant, heat can continue to be extracted.

For context, the average air temperature in the UK almost never gets below 0°C – so that leaves at least 28°C of potential warming in the worst of cases!

Now, a possible drawback here is that they need slightly more electricity than alternatives to run the fans, the compressor, and to defrost components – but because they’re not the source of energy, they can operate at more than 100% efficiency.

Quite frequently these days, an air source heat pump can operate at over 400% efficiency (which means that for 1kW of electricity, you get over 4kW of heat).

The operational cost, therefore, fluctuates based on the current price of electricity – and needs to be weighed against the current cost of gas. But taking the plunge and installing domestic solar (or switching to a green tariff) therefore creates a super reliable, 100% sustainable way to heat your home.

In theory, a fully-solar setup with an expertly installed air source heat pump system gives you free energy with minimal maintenance for upwards of 20 years.

Air Source Pros:

  • Far lower installation costs compared to ground or water source heat pumps
  • Consistent, reliable heat extraction year-round in most of the UK
  • Easily tie into existing central heating systems (with air-to-water heat pumps)
  • Easy access makes servicing & repair cost-effective
  • Can run completely from renewable energy, completely off grid, and in remote locations
  • Expected operational life at least 20 years, with many heat pumps still working perfectly well (though much less efficiently than modern units) since installation in the 1970s

Air Source Cons:

  • They sometimes need more electricity than alternative heat pumps (primarily in extreme weather due to backup components)
  • Inexperienced installation & setup can seriously throttle system efficiency

Air-to-air vs air-to-water heat pumps

So far, we’ve only talked about ways in which heat is extracted (that is, how heat gets from one place to the heat exchanger) – but there are two main ways of getting that heat around your home or workplace in order to actually heat it.

Systems usually use either air or water to heat your home. Let’s look at these in more detail.

Air-to-air heat pumps

These are slightly more common in UK commercial heating applications (such as for office buildings or retail spaces) compared to air-to-water, but are extremely rare in UK domestic applications. Primarily, this is due to a quirk of history (more on that in a sec).

Air-to-air systems take the heat from the heat exchanger and heat up air (usually from within your home) that’s then circulated around through ducting or via separate room units to provide heat where it’s needed. A key benefit of air-to-air heat pumps is that they can also be run in reverse to cool a space in the summer. It’s precisely this reason that they’re so common for domestic use in the USA (where they’re more commonly called ‘HVAC’ systems).

So why aren’t air-to-air heat pumps popular in the UK?

Well, for five main reasons really.

Firstly, UK homes don’t need a huge amount of cooling during the summer months. While cooling is certainly nice, we only tend to have a few weeks every summer where we’ll have to have windows open and fans running. Homes in, say, Los Angeles, have sweltering summer temperatures and therefore need a great deal more cooling.

Secondly, air-to-air heat pumps aren’t an energy-efficient solution – at least in environmental terms. Heating in winter and cooling in summer doesn’t reduce energy consumption but effectively doubles it! This, in turn, means that you have to rely on more energy from the grid or larger solar PV arrays to run them effectively if you want to power them with renewable energy.

Thirdly, most UK domestic properties were built with water-based central heating systems. So removing all the heating pipes & radiators before installing individual room units or ducting (which usually has to be quite large) is so much work that it’s almost always financially unfeasible. Why not just use the central heating pipes & radiators that your home already has, right?

Fourthly: hot air heating is quite unpleasant! Heating a home with hot air creates a really dry, “stuffy” environment – which most people find quite unpleasant.

Lastly, air-to-air can’t heat your water. A key drawback of air-to-air systems is that they can only heat your home’s air, you end up needing a whole separate system for heating water for your taps & showers or baths.

Air-to-water heat pumps

We’re biased, but air-to-water heat pumps are an absolute joy.

As we’ve seen, air-source heat pumps are much more cost-effective to install because they can be installed on a sturdy surface next to your home – with little or no excavation equipment required and without the need to bore deep holes and create potentially unforeseen environmental problems.

Air-to-water heat pumps tie into existing central heating systems to essentially just replace the gas (or oil) boiler. This makes them a really sensible option for UK properties because you’re able to save on not having to drill huge holes in the ground and not having to replace your whole central heating system – a win-win!

In fact, we only install air-to-water heat pumps because we’ve found them to be the most effective, dependable systems available in the UK – they’re great for our climate and have relatively small energy consumption compared to alternative heating systems.

But this is just looking at the core design and units themselves – modern air-to-water heat pumps have the added benefit of really advanced controls.

It used to be the case that your heat pump system was either on or off. So you’d end up with sudden, short blasts of heat until the desired temperature was reached (which is still the case with many gas-based central heating systems). But modern components & innovations allow for really fine-tuning of energy use & heating. You can, for example, set a low but consistent internal temperature so that your home stays comfortably warm without huge fluctuations in room temperature.

This lets you get the most heat with the least amount of electricity used – so you can make the transition to sustainable heating without having to anxiously watch your meters. In fact, a great air source heat pump system installed by experts is something so effective that you quickly don’t even think about it – it just works!

How to get an air source heat pump

If you’re thinking about taking the plunge to sustainable, reliable heating, we’re here to help. Our expert installers are all fully qualified for heat pump installations and draw from over 20 years of plumbing and central heating experience.

Expertise & support you can rely on

Because air source heat pumps are more straightforward to install than ground or water source, there are unfortunately a lot more cowboys in the market doing half-jobs or ineffective installations – both of which are more costly to fix than just getting it right the first time.

We can effectively size systems, install them, and have a helpful and friendly customer service team.

Let’s talk about getting your heating sorted for good.

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