One of the big drivers for taking on a major home building project is to achieve a low-energy, low-bills household. One piece of the puzzle is how you choose to provide central heating and hot water. A popular option for those undertaking an eco build is to install a heat pump – but is it the right choice for you?
These appliances take low-grade energy from the ground or air and convert it into usable, higher-temperature warmth that can be pushed into the living environment and used for space and water heating. They basically work on the same principle as a refrigerator, but operating in reverse.
It takes electric energy to run the pump, so while it does make use of renewable heat, it’s not a completely carbon free system. For each unit of power used by the appliances, you get more than one unit of heat output – and this is what makes them an attractive option.
Heat pumps work best when they’re used to run low-temperature distribution setups, such as underfloor heating (UFH). This is a great option for new homes, and can now be relatively easily installed in many existing buildings thanks to the proliferation of low-profile UFH systems.
This converts your entire floor into a heat emitter; the increased surface area allows for a much lower flow temperature to create a cosy environment – so less energy is required from the pump to raise the warmth of the water. An alternative is to go for oversized radiators.
It’s also possible for heat pumps to use water (from lakes or rivers) as the energy source; but in practice the opportunities to do this are rare. So in this article we’ll focus on air, ground and hybrid appliances.
This type of pump is the most affordable and least disruptive to install. It works by repurposing energy from the external air, concentrating it and outputting it to warm water. It’s an attractive renewable technology for many self builders and renovators, but there are some key considerations to bear in mind before you specify.
While it’s technically possible to extract heat from anything down to absolute zero (-273°C), it takes more and more energy to do this the lower the temperature falls.
The point I’m making, of course, is that ASHPs work most efficiently when the air temperature is higher – which is not ideal if you’re trying to deliver space heating (because you don’t need the extra warmth in summer).
However, they remain significantly more efficient than electrical resistance heating – so if your plot or property is off the gas grid, they can be a very sensible choice.
Because ASHPs do not perform quite so effectively when the air temperature is very low (in the coldest parts of winter when you really want the heating on), there is growing interest in hybrid heat pumps. These include a fossil fuel burner (mains gas, LPG or oil) that takes over when the air temperature drops.
The system automatically selects which mode is the most efficient to use. In this scenario, the ASHP component would be used mostly during the shoulder months of the heating season – ie at either end of winter – to deliver efficient, cost-effective warmth.
Dig down slightly into the earth, and the temperature is about the same as the average in the air. Over the course of a year, this approximates to 12°C in the UK (slightly higher down south and slightly lower in sunny Scotland).
A GSHP cashes in on this by extracting warmth via a collector circuit that’s dug into the earth. Of course, the ground and the air are both heated by the sun – so GSHPs are actually making use of solar energy.
How eco-friendly is a heat pump?
The baseline when looking at how sustainable a renewable heat source will be is to compare the predicted carbon emissions with those of using mains natural gas.
In the case of a heat pump, it’s not possible to achieve any savings unless the actual Seasonal Performance Factor (SPF) of the appliance is well above 2.5. This is because electricity creates approximately two-and-a-half times the CO2 emissions of gas per kWh.
Over the next two decades, the UK government plans to decarbonise the National Grid by increasing the proportion of mains electricity that is generated from nuclear and renewable sources, rather than fossil fuels such as coal and gas. So it’s possible that heat pumps may eventually become more effective in reducing carbon emissions. However, at the moment the potential savings are minor.
On top of this, installing a ground source heat pump has a much higher environmental impact than fitting a gas boiler. And as we drive down heating demand by improving the insulation and airtightness of our buildings, this embodied effect becomes just as important as impact in use.
If mains gas is not available for your plot or property, and sustainability is important to you, then it’s interesting to note that the form of heating with the lowest emissions is normally biomass (burning logs, wood chips or pellets). This holds true as long as the woodlands where the fuel is sourced are replanted and responsibly managed.
However, heat pumps do deliver significant savings in emissions and other pollutants when compared to oil – and a vast improvement compared to storage heaters or other electrically powered options. As with running costs, when off the gas grid, the embodied impact of installation is better than the alternative boiler systems (oil, LPG or biomass).
Most domestic ground source heat pumps use closed-loop heat collector systems. With this method, a collector fluid (a mix of water and antifreeze) is pushed around a sealed loop, drawing in warmth and returning to the pump at a higher temperature.
It’s then compressed to raise the temperature even further, before the warmth is transferred into the distribution system (usually UFH). The pressure in the loop is then reduced (the fluid is expanded), thus reducing its temperature before it returns to the underground collector.
Where there’s sufficient land available, a horizontally-laid loop is used, with the pipes fitted in trenches approximately 1.5m below ground level (usually in a slinky format to get sufficient surface area). This is disruptive work, so it’s best suited to new builds where the garden is not yet established. If space is tight, a vertical collector loop can be installed in a borehole, typically running between 50m and 90m deep.
The effectiveness of a heat pump is important because this will determine whether you can save money or reduce your household’s carbon emissions in comparison to using other methods of heating your home.
The ratio of how many kilowatts (kW) of heat energy are produced by the system for every single kW of electricity needed to power the pump is known as the coefficient of performance (COP). If a system runs at a COP of 3, then it produces 3kW of heat for every 1kW of electricity input. Crucially, however, efficiencies change over the course of a heating season as temperatures vary outdoors.
When summed up over this entire period, the ratio of total energy input to total heat output is known as the seasonal performance factor (SPF). The SPFs quoted by some manufacturers can be higher than what’s achieved in practice, as it’s the efficiency of the whole system that matters.
Published research on actual installed heat pump effectiveness puts the average figure at around 3 for GSHPs and approximately 2.5 for ASHPs. Performance is also dependent on the amount of domestic hot water being delivered by the heat pump.
To supply this at the required 50°C-60°C, the appliance will work much less efficiently, as it has to raise the temperature far higher than the 35°C-45°C needed for space heating. Even at the lower limit, it will do a better job than an immersion heater (which is simply electrical resistance heating, the most expensive and highest-emission way to warm a living space).
Heat pumps are similar in running costs to mains gas boilers. They compete well with LPG, oil or biomass, too, making them a sound choice for off-grid situations.
To illustrate the point, electricity costs about three times as much as mains gas per kWh. So if your system has an SPF of 3, you’ll get three units of heat from the appliance for the cost of one unit of electricity (currently about 15p). By comparison, natural gas costs around 5p per kWh, so three units of heat would also come in at 15p.
The potential game-changer in terms of running costs is the Renewable Heat Incentive (RHI). This is a government-led cashback scheme whereby quarterly payments are made directly to homeowners for qualifying installations over the course of seven years. Of the options mentioned here, only biomass boilers and heat pumps are eligible for the subsidy (which is funded through general taxation).
There are a number of hoops you need to jump through to ensure your installation is going to be eligible for the RHI, including using only products and installers certified by the Microgeneration Certification Scheme (MCS).
On 22 May 2018, a new eligibility stipulation was introduced that now requires heat pumps to be fitted with meters to monitor the electricity used – although this currently has no impact on the rate paid. Visit for an in-depth guide.
For domestic RHI installations, the amount of heat generated is not measured but deemed. This means it’s based on the household’s estimated annual space and water heating need stated on your Energy Performance Certificate, adjusted by the SPF. So the amount of energy you use makes no difference to the money you receive.
To limit exposure for the taxpayer, payments under the RHI are now capped by an annual heat demand limit as follows:
(for eligible installations before 30th September 2018)
Annual heat demand limit
A ground source heat pump with an SPF of 3.5 that qualified for the maximum heat demand would generate a subsidy of £4,384 per annum. Over the seven years of RHI payments, this would more than offset the capital price of installing the GSHP (which is likely to cost at least £12,000 due to the expense and disruption that will be involved in installing the ground loop).
Bear in mind, however, that most new homes should not have such a high heat demand, as they will be highly insulated – plus you’ll have to offset the price of the electricity used to run the pump.
To stay up-to-date on RHI tariffs and regulations, check out the Ofgem website. For comparison, ASHPs are much cheaper to install than GSHPs (hence the lower tariff), but still considerably more expensive to fit than gas boilers.
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