An Air Source Heat Pump (ASHP) allows you to heat your home and hot water without burning anything! The heat pump takes heat from the air and uses a compressor to make that heat hot enough to heat your home and water.
Some systems are reversible, so can provide air conditioning as well as heating. Heat pumps have been widespread in Sweden, Europe and North America since the 1980s, but have become more popular recently as they have a very big impact on your carbon footprint.
This article aims to answer all of your nitty gritty questions on air source heat pumps:
- Is an air source heat pump suitable for my home?
- Will my bills go down?
- What does the installation process look like?
- Should I install a heat pump now, or wait until I've improved my home's insulation?
- Are air source heat pumps noisy?
- How do heat pumps work?
- Different types of air source heat pump
- Technical details
Is an air source heat pump suitable for my home?
Your home must meet the following criteria in order to for you to be able to install an air source heat pump. In most cases it's relatively easy to fulfil these criteria, but there are some challenging cases as we have flagged below. If we have included a heat pump in your pathway, it means we think a heat pump is a good option for your home, but the following criteria may require further evaluation.
Sufficient space for the outdoor unit
An air source heat pump always has an outdoor unit that collects heat from outdoor air. You need enough space for this outdoor unit. There are various regulations that govern where you can put a heat pump. It cannot be mounted on a wall facing a highway and in Wales it currently must be more than 3m from a boundary, which makes installation difficult for many terraced houses.
Sufficient space for a hot water cylinder
A heat pump needs a hot water cylinder in order to provide hot water. If you currently have a combi boiler then you will not usually have a hot water cylinder. Combi boilers do not require a hot water cylinder because they are extremely powerful and can instantaneously heat up your water when you need it. A heat pump can't provide heat that fast, and so it needs to gradually heat up water and store it for when you need it. Common locations to add a hot water cylinder include an airing cupboard, utility room or a room in the loft if it is suitably insulated. If vertical space is a challenge, then a horizontal cylinder can be used, or higher density phase change batteries can be a solution, however they do slightly reduce the heat pumps efficiency.
If you already have a cylinder then that's great. It may even be possible to re-use your existing cylinder with an external plate heat exchanger to save the cost of a new cylinder.
Adequately sized radiators and pipes
Heat pumps work most efficiently at low flow temperatures. The more efficiently your heat pump runs, the less you will spend on your bills. In order for a heat pump to provide your home with enough heat while running at a low flow temperature, your radiators and pipes need to be big enough.
It is likely that your existing fossil fuel heating system runs at a relatively high flow temperature and so your radiators and pipes may be too small to heat your home at a lower temperature. This isn't always the case - sometimes the radiators have been oversized for your old system, or your home could have been improved since the radiators where calculated meaning that some, or all of your existing system could be perfect for a heat pump. If your current heating system only has to run for a few hours on even the coldest days then your radiators may be perfectly adequate for a heat pump. If your radiators are too small however, you have two options. You can either add radiator area by installing bigger (taller, wider or simply deeper) radiators or more radiators, and/or you can improve your homes insulation so that your home doesn't need so much heat and hence your existing radiators are sufficient. The systems that transfer the heat into the air in your home are called 'emitters'. While radiators are the most common in the UK, there are alternative options that also work with heat pumps, such as underfloor heating and fan coil units.
It's theoretically possible to heat a home with small radiators and a high temperature heat pump. The issue is that your heat pump will run inefficiently and your bills will be much higher than they could be if you had bigger radiators.
Enough space on your electricity panel
If you don't already have a 100A fuse you will need to ask your network operator for an upgrade before starting any work. It is very unlikely that you will require a three phase connection for a heat pump!
(Sometimes) space for a buffer tank
In some cases you may want to install a buffer tank on your heat pump to store extra heat. Adding a buffer tank improves the flexibility of your heating system such that you can run your heat pump when electricity is cheap and then use the heat later. Buffer tanks are not always required and will inevitably lose some heat and can also reduce the overall efficiency of your heating system, so not having space for a buffer tank definitely isn't a deal breaker.
Will my bills go down?
Gas and electricity prices have shot up recently, but gas prices have gone up more than electricity prices have. As a result, a well designed heat pump system can be cheaper to run than a main gas boiler and much cheaper than LPG or solid fuels.
Whether or not you save money when you switch depends on the price of electricity vs. gas, on the efficiency of the system, and on whether you stop using gas completely. Electricity currently costs about 4 times more per kWh than gas does, and so to get a saving on the 'per unit' piece of your bill, your heat pump would need to be 4 times more efficient than a gas boiler. If your gas boiler runs at an efficiency of 85%, then you would need your heat pump to run at an efficiency of 350%. This is achievable, but it requires a very well designed system. If you stop using gas altogether than you also save a significant amount on the gas standing charge - currently 27p a day, or £99 a year, and so your heat pump can run at an easier-to-achieve efficiency of 300% and you can still save money.
This relationship means that you want to maximise the efficiency of your heat pump. The more efficient your heat pump, the lower your bills will be and the higher the chance that you will save money relative to gas. As we explained above, the lower the temperature of your radiators, the more efficient your heat pump. So to save money you want your installer to ensure that your heat pump can run at as low a flow temperature as possible.
There are number of ways you can reduce the price you pay for electricity. On a flexible tariff such as Octopus Agile the electricity rate varies half-hourly. This means that you can, to some extent, shift your heat pump's operation to lower cost hours. Installing a battery and/or a large hot water cylinder enables you to make better use of these fluctuations. Installing solar photovoltaics means that you also generate your own electricity for free. While your solar generation won't perfectly match your heat pump's electricity use, solar plus a battery plus a heat pump is a fantastic combination for lowering your bills.
It is hard to predict what gas, oil and electricity prices will do in the coming years of course, so we cannot be sure what the bill impact will be over the life of the heat pump. If government moves policy costs from electricity to gas or to general taxation, restructures the electricity market so the marginal unit doesn't set the price, or adds a carbon price on natural gas, then electricity rates will fall further relative to gas rates. In any case, maximising the efficiency of your heating system will always be a good idea.
What does the installation process look like?
An ASHP installation starts with a detailed room by room heat loss survey, a hot water demand survey and a quote that includes the hydronic design with low flow temperature (<45C) emitters, pipework flow rates, cylinder heat loss per day, fan noise level & electrical diversity calculation. This level of detailed design will maximise the heat pump’s efficiency and so minimise running costs. Your installer can then order the exact equipment to fit your home. During the installation a team will fix the main heat pump unit outside your home. Then a system of insulated pipework will be installed to connect the heat pump to either new or existing radiators, underfloor heating and hot water cylinder.
Should I install a heat pump now, or wait until I've improved my home's insulation?
This is a contentious topic. Here at Sero we think it depends on your objectives, and on the remaining life of your existing heating system.
Your existing heating system
Deciding to take out a functional boiler is rather different to making the choice between a new heat pump or a new fossil fuel boiler. We think you should avoid installing a new boiler. If your existing boiler is struggling but you don't feel ready to install a heat pump, see if you can find a great heating engineer to repair your existing boiler and keep it going until you are ready to switch to a heat pump. That way you won't waste money on a new boiler that you will only use for a few years.
If your current boiler is working fine you can still install a heat pump right away if you want to - there is just less urgency in making that decision. If you do want to keep your current system running for the moment, it's worth getting a heating engineer to check that it's running as efficiently as possible. A lot of gas boilers are running at unnecessarily high flow temperatures resulting in low efficiencies and unnecessarily high bills
If you want to reduce your home's carbon footprint as quickly as possible but you can't afford to do everything all at once, then you should do easy insulation measures such as loft insulation, and then install a heat pump. If your home has a high heating demand you will have to install a powerful heat pump that can provide heat as fast as your home looses it. You will also need to ensure that your radiators (or other emitters) are sufficiently sized so that heat pump can distribute heat at that rate. This is likely to mean that you need to increase the thickness of your existing radiators and/or add more emitter area (in the form of more radiators, underfloor heating or fan coils). If you later improve your home's insulation then your heat pump and radiators will be oversized. Having oversized radiators isn't an issue functionally - you will just have spent money that you didn't need to. Having an oversized heat pump can be problem though. Again you will have spent (slightly) more money on the heat pump than you needed to, although heat pump prices don't actually vary that much with size. More importantly, if the heat pump cannot 'turn down' enough to match your home's new lower heat loss, then it will turn on and off more. This frequent turning on and off, referred to as cycling, can reduce the efficiency and the lifespan of your heat pump. Some heat pumps have a much greater 'turn down' ability than others though, so one option is to select a heat pump which covers your heat load now, but which has a really good ability to turn-down so that it doesn't cycle too much when you do insulate your home. Ask for the manufacture's minimum kW capacity for the unit recommended. If you go down this route, remember to think about how your heat pump install will fit physically with measures you plan in the future - such as external wall insulation.
If you want to avoid installing too large a heat pump, then you should do all of your insulation measures first, and only install a heat pump at the end once your space heating demand has been minimised. This will likely mean you don't have to spent any money upgrading radiators. The downside of this approach is that you continue burning fossil fuels in your home for longer.
Are air source heat pumps noisy?
Older heat pumps and air-conditioners have suffered from noise problems. Newer heat pumps are much quieter. A modern, well installed heat pump will make very little noise and you certainly shouldn't hear it from inside. By law it must create less than 42dB at 1m in front of a neighbours window, which is the same sound level as a library.
How do heat pumps work?
A heat pump captures heat from outdoor air, uses electricity to make that heat hotter, and transfers that heat into your home. It uses the renewable thermal energy in outdoor air (originally from the sun!) to heat your home. The idea of using the low temperature heat in outdoor air to make high temperature heat in your house sounds kind of magic, and it is, but it's also very common. Fridges and air-conditioners use exactly the same technology to move heat from a cold place (inside the fridge or the cooled building) to a hot place (outside the fridge or the outdoor air on a hot day).
The technology that performs this magic is called a refrigeration cycle. Refrigerant in the heat pump is first expanded to a really low pressure. The process of expansion makes it very cold. That very cold refrigerant then runs through a heat exchanger while outdoor air is blown passed the heat exchanger. Because the refrigerant is colder than the outdoor air, heat is transferred from the outdoor air to the refrigerant. We have now successfully captured the renewable heat! The next step is for the compressor to compress that refrigerant to a much higher pressure. This is the part of the cycle that uses electricity. The process of compression makes the refrigerant very hot. The hot refrigerant then runs past another heat exchanger where it transfers that heat to the water circulating around your home. If you home needs heating then that water either circulates around your radiators. If instead you need hot water then it goes to yet another heat exchanger in your hot water tank and transfers the heat into your hot water tank.
Different types of air source heat pump
Split vs. monoblock
Heat pumps can either be 'split units' or 'monoblock' units. As the name suggests, split units consist of two separate components: an outdoor unit that collects heat from outdoor air, and an indoor unit that transfers that heat to the water circulating around your radiators. The two units are connected by refrigerant piping which carries the heat from the outdoor unit to the indoor unit. The outdoor unit consists of a heat exchanger (to extract heat from the air) and a fan (to push air over the heat exchanger so that it can extract as much heat as possible). The indoor unit is a similar size to a gas boiler and consists of a second heat exchanger where the hot refrigerant transfers heat to the water in your heating system. Monoblock units have this second heat exchanger within the outdoor unit, so you do not need an indoor unit. In this case hot water is just piped directly from the outdoor unit.
Air-to-water vs. air-to-air
Air source heat pumps can either be 'air-to-water' or 'air-to-air'. The first word describes where the systems get their heat from, so will always be air for an air source heat pump. The second word describes the substance that the heat pumps are heating up. In a typical British home, our heat is delivered by water flowing through radiators, and so our heat pumps need to heat up water. They are therefore air-to-water heat pumps. In this case the heat pump heats the water, which circulates through the radiators and heats the air in the room. An alternative is to heat the air directly - either with in-room 'fan coil' units that look like hotel room A/C units, or by carrying air around in ducts (like the things people crawl around in spy movies). Air-to-air units are common in commercial buildings, and in homes in countries where air-conditioning is widespread like the U.S. In the UK they can be a good choice for heating isolated rooms, or for homes that are likely to need cooling, such as flats or homes with loads of windows. They are typically cheaper and more efficient than air-to-water heat pumps (because they don't need to make the heat as hot), but they do require a suitable distribution system and they won't heat your hot water, so you'll need a separate heat pump water heater for that. Air-to-air units do not meet the criteria of the Boiler Upgrade Scheme grant, however fan coils supplied by an air to water unit do meet the criteria.
Do you need an immersion element in the hot water cylinder?
Low temperature heat pumps will likely require an immersion heater to provide hot water legionnaires cycles. High temperature air source heat pumps can actually provide hot water up to 70C, so they are able to ramp up the temperature more efficiently therefore not needing an immersion heater element for general operation. You may wish to still include an immersion heater as a back up and boost option.