33. Air Source Heat Pump MythBuster
- suerowlands47
- 23 hours ago
- 9 min read
Updated: 10 hours ago
Research shows that 94% of UK homeowners with heat pumps are satisfied or extremely satisfied with their heating. This compares to 85% for the most common heating system in the UK, the mains gas boiler.
Yet we constantly see in the media and on social media claims that heat pumps are noisy, expensive to install, expensive to run, don’t work in old houses and don’t work in the cold.
What’s the truth?
I’ve been living with an Air Source Heat Pump (ASHP) for 18 months now. My experience is different to most, as the ASHP was installed as part of a whole house retrofit where the building was super insulated and made very airtight. This makes my ASHP much more efficient than average. However, there is lots of research and plenty of case studies out there that show that ASHPs work well in all sorts of buildings. I’ve used this information together with my own experience to look at some of the more common myths.
They’re expensive to install
There’s no doubt that Air Source Heat Pumps cost more to install than gas boilers. This is a fact.
The Energy Saving Trust says that the average cost of installing an ASHP is around £11,000. Generally, installations range from £8,000 to £18,000 depending on the size of the home and specific requirements, such as replacing single panel radiators with more efficient double panel. A replacement gas boiler can be as low as £3,000.
However, the Boiler Upgrade Scheme (BUS) provides funding of £7,500 to reduce the cost of installing an ASHP in England and Wales, making the cost comparable to a gas boiler. This is straightforward to claim once the system has been commissioned and installed. But it’s not an up-front payment – householders need to fund the purchase and installation first. This is an obstacle for householders without the ready cash. Coming at the end of my build, cash flow was tight and the time taken for the BUS payment to arrive made life painful for a while. This needs to improve to incentivise homeowners to install heat pumps.
They’re noisy
ASHPs emit a low noise during operation, similar to a fridge. Is your fridge noisy?
If it is causing a loud noise, there is something wrong with the installation. It’s important that ASHPs are installed absolutely level to ensure the fan runs smoothly, otherwise they may create unwanted noise (and wear out the fan).
You need planning permission
Planning permission isn’t normally required for an Air Source Heat Pump, as they’re ‘permitted development’ for most householders. There are some restrictions, and details can be found on the Planning Portal website here.
You need a big space
ASHPs aren’t suitable for every property, as you do need sufficient space around them. So very tight terraced houses with very small gardens might be problematic. The amount of space needed varies according to the size of the unit and the positioning. Mine is at the side of the house, with just over 1 metre to the blank wall of the next door house: a fairly tight space. They’re best located on the ground close to the property or attached to an external wall, so that the pipe runs are kept to a minimum.
You have to have the heating on all the time and so they cost a fortune
Air Source Heat Pumps work differently to the gas boilers we're all used to. ASHPs work efficiently by being steady and slow: they need to operate for long periods at low temperatures. Gas boilers work short and fast: once they’re on, the temperature inside the home rises rapidly.
So, yes, it’s true that you need to have the ASHP switched on all the time. But that doesn’t mean its heating your home all the time – it will provide heat when your home needs it (this is ‘Weather Compensation’ – explained later on in this Blogpost).
Let’s have a look at a day at the EcoBungalow from almost a year ago: 27th November 2024. It was a cold day: around -2ºC overnight and into the evening, and only creeping up to around 6ºC in the middle of the day. The living room stayed at a pretty consistent 20ºC during the whole day.
The information from the ASHP app for that day shows the unit consuming electricity over the whole day, with a few peaks where the hot water was heating or the system decided the house needed more heat in response to cold external temperatures. The total electricity consumption for the day is 7.12kWh. This produced a total of 40kWh of thermal energy in the form of heating and hot water. That’s 5.6kWh OUT for every 1kWh IN. That’s an incredibly efficient Coefficient of Performance (COP) of 5.6. Compare this to a typical gas boiler, where the COP is typically 0.8 to 0.9.
On my variable Cosy Octopus tariff, the average cost per kWh at this time was 21p along with a daily standing charge of 47.6p. So the cost of heating and hot water for the day was £1.97.
As a rough estimate, with a boiler working at 90% efficiency, I’d have needed about 45kWh of gas to give the same amount of thermal energy. At the then rate of 6.24p per kWh and a daily standing charge of 31.66, the cost would have been £3.12 for the same level of heating.
So, although electricity is more expensive than gas, the efficiency of the ASHP means that it costs less – in my case, a LOT less. It’s important to stress that my figures are for an unusually efficient system because of the additional improvements that have been made to the house.
You can’t heat the house up quickly
True.
Air Source Heat Pumps aren’t designed to work in the same way as gas boilers. They are steady and slow, keeping the home at a consistent temperature with no big fluctuations. In contrast, the way we use gas boilers is typically to heat the house in the morning and the evening. This means that the temperature fluctuates widely, with the house feeling chilly before the heating comes on.
So don’t expect to quickly boost your home’s temperature if you feel cold. Instead, make the ASHP work efficiently by keeping the houses constantly at a comfortable temperature. Don’t let it get cold in the first place, and you won’t need to boost the heat.
The UK gets too cold for a heat pump to work
Not true.
Heat pumps work in temperatures below freezing, down to around -25ºC. However, their efficiency does drop as the temperature decreases. But this doesn’t stop people in cold countries using heat pumps. Norway is a great example: with an average winter temperature of -6.8ºC, around two thirds of households have a heat pump.
With an average winter temperature of more than 5ºC, heat pumps don't have a problem in the UK.
An ASHP isn’t suitable for older buildings
As with any form of heating, if the building loses a lot of heat because it’s badly insulated and draughty, the heating system will need to work harder to keep the home warm. This means it will be less efficient and cost more. However, heat pumps can work efficiently in older properties that fall below modern insulation standards.
This report from Historic England provides case studies of small-scale older buildings. It concludes that ASHPs work well in a range of different historic building types. It notes that when poorly performing systems are reported openly without exploring the underlying issues, people may conclude that heat pumps are not a suitable replacement for fossil fuel heating systems in older buildings. Underlying reasons why a system doesn’t perform well often include poor briefing of users on how it works and/or what to expect.
The report has more than likely influenced the recent refurbishment of Lambeth Palace, which is now heated by ASHPs. If Lambeth Palace can do it, surely other old buildings can?
The water doesn’t get hot enough
You need a hot water cylinder with an ASHP. My water is heated to 45ºC – plenty warm enough for washing up, baths and showers. I particularly like a long, hot shower after a day out in the hills of the Lake District. No problems with this.
I now find it bizarre when I stay at other people’s homes and add cold water to the hot when washing dishes. What a strange (and pointless!) way to use energy – make water so hot you have to cool it before you can use it.
One thing to note: there is a risk of Legionnaire’s Disease with heating water only to these relatively low temperatures. All systems have a ‘cleansing’ mode whereby the water is heated to a higher temperature every couple of weeks to eliminate the risk. I control the timing of this via an App, and vary it to coincide with when I’ll have high demand for hot water – e.g. I’m planning on cleaning the house.
They’re complicated
It’s true that ASHPs are more complex for the homeowner to understand than standard gas boilers. Setting the controls so that the ASHP runs as efficiently as possible is initially done by the commissioning engineer. However, this may need tweaking, as each house performs differently. It’s a good idea to get to grips with what it all means so that – as a homeowner – you can continue to ensure that the system runs efficiently. The key to it all is ‘weather compensation’.
What on earth is ‘weather compensation’? By far the best explanation I’ve seen is given by Heat Geek:
The key thing to understand when you’re trying to maximise the efficiency of heat pumps … is that the lower the flow temperature, the higher your heat pump COP (or efficiency).
Now, the temperature your radiators need to be to maintain your desired room temperature isn’t the same all year round. For example, your radiators might need to be 50°C when it's -2°C outside, but only 35°C when it's 12°C outside in spring and autumn.
If they’re still running at 50°C during mild weather, the thermostat will keep switching the heat pump on and off to prevent overheating – which wastes energy. You don’t drive your car at 70mph and 0mph to average 35mph, right?
To minimise flow temperature throughout the year, all heat pumps should use a feature called weather compensation.
Weather compensation simply measures the temperature outside and adjusts the required flow temperature: cold outside = hotter radiators, warm outside = cooler radiators. This minimises cycling and keeps your system running at maximum efficiency.
The fine-tuning of these flow temperatures relative to the outside temperature is where hidden efficiencies lie.
Weather compensation is set by the ‘heating curve’. Here’s the heating curve on my Viessman App – it’s set at a slope of 0.6, which gives a fairly linear increase in flow temperature as outside temperatures decrease. This works very well for my super-insulated home, and – because of this - I reduced the ‘level’ (the flow temperatures) by -3 as I didn’t need it to be so warm.
For older buildings with radiators, Viessman recommends a heating curve of 1.4 to 1.6. This gives a different curve profile, with higher flow temperatures. This means that the ASHP needs to work harder, making it less efficient and more expensive to run than in the super-insulated EcoBungalow.
Living with the system for a while enables the fine-tuning that Heat Geek recommends. By experimenting with the settings, homeowners can get the system to work more efficiently and provide the right level of warmth. For example:
If the room temperature is generally too low: increase the level.
If the room temperature is too low, especially on cold days: increase the slope.
If the room temperature is too low in the transition period, but sufficient on cold days: lower level and increase slope.
What you DON’T do is attempt to increase heat simply by turning up the target room temperature. If you’ve set it to an appropriate level for comfort (e.g. 21ºC) and the system isn’t delivering that temperature, then it’s a problem with the flow temperature relative to outdoor temperature. So it’s the heating curve and level that need adjusting.
All quite mind boggling until you realise that – once you’ve got the right heating curve for your home – you can simply leave the ASHP alone to do its thing.
Air Source Heat Pumps are great: efficient and reliable. They're a proven technology, with years of working well in climates much colder than the UK. Crucially, they provide a massive opportunity to free ourselves from the shackles of gas in an increasingly uncertain world. And, of course, they help to reduce the huge environmental footprint of heating our homes.
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