Heat Pumps

Air Source Heat Pumps (ASHP)

There is a lot of talk about air source heat pumps, in the media and in the pub and some of it is quite negative. But, as with everything there are two sides to the story so here we will try to unravel it for you.

The Good

There is a sliding scale with air source heat pumps (ASHP) – they work brilliantly in well-insulated new build properties, keeping you warm, giving you plenty of hot water and being the cheapest form of heating your home. That is why from 2035 no new-build property in England will be able to install a fossil-fuel boiler. Under the “future homes standard”  all new builds will have to have to have low-carbon central heatingsystems, such as electric heat pumps.

… and the not so good

At the other end of the scale, they don’t work as well in a leaky, old, uninsulated properties. We can make them work in such a property but you may need two or more heat pumps, you’d need to upgrade your insulation where possible, you would need to install larger rads and it would be more expensive to run, more than likely costing more to run than a fossil fuel boiler.

The usual

Most homes are somewhere in between and if you have cavity walls that are filled, decent loft insulation and double-glazing your house would be suitable for a heat pump.

Lower heating bills

Based on a well-insulated 200m² home a heat pump could cost you:

  • 70% less than electric heating
  • 54% less than an oil boiler
  • 29% less than a gas boiler

Design is key

The key is doing a proper heat loss survey of your property and coming up with the right design. This is something we excel at. We will provide a proposal that will set out the costs of installing a heat pump system and the expected running costs, so you can make an informed decision.

We have even been known to recommend that customers keep their current heating system and spend their money on better insulation and controls to cut down their use of fuel and save money.

Is it a boiler?

There is some mystique about heat pumps, but you can think of a heat pump as your new boiler – they just provide hot water for heating and hot water. They produce hot water for your heating circuit, whether underfloor heating or rads, and also for your domestic hot water, taps and showers. (Just to note we are talking about air-to-water heat pumps, not air-to-air heat pumps).

The basic principles of an air source heat pump system
The basic principles of an air source heat pump system.

The difference is they run at lower flow temps than a fossil fuel boiler, typically at 50˚C rather than 70˚C. And that is why it is important to get a proper design done and the reason you may have to upgrade your radiators.

Insulation is key too!

They also heat your house in a different way. They run low and slow giving you a continuous ambient heat, rather than blasting huge amounts of heat round the rads intermittently like fossil fuel boilers do. Great for your carbon footprint.

That is why decent insulation is important, a heat pump will get your house up to temperature and then keep it there by working gently in the background. The better the insulation the less heat escapes.

The efficiency of an air source heat pump is based on the flow temperature. They are cheaper to run if the flow temp is 40˚C than if it is 50˚C. The better the insulation the lower the flow temp needs to be to keep you warm and the lower your bills.

How does it actually work?

Air to water heat pumps are stand-alone units, roughly the same size as an air conditioning unit, which are placed outside your home or business, so normally no digging or underground pipework is required. They provide hot water to a buffer vessel for your heating or to a cylinder for your hot water – they are not combis; they need a cylinder!

Is it a fridge in reverse?

ASHPs don’t create heat, they move it and, in this sense, work a bit like your fridge. Your fridge takes the heat out of the inside and dumps it out the back – put your hand on the grille on the back of your fridge it’s warm.

A heat pump takes the heat out of the air and dumps it into your house in the form of hot water. It does this using a refrigeration cycle. That’s the techy bit which can be explained in the following video.

Where is the “renewable” bit?

Heat pumps run on electricity. The electricity drives the compressor circuit which generates the heat.

An electric 1-bar fire takes 1 kilowatt (KW) of electricity and provides slightly less than 1KW of energy. A heat pump takes 1KW of electricity and generates about 3.5KW of energy by concentrating the heat from the air and moving (pumping) it into your hot water.

So we get a “seasonal coefficient of performance” SCOP of 3.5 meaning 1KW of electricity in and 3.5KW of energy out. Thus, a heat pump is roughly one-third the price to run compared to direct electric heating.

Why efficiency is important

The efficiency of your water heat pump determines the running cost, and this is why the type of air source we specify are good quality European heat pumps such as Vaillant or Stiebel-Eltron – they have the highest efficiencies when you need them most, in the middle of winter.

The efficiency of an air source heat pump is affected by the outside temp and how hot you would like the water. Most heat pumps advertise their best SCOP (seasonal co-efficient of performance) and this is measured on an industry standard of Air Temp 7˚C and Flow Temp of 35˚C. In these conditions the heat pump isn’t working that hard and SCOPs can look impressive.

If we look at an Air Temp of minus 3˚C and a flow temp of 50˚C we get a better indication of how the heat pump will perform in winter.

Vaillant’s new 12KW aroTherm Plus has a SCOP of 3.92 when it’s -3˚C outside and it’s producing water at 50˚C.

This means each kilowatt of electricity used to run the heat pump will provide us with 3.92KW of energy.

Which also means that if our electricity price is 20p per KWh (kilowatt hour) we are in effect paying a touch over 5p per KWh for our heating (20p divided by 3.92). Suddenly we are heating on a par with gas per KWh!

How does a heat pump work in winter when it’s freezing outside?

When it’s below zero we know it’s cold, so how can an air-to-water heat pump take heat out of cold air and provide our heating and hot water?

Well, we humans use the freezing and boiling point of water (0˚C and 100˚C) as a handy way to measure temperature. But for heat pumps we need to think in terms of energy.

The air contains energy at 0˚C and below, indeed it contains energy down to “absolute zero” which is minus 273C – when the molecules stop moving!

So, a heat pump can take energy out of the cold air and keep your house warm, down to about -15˚C, which is why you will find lots of them in the cold countries of Scandinavia. That is why when installing an air source heat pump we fit the type of air source best suited to northern European climates.

Spreading the cost

Obviously at -15˚C they are working like the clappers and can be expensive to run but the majority of our winters here in the UK are between 0˚C and 8˚C and it is very rarely actually freezing during the day.

At lower winter temperatures an air-to-water heat pump will work harder to get your heating and hot water water up to 50˚C plus and your bills in winter will be higher – as with any central heating system.

Conversely in Spring Summer and Autumn when temps are milder or warm, they can work much less hard and are cheap to run. Over the year they can be cheaper to run than an oil boiler and even a gas boiler.

When we do a proposal for installing an air source heat pump, we’ll give you some comparisons of running costs versus your current heating fuel – although much will depend on the relative prices of electricity (to run the heat pump) gas and oil at the time. All three have been volatile of late.

Why Biosun Energy?

  • As an MCS (Microgeneration Certification Scheme) company before installing an air source heat pump we carry out full heat loss calculations and design a heating system to meet your needs. We provide a full Estimate and cost analysis, including predicted running costs, so you can make an informed decision.
  • As an MCS company we can apply for the £7,500 Boiler Upgrade Scheme grant on your behalf.
  • We only install good quality air source heat pumps from leading European manufacturers with the highest efficiencies. They are not air-con units that have been engineered in reverse they are built as heat pumps for European winters.
  • Since they are built with top-quality components we expect our heating systems to last for 20 years – with very little maintenance.
  • They come with a 5-year warranty as standard.

Please contact us for air source heat pump installation.


As a long-standing partner with Vaillant and Stiebel-Eltron, BioSun Energy receives unrivalled design and technical support from both and can therefore offer customers:  

  • Bespoke heat pump and hot water systems, tailor-made to your property and circumstances
  • Excellent design and installation service
  • Full Vaillant and Stiebel Eltron internet service, allowing you to control the heating, ventilation, and hot water in your home – wherever you are.
  • Both internet links also allow the companies to monitor your system and receive alerts should anything not be working perfectly – ensuring we can spot an issue and correct it before it becomes a problem.
  • A full seven-year warranty on Vaillant and Stiebel-Eltron heat pumps

Further help and advice

If you would like to find out more about heat pump systems, please visit John Cantor’s Heat Pumps.

John has been in the heat pump industry for more than 40 years and has an enviable reputation for providing impartial advice on the pros and the cons of heat pump systems, providing warts and all advice in layman’s terms.

Like us he wants to demystify heat pumps and give solid, no-nonsense advice, and is happy for us to refer people to his site.

As John says:

"My site is basically a download of my thoughts and experience gained from forty-two years working with many different types of heat pump. I am not selling anything here and aim to be hype-free and explain in a neutral manner. If I think something won’t work well, I will say so."

"I try to outline the simple basics in a jargon-free understandable manner. So, it should be ideal for beginners to the subject."

"However, I do branch-off into the finer details of various aspects of different heat pump system, and hope there are enough facts and figures to use as reference for those of you actually designing and installing things."

Let's talk about your project

If you have a project in mind or are thinking of installing renewable energy, please contact us for an initial informal chat.