The trouble with heat pumps

"Heat pumps don't provide as much heat as boilers."

"If you get a heat pump you'll need bigger radiators."

"Heat pumps are too expensive."

You have probably seen and heard comments like these lately from people who know something about home heating, and what they say has some truth to it, but there's telling the truth and there's telling the whole truth.

Boilers do have a bigger heat output than heat pumps. Boilers are typically from 25kW up while heat pumps may be 5-15kW. But how much heat do you need. A modest size well-insulated house might only lose around 2kW even when it's well below freezing outside, and even most bigger, older homes are unlikely to need more than 10kW.

Let's take a heat load of 5kW in the coldest weather for a typical house. Over 24 hours this is 120kWh of energy. A 25kW boiler can provide this amount of energy in less than five hours and can circulate very hot water round the radiators to heat the space reasonably quickly. A 5kW heat pump, though, would need to run all 24 hours and would not get the water as hot as a boiler. This is where the 'bigger radiators' comment comes from. But though the radiators wouldn't be as hot, they will be warm all day and not just five hours. This is in the coldest weather - usually less than two weeks a year. Most of the year, heating systems are running well below capacity if at all.

Both systems provide 120kWh of heat on the coldest days. How much energy do they need to do this? The latest gas boilers are very efficient - around 90% - so will use perhaps 135kWh of gas. Heat pumps use electricity (and a little magic) to borrow heat from the outiside air, the ground, or occasionally a watercourse, raising its temperature to heat the home. They can provide around three times as much energy as they consume - 300% efficiency - so 120kWh output would need only 40kWh of electricity. Leaving aside the matter of the climate - the reason we are being encouraged to switch to heat pumps - and the pros and cons of burning fossil fuels or using clean renewable electricity, heat pumps are way more energy efficient. What about cost? Gas boilers have have been sold in vast numbers for many years and are pretty cheap, but heat pumps are relatively new and there aren't many around yet, so they are relatively pricey, but as numbers increease prices will fall. Gas is very cheap in the UK - currently less than 4p/kWh - while electricity is overpriced at around 16p/kWh. This gives us <£5.40 for gas or £6.40 for electricity.

So there we have it. The trouble with heat pumps is that they are not a simple 'fit and forget' replacement for boilers. They are best in homes designed for them - well insulated, airtight and with underfloor heating - though they are capable of heating older houses with radiators provided they are used in the right way: always available and controlled by thermostat to keep homes at constant comfort temperatures.

The trouble with many sceptics is that they cannot think beyond on/off boilers and red-hot radiators, and need to adjust to other ways of delivering heat.

And the trouble with energy prices is that climate-destroying fossil fuels are much cheaper per unit of energy than electricity - increasingly provided by renewable sources. Fossil fuels must be taxed to both incentivise and finance a transition to clean, renewable energy.