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:

  • Heat pumps are cheap to install (relative to other renewables and not much more expensive than oil boilers)
  • They are a ‘clean’ energy
  • Are a fit-and-forget technology.

The general level of understanding on the part of installers and wider industry 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, which will remain in place until March 2021, have moved in their favour, too. The latest round of changes to the Renewable Heat Incentive (RHI) means that the tariffs are:

  • 10.18p (p/kWh) for air-source heat pumps
  • 20.46p (p/kWh) for ground-source heat pumps.

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

lake with a heat pump in it for a Grade II listed farmhouse

Keith Clarke replaced his gas boiler with a Kensa 24kW Single Phase Twin Compressor to harness heat energy from a nearby lake to provide space heating and hot water to his farmhouse

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:

Ground-Source

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

Air-Source

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

As a general rule, heat pumps work best when flow/return temperatures are lower than the 60°C to 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, they should be a serious consideration.

It is clear that heat pumps are here to stay and will continue to be sold. So how do you get it right, and what are the latest things to look for?

Calculating Efficiency

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.

Highfield Cowper House plant room for heat pump

Steven Bowes decided on a ground-source heat pump with solar thermal panels for his off mains barn conversion. The NIBE system has reduced his bills by two-thirds compared to oil. 

What is an Inverter Heat Pump?

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.

What are Hybrid Heat Pumps?

A hybrid heat pump (sometimes known as a bivalent system) 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 (low capital cost) that operates in spring, summer and autumn, when the weather is mild (highest efficiency)
  • and a conventional boiler (also low capital cost) 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.

Conclusion

Air source heat pumps

Used in the right way, in the right place, ASHPs 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

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 GSHP system is designed to properly meet the ground conditions, and the demands of the house, and will run happily for 20 to 30 years.

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