Air Source Heat Pumps Explained

Air source heat pumps are a popular renewable solution for homeowners looking to keep a lid on rising fuel bills. Rebecca Foster explains why this kind of technology might be the ideal option for your project
Rebecca Foster
by Rebecca Foster
6th February 2019

Creating a sustainable home with low running costs is one of the great attractions of undertaking a major project. But this isn’t just about deciding you want to use an air source heat pump and getting one installed. Renewable technologies can be a great choice, but you need to determine whether they’ll perform well in your project.

The first step is to get the building fabric right. Once you’ve got the key ingredients of good insulation and airtightness levels in place (to prevent warmth escaping from your living space) you can start to think about how to power your central heating system. And that can include renewables.

One of the most popular solutions, especially if you’re off the mains gas grid, is to fit an air source heat pump (ASHP). Straightforward and relatively affordable to install compared to some renewables, if specified correctly this tech can help you achieve significant savings on household energy bills.

Pairing the appliance with other sustainable options, like solar thermal panels, could help to further reduce running costs and increase the overall efficiency of your heating system.

A range of factors will contribute to your decision on whether an ASHP will work for you, including the size of the house, insulation levels and the type of emitters you’re using.

How air source heat pumps work

Essentially, air-to-water heat pumps operate in a similar way to fridges – but in reverse. A fan-powered unit is used to extract warmth from the outside air to provide space heating and hot water for the home.

“The fan blows air over a coil, which contains a working fluid that absorbs the warmth. This is condensed using an electric compressor to increase its temperature, before being passed through a heat exchanger to warm water for the central heating system,” says Brian Horne, domestic energy expert at the Energy Saving Trust.

An appliance’s efficiency level is measured according to its coefficient of performance (COP), which indicates the amount of usable energy extracted from the air compared to how much electricity is used to power the pump. For example, a COP of five means that for every one kilowatt (1kW) of electricity used, 5kW of heat is produced.

In addition to their excellent efficiency, ASHPs offer a raft of other benefits. “Modern versions are very quiet and come with energy-monitoring systems and internet-based smart controls,” says Max Halliwell, product marketing manager for heating at Mitsubishi Electric. “Many of the best models are inverter-driven, which means they modulate power consumption to suit the needs of the house at that moment.”

If installed correctly by an MCS (Microgeneration Certification Scheme) accredited engineer, homeowners can expect their device to deliver effective performance for approximately 15-20 years – which compares well to boilers and other renewable heat source options.

Dimplex air source heat pump

Available in a range of outputs from 8kW-12kW, this model has a maximum water flow temperature of 65°C. A-class air source heat pump, Dimplex

Is an air source heat pump right for you?

Most air source heat pumps deliver a lower flow temperature than conventional appliances, such as gas boilers. This makes them an ideal partner for well-insulated new builds that maintain a constant interior temperature.

“Preventing warmth from escaping your home is particularly important if you’re planning on installing an ASHP,” says Scott Greening, commercial manager at Ice Energy. “Heat pumps aren’t designed to take a property from a cold to warm state as quickly as possible. Instead, they gently heat the fabric of the building and then continually provide warmth at the same rate it’s lost.”

The most effective setup tends to be an ASHP alongside underfloor heating (UFH). This creates a more comfortable living environment because the pump only needs to provide a low flow temperature of 30°C-35°C.

Older and draughtier properties require a higher operating temperature to maintain a pleasant internal climate. “Although ASHPs are capable of operating at 45°C-50°C, this reduces their efficiency, which will have an impact on the environmental savings you could make,” says Jeff Tomlinson, sales and marketing director at Waterkotte UK.

Why air rather than ground source?

One of the big advantages of this tech is that it can be easily fitted on both new builds and renovations.

“ASHPs don’t require loops of underground piping like their ground source cousins. Where space is limited, air source will always be a more cost-effective and less disruptive option than drilling boreholes,” says Scott.

However, if you’re self-building, the extra digging required to install the loop in your garden can be done at the same time as the other groundworks.

“Air source technology is often preferable over other renewables when the end-user doesn’t want to be hands-on with the appliance. For example, biomass boilers require the delivery of fuel pellets or chips and will need regular topping up,” says Jeff from Waterkotte.

As such, ASHPs are frequently the tech of choice for those who would prefer a low-maintenance system. They’re also easy to integrate into a scheme, as the internal part of the pump is no larger than a standard boiler.

Installing an air source heat pump

The heat pump itself is usually fitted outside in a position where it will receive good airflow. It then supplies hot water to the indoor unit (usually stored in a utility room or airing cupboard) which in turn delivers it to your chosen central heating emitters. It can also supply a cylinder for domestic hot water.

NIBE Air Source Heat Pump

Air Source Heat Pump by NIBE

Your heating engineer will be able to survey the house to determine the best size of pump, but as a guide a 4kW ASHP will provide the sufficient output for a typical three or four-bedroom home. “They will consider factors such as the building fabric, glazing, room dimensions and heat loss when calculating the required output” says Karen Trewick from Dimplex.

Because of the lower flow temperature ASHPs supply, they work best alongside emitters with larger surface areas, such as UFH or specially-designed oversized radiators.

It is possible to retrofit an air-to-water heat pump in an older property. However, structures that aren’t insulated to current Building Regs may benefit from a hybrid model. “These appliances can bridge the gap in houses where heat loss is too high for the building fabric to be warmed by an ASHP alone,” says Stephen Bancroft, heat pump team manager at EES Renewables.

The hybrid uses a combination of a heat pump and a gas boiler, calculating the efficiency from both sources and enabling whichever appliance will deliver the most cost-effective option. “Once the efficiency threshold is breached – which is basically when the outside climate drops below 1°C-2°C – the ASHP relinquishes all heating demand to the boiler because at this point, higher flow temperatures will be required to maintain indoor comfort levels.”

Heat pump costs

You can expect to pay £5,000-£8,000 to install an air-to-water heat pump system in a well-insulated three-bedroom home. For properties that require a higher heat output, this sum could rise to as much as £12,000.

Read more: Practical Guide to The Renewable Heat Incentive

However, the low running costs that are associated with this form of renewable tech are particularly attractive. According to the Energy Saving Trust, switching from an older, non-condensing gas boiler could save you between £295 and £425 on your annual fuel bills.

Ice Energy air source heat pump

This Build It Award-winning ASHP offers a seasonally-adjusted COP of up to 4.84, as well as industry-leading sound performance. IVT AirX air source heat pump, Ice Energy

Air Source Heat Pump FAQ

This renewable tech takes warmth from the air and repurposes it for space heating. But is it the right option for your project?

Our expert Q&A goes beyond the basics to answer the in-depth questions you really want to ask.

What should you look for to make sure your ASHP is specified correctly?

Robin Adderley, from NIBE: Always appoint an installer who is registered with the Micro Generation Certification Scheme (MCS). This will ensure you get the best advice and gives you access to the Renewable Heat Incentive (RHI).

Phil Birchenough, from Sime: Your installer should always look at the efficiency of the house and what you need from your heating solution in order to offer you the best solution.

As with any system, upgrading insulation in existing homes is important. You should also be confident your engineer fully understands the product they are fitting.

Andrew McLauchlan, from Global Energy Systems: The unit should be specifically sized to account for local temperatures, too. It needs to be able to provide all the warmth a property demands, right down to this design temperature.

If an ASHP isn’t providing enough heat, then it’s been sized incorrectly.

In 2013, the Energy Saving Trust (EST) reported that most ASHP installations weren’t performing as well as expected. Has much changed since then?

David Hilton, from Heat and Energy: The EST report looked at the System Performance Factor, taking into account the heat losses of the whole house – it wasn’t just measured across the inlet and outlet of the heat pump itself.

So one of the key things to come out of it is that we should always compare like-with-like when we’re looking at things like the Seasonal Coefficient of Performance (SCOP).

The other major finding concerned installation quality. At the time, ASHPs were often being fitted as if they were boilers, rather than taking into account the tech’s unique performance criteria.

We’ve moved on quite a bit, and there are now more installers with specific heat pump experience.

Why do some suppliers still list the COP (coefficient of performance) rather than the SCOP. Isn’t the latter a better reflection of the appliance’s efficiency?

David Hilton: Suppliers will work with the data that the manufacturers give them. As a consumer, what’s important is that you make sure you’re comparing the same performance factors when looking at different models.

And remember that, while the SCOP is more detailed, it’s still a benchmark that’s been developed under controlled conditions rather than a real-life scenario.

Andrew McLauchlan: If you buy a car, you’re told its top speed to show its maximum performance. But you also look at the miles per gallon to get an idea of how it’ll perform in the real world.

We provide both the COP and the SCOP on our data sheets to make sure customers have all the information they need to make an informed decision.

One of the criticisms of air source heat pumps has been that they’re noisy. Has anything been done to improve this?

David Hill, from Carbon Legacy: If you go back 10 years, you’d have had to position your ASHP round the back of the garage – they were so noisy you wouldn’t have wanted them anywhere near the house.

The good manufacturers have looked very hard at this aspect, introducing clever fan technology with new shapes and blades. They’ve also focused on how the air is drawn in to the unit and how it comes back out.

It’s important to check exactly what claims are being made: is that 45dBA rating for when it’s running at full power, or does it shoot up to 60dBA?

The former is around the level of a washing machine or loud fridge – but at the 50-60dBA range, it will be a lot noisier.

An MCS-approved installer should give you a calculation to show how your appliance rates – if they don’t provide this, it’s a bit of a red flag.

Air source heat pump exchanger

Worcester’s Greensource air to water heat pump external unit

Is it ever a good idea to fit an air source heat pump to an old boiler-fuelled radiator and pipework system?

Phil Birchenough: This depends on the operating temperature of the system.

Older emitters are designed to work with a conventional appliance (ie a boiler) that runs at a higher flow temperature. Connecting these to a heat pump will reduce their Delta T (the difference between the room temperature and that of the water within the central heating system).

Consequently, the warmth produced might not be sufficient to heat the entire house. So if you’re connecting an ASHP to an old pipework and radiator setup, an additional source of energy would be needed.

The most efficient approach is to fit a modern emitter system, such as underfloor heating or radiators designed to work at a lower flow temperature.

What impact does the heat transfer fluid have on ASHP sustainability?

David Hilton: Because there are so many different metals inside a heat pump and it’s a closed loop system, the transfer fluid usually includes some anti-corrosion components.

Vegetable-based eco versions are available, but in my view it’s better to go with a solution that protects the system and gives it longevity – an important part of sustainability. Some transfer fluids contain glycol and similar products that can be potential contaminants.

The Environment Agency won’t want to encourage any leaking of these. However, there are in-between solutions that biodegrade and still contain the anti-corrosion elements needed.

Your installer should specify an option that’s compatible with the manufacturer’s instructions and local Environment Agency guidelines.

How much guidance do installers give homeowners on getting the best performance from their heat pump?

Phil Birchenough: Post-installation, an engineer should do a thorough handover and advise the homeowner on how to make the most of their system.

They will have designed the setup to deliver maximum efficiency and output for the requirements of that property and should show you what settings are best for your needs.

Normally, they’ll tailor it to your preferences and give a full run-through of how you can change the operational settings. Ideally, they should also take you through some basic troubleshooting tips.

Read more: How to Combine Renewable Energy Sources

Can you use a heat pump to provide hot water – doesn’t the higher temperature needed just slash the COP?

David Hill: Different models offer different efficiency levels for supplying domestic hot water (DHW).

The first step is to pair the appliance with a special cylinder that works on a low flow temperature. This is fitted with a larger-than-usual heat exchange coil to transfer enough energy from the ASHP (the highest output temperature is 55°C) to get the water in the tank up to 50°C. Some installers might plug the ASHP into your existing cylinder, which almost certainly won’t work.

A new, well-insulated house might need around 20,000 kWh of heat per year for space heating, but only 3,000 for DHW. So it’s a small proportion of your overall usage.

The appliance will only run at its highest temperature for about two hours per day to provide this hot water. It’s automated, so a sensor keeps tabs on the tank all day and boosts the temperature if it drops below 45°C, before switching back to space heating only.

High quality versions will feature two sensors in the tank. This is important, as it reduces the risk of overheating the cylinder and wasting energy.

There’s a lot of talk about hybrid heating systems, combining boilers with ASHPs – is that an admission that heat pumps on their own don’t work?

David Hilton: No, because hybrid systems are designed for larger or harder-to-treat dwellings.

For example, you might have a stone barn where an ASHP would adequately heat it until external temperatures got down to maybe 5°C.

Every time it goes below that level, which might only be 10 or 20 days a year, the boiler kicks in. So we’re using the renewable tech in order to cut down oil or LPG usage, for instance.

There are also some ASHPs intended to work in slightly lower temperatures. In that case, you might have a hybrid setup where the heat pump would take care of the space heating and the boiler powers the hot water supply.

Robin Adderley: Hybrid systems have their place. However, in a new build dwelling that is sufficiently well insulated, an ASHP alone should provide all the space heating and hot water requirements.

In the case of retrofit systems, possibly in older properties that have a higher heat loss, combining the pump with an additional heat source may become more attractive in terms of capital cost.

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