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Heat Pumps: What Type of Heat Pump is Right for Your Project?

air source heat pump
(Image credit: getty images)

If you're thinking of installing a heat pump in your home, then there is a lot to consider and specialist advice to be sought to ensure you choose the right type of heat pump for your home.

Heat pumps (both air-source and ground-source) are established as the go-to solution for those off mains gas looking for a renewable alternative to oil:

  • They are a ‘clean’ energy
  • Are a fit-and-forget technology.

The general level of understanding on the part of installers and wider industry when it comes to heat pumps has come on in leaps and bounds in recent years, although many homeowner find them quite difficult to understand.

Finding a good installer is absolutely critical to the success of the entire venture.

The Government incentives, including the Green Homes Grant, which will remain in place until March 2022, have moved in heat pumps favour, too. The latest round of changes to the Renewable Heat Incentive (RHI) means that the tariffs are:

  • 10.85p/kWh for air-source heat pumps (capped at 20,000kWh per year)
  • 21.16p/kWh for ground-source heat pumps (capped at 30,000kWh per year)

The RHI is payable for seven years from the date of installation.

(MORE: Get a quote for your heat pump)

Heat Pumps: Quick Facts

We all own a heat pump in our home already, in the shape of a fridge. Heat pumps simply move heat from one place to another. There are two main types used in UK homes: 

With a ground source heat pump, pipes extract latent heat from below the ground (either in trench-based ‘slinkies’ or more expensive boreholes).

  • Typical cost: £15,000 to £16,000
  • Running costs for four bedroom house: £500 a year

With an air source heat pump, a unit similar in look to an external air conditioning kit extracts heat from the air and runs it through a heat exchanger.

  • Typical cost: £10,000-£11,000
  • Running costs for four bedroom house: £703 a year

air source heat pump

Air source heat pump (Image credit: getty images)

As a general rule, heat pumps work best when flow/return temperatures are lower than the 60-70°C range required for older radiators and, of course, hot water.

For anyone building from scratch off mains, especially if they are planning to use underfloor heating, a heat pump should be a serious consideration.

Editor's Note: Homebuilding.co.uk partners with the UK's best heat pump specialists to match your requirements with their products and services. Simply answer a few questions on what you need from your heat pump and we’ll put you in touch with a suitable partner.

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Calculating Performance of a Heat Pump

The performance of the heat pump after installation will be expressed as a Seasonal Coefficient of Performance (SCOP). This number is a factor not just of the kit but of the specific design (e.g. use of underfloor heating, etc.).

Typically, air-source heat pumps would expect to enjoy a SCOP of around 3.2 (i.e. 1kW of electricity creates 3.2kW of heat) and ground-source more like 4. As electricity, used to power the pump, is around three times as expensive as gas, heat pumps are usually not viable for those properties on mains gas.

How Do I Choose the Right Size Heat Pump?

Getting the size right is critical, but the standard plumber is not equipped to calculate what the right size is. They have never had to do it because a conventional boiler is far more tolerant.

It needs a specialist heat pump installer, backed-up with good manufacturer software, to get the calculation right. Judging an installer’s attempts at sizing your heat pump correctly is a great way to start understanding whether they are the right installer for you.

An oversized heat pump is outputting more heat it needs to operate for a shorter period of time to reach the desired temperature — this is called short-cycling. An undersized heat pump will be working at its maximum, and least efficient, output for most of its operational time.

Are Other Types of Heat Pump Available?

Inverter Heat Pumps

Until relatively recently non-inverter heat pumps were the norm. They operate by being either on or off. 

Inverter heap pumps act more gradually, increasing or decreasing output to meet the prevailing conditions and demand. 

Each time a heat pump starts up it consumes energy in balancing the pressure in the system and bringing the refrigerant to the right temperature before the heating process can start. 

An inverter heat pump can operate 24/7, never or seldom switching off, eliminating the losses caused by stopping and starting. A continuous 24/7 operation may seem odd to those of us used to being in control of the boiler, but some manufacturers are claiming up to 30% reduction in running costs by using this method.

It also allows the homeowner to use an app on a smartphone or tablet to adjust the temperature in the house rather than just simply switching the heating on or off.

Hybrid or Bivalent Heat Pumps

A hybrid heat pump is one with two heat sources — a heat pump and a conventional boiler, for example.

The ideal system would be:

  • an air-source heat pump that operates in spring, summer and autumn, when the weather is mild (highest efficiency)
  • and a conventional boiler that operates in winter (at good efficiency)

Heat pumps now have weather compensation as standard and so can be programmed to shut down when the outside temperature drops below 7°C (when it starts to get inefficient), and allow the gas or oil boiler to kick-in.

What Type of Heat Pump Should I Choose?

Used in the right way, in the right place, air-source heat pumps provide a good, viable and cost-effective option. Many of the newer models are much quieter than earlier models, which had a reputation for being noisy.

Ground source heat pumps are expensive to install but they offer the highest RHI return and the lowest running cost of any renewable energy. Systems are generally reliable and long-lasting. A good, qualified, experienced installer will ensure the ground source system is designed to properly meet the ground conditions, and the demands of the house, and will run happily for 20 to 30 years.

Tim Pullen

Tim is an expert in sustainable building methods and energy efficiency in residential homes.