Heat pumps are now a long-established technology in the UK. The general level of understanding on the part of the installers and wider industry has come on in leaps and bounds in recent years — but for the homeowner they remain quite difficult to understand. This results in a huge reliance on the installer to give correct advice, which means finding a good installer is absolutely critical to the success of the entire venture. For the most part, 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.

And quite rightly so. They are cheap to install (relative to other renewables and not too much more expensive than oil boilers), are ‘clean’ and, above all, are a fit-and-forget technology. Not only has the general expertise among installers improved but the Government incentives have moved in their favour, too.

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

Heating a Period Home: 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 Grade II listed farmhouse

The latest round of changes to the Renewable Heat Incentive (RHI) on 1 October 2015 brought little actual change from the existing (positive) position. The tariff for air-source heat pumps remains at 7.42p, ground-source at 19.10p and solar thermal at 19.51p. Only biomass has been affected with a cut to 6.43p. But the generally accepted wisdom in the industry is that the RHI for all renewable heat sources will end, if not in April 2016 then April 2017. The Department of Energy and Climate Change (DECC) have said that the gradual degression of tariffs will allow the budget to remain until April 2017.

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 and there are two main types used in UK homes:


Pipes extract latent heat from below the ground (either in trench-based ‘slinkies’ or more expensive boreholes).

Typical cost: £10,000-£15,000


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: £6,000-£8,000

The general problems with heat pumps have been in poor design installation; effectively, the wrong heat pump in the wrong house. 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. But for anyone building from scratch off mains, they should be a serious consideration.

Mitsubishi Electric have been selling heat pumps in the UK for 10 years. They are, in terms of number of units sold, the biggest supplier in the UK. They sold air-source heat pumps before the RHI was introduced and are confident they will continue to sell them after the incentive ends. Ben Hodges of Source Energy – one of Mitsubishi Electric’s biggest installers – says: “Installing an air-source heat pump has always been as much an emotional decision as a commercial one.”

Whatever the motivation, it is clear that heat pumps are here to stay and will continue to be sold. It is also clear that the demise of the RHI scheme will see a shake-out of the – shall we say – less than committed suppliers. So how do you get it right, and what are the latest things to look for?

Inverter Heat Pumps

Until relatively recently inverter heat pumps were the exception. Non-inverter heat pumps were the norm and 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 counterintuitive to those of us used to being in tyrannical control of the boiler, but some manufacturers are claiming up to 30% reduction in running costs by using this method of operation. It also enables effective remote control; using an App on a smartphone or tablet to adjust the temperature in the house rather than just simply switching the heating on or off.

Calculating Costs and Efficiency

Heat pumps enjoy a payment through the Renewable Heat Incentive (RHI) which, as mentioned elsewhere, is likely to be reviewed by the Government shortly.

Typically, on a 20,000kWh annual heat demand, an air-source installation, costing around £6,500, would expect around £1,400 in annual payments (every year for seven years). A ground-source heat pump, costing closer to £15,000 to install, would expect annual payments of £3,820 (again, for seven years) — both clearly more than pay their own capital cost back.


The performance of the heat pump after installation will be expressed as a Seasonal Performance Factor (SPF). 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 around 2.8 (i.e. 1kW of electricity creates 2.8kW 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.

The Right Size Heat Pump

The effect of an oversized heat pump is similar to that of a non-inverter heat pump; because it is outputting more heat it needs to operate for a shorter period of time to reach the desired temperature — this is called short-cycling. Equally, an undersized heat pump will be working at its maximum, and least efficient, output for most of its operational time.

Getting the size right is therefore critical and the problem is that 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.

Highfield Cowper House plant room for heat pump

Ground-Source Heat Pump Installation: Steven Bowes decided on a ground-source heat pump combined with solar thermal panels for his off mains barn conversion in North Yorkshire. The NIBE system has reduced his bills by two-thirds compared to what they would have been if he had chosen oil. Thanks to the RHI, the system will pay for itself in seven years

Bivalent Systems

Bivalent is a bit of jargon for a system with two heat sources — a heat pump and a conventional boiler, for example. “We can design a bivalent system to get 85% of the RHI with 50% of the capital cost,” says Max Halliwell, Business Development Manager at Mitsubishi Electric.

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 boiler to kick-in.

The Issues

Heat pumps have a very good level of success, but where they tend to disappoint in installation (usually resulting in higher-than-expected electricity bills) it is down to two reasons:

Poor Design

Installing only to the right kind of property. Heat pump manufacturers take a somewhat broader view of what constitutes the right kind of property but there is agreement that installing to the wrong kind of property leads to a bad reputation.

Poor Usage/Controls

Users need to know how to use a heat pump. It is not like a gas or oil boiler and you need to read the manual, watch the DVD, pay attention during the training and learn how to operate the heat pump in the most effective and efficient way.


It is fair to say that heat pumps do not have the best of reputations. It is also fair to say (and supported by an Energy Saving Trust field trial) that most of the problems can be traced to poor installation and inappropriate use. The industry is dealing with the issues with solutions such as quieter machines, an electrical start-up load that means they could run on a solar PV system, better training for installers and end-users, and with technological innovation.

The industry wants heat pumps that have wide appeal and that are justified in their own right, without recourse to Government support or to an emotionally motivated decision. Looking at the heat pumps now on offer and the competence of the dedicated installers, they seem to have achieved it.

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