A Load of Hot Air?

Clean energy will make us heat pump fans

In his spring statement in March 2019 the then Chancellor, Philip Hammond, announced that in all new homes built after 2025 gas boilers would be replaced by low-carbon heating systems.

This was a bit of a jolt for the housebuilding and heating industries. Eighty-five per cent of all UK heating systems run on gas and around 1.5 million gas boilers are installed every year. Low-carbon or renewable heating systems account for just 2% of the market.

The year 2025 may seem comfortingly distant, but to a selfbuilder about to begin a plot search those intervening years can zip by. It’s well worth, then, considering your future heating options from the start, not least because they could have implications for the design of your new home, its construction method and even its location.

So what exactly does the government mean by non-carbon heating methods? A good place to start is with the government’s Renewable Heat Incentive (RHI). This encourages householders to install renewable heating systems rather than cheaper conventional systems by paying a quarterly tariff for seven years after installation. It covers solar thermal panels, heat pumps and certain biomass systems.

Biomass systems burn wood chips, wood pellets or logs – useful if you have a rural plot, ideally with or close to available woodland. Wood pellet stoves with back boilers can power a central heating system, be fed automatically and ignited via a thermostat or timer, just like a gas boiler. Biomass boilers are larger, requiring a dedicated space for a hopper and fuel storage.

Because they operate most efficiently at high temperatures for continuous periods, they are often combined with a large, well-insulated thermal store (basically a super-sized hot water cylinder) to soak up surplus heat. But that adds to the space required as well as the initial cost, which can range from £7,000 to £25,000.

This is one reason why government has pinned its renewable hopes on heat pumps. A heat pump is effectively a fridge in reverse. But instead of extracting heat from its interior and releasing it outside, a heat pump extracts heat from the outside world and releases it into the home.

The only power it requires is a domestic electricity supply. Units are easy to install, low maintenance and can last over 20 years. But where exactly does the heat come from, especially in winter?

There are two main sources. The most efficient is the earth itself. A metre and a half down its temperature remains a constant 10°C to 12°C throughout the year. Less efficient is the atmosphere because its temperature fluctuates constantly.

Ground source heat pumps (GSHPs) and air source heat pumps (ASHPs) are similar in cost – starting from around £7,500. But ground source installation costs can be double that. This is because the underground pipework required needs enough space to extract ground heat in the most efficient way. Kaspar Bradshaw, project engineer at low-energy specialists Enhabit, suggests around an acre for a standard detached house.

For a smaller plot a single borehole can be used, but the cost can double again for drilling a typical 80-90m deep hole.

Air source heat pumps, however, can be fitted on an external wall, a roof or even at the end of a garden. All they require is clear space around them so that air can circulate freely.


'Free' energy

For every kilowatt of hopefully renewable electricity used to power a heat pump, between two and four kilowatts of ‘free’ energy is extracted from the earth or the air. It’s a level of efficiency unmatched by even the most energy-efficient gas boiler.

But there are provisos. The most energy is extracted when interior and exterior temperatures are closest. In cold weather an air source heat pump needs to work harder to extract heat from outside.

It’s also most efficient in producing low-temperature heat – typically 35°C to 50°C – rather than the 70°C to 80°C produced by a gas boiler. As a result, it best suits a well-insulated house with good airtightness and a low-temperature distribution system, either underfloor heating or over-large radiators.

Heat pumps received their biggest boost this May with the announcement of the government’s Clean Heat Grant Scheme. Beginning in 2022, it proposes swapping the Renewable Heat Incentive for a one-off £4,000 grant, covering heat pumps and biomass boilers where a heat pump isn’t suitable.

Darren McMahon, marketing director of Viessmann, which manufactures both heat pumps and gas boilers, sees this as a key factor in helping the transition to heat pumps.

“It’s much more attractive than the Renewable Heat Incentive,” he says. “Retrospective payback doesn’t really work for anyone who can’t afford a heat pump in the first place.”

He predicts the UK heat pump market could triple by 2025, but argues the transition may well be eased by hybrid systems where a heat pump works in tandem with a gas, LPG or oil boiler, either as a combined unit or as a bolt-on to an existing system.

“The heat pump is the lead technology,” explains McMahon, “providing heat and hot water until it becomes inefficient and the conventional system takes over.”

Last year for the first time renewable and low-carbon sources produced more electricity in the UK than fossil fuels. As their contribution rises, electricity prices are likely to fall and the popularity of heat pumps to grow, creating economies of scale and, hopefully, lower prices.