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Heat Pumps (referred to as ASHPs): Air to Air vs. Air to Water

Heat pumps (ASHP) have been getting a lot of press recently, but most are still a bit of a novelty in the UK. In 2019 - the most recent year for which statistics are available - there were 38m of these devices across the EU. But, while Italy had nearly 20 million of them, the UK had less than a quarter of a million!

Given that UK heat pumps are thin on the ground, it's likely that you've never seen one at work. But you've probably seen an air conditioning unit (Air to Air heat pumps).

We wrote this article to help you jump from familiar to unfamiliar technology.

How is a heat pump like an air conditioner?

The devices don't chill or heat the air. Instead, they move heat from one place to another.

It's a subtle distinction, but an important one. On a hot day, your office air conditioner takes heat from inside the building and moves it to a heat exchanger on the outside. A heat pump uses the same method to move heat from outside the building to inside. The two devices work in much the same way...and sometimes look so similar that it takes an expert to tell them apart!

For both devices, they use a metric called 'CoP' (coefficient of performance) which indicates how efficient they are at moving conditioned air, either heated or cooled, around. We'd expect a modern heat pump or air conditioner unit to operate with a CoP of between 4/3 to 1 – so a heat pump uses less than a third of the energy that you'd burn in a gas boiler to get the same amount of warmth.

No wonder everyone's getting interested...

How is an air-to-air heat pump different from an air conditioner?

Although air source heat pumps look very similar to air conditioners, their version of the heat-moving trick is cleverer. They use technology borrowed from kitchen fridges to reverse the usual flow of heat. In winter, a heat pump scavenges heat from the cold outside world to further warm your home or office.

The kicker is that CoP drops very slightly when and if the outside temperature drops below -15 degrees. As a result, air source heat pumps get can become marginally more expensive to run as the temperature falls, although you can help them along by installing top-notch insulation. (That's why we at Cold Control are so keen to check your home for drafts and cold spots before we carry out a heat pump installation.)

Classifying air-source heat pumps

You need to be aware of two versions of the air source technology - air-to-air (ATA) and air-to-water (ASHP).

Air-to-air heat pumps are the ones that look exactly like air conditioning units. They provide space heating by blowing out warm air, and in most cases, can be switched to cooling duties during the summer months. They're easier to install, relatively inexpensive and highly effective if sized correctly in all properties.

But an air-to-air heat pump cannot be used for heating water.

If you want to use heat pump technology to warm house and you want to take care of hot water as well as heating, you'll need to look into air-to-water options.

An air-to-water heat pump takes energy from outside and uses it to heat water for radiators, underfloor heating and for your kitchen and bathroom. In fact, it behaves rather like a gas boiler...although you'll probably need to install new, larger radiators to get the best out of it.

As you'd expect, an air-to-water heat pump installation is an expensive process, especially if you choose the 'bells and whistles' option and integrate a conventional boiler into the system as a backup.

But, there are government initiatives to help towards to cost of your new system. Click here to find out more.

Whether you opt for an air-to-air or an air-to-water system, an air source heat pump is a long-term investment that will save significant amounts of money over the years and decades to come.

To learn more, give us a ring on 0333 305 0573 or email


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