4: Decarbonising domestic heating and cooking

This week’s entry looks at the challenge of decarbonising heating and cooking in Britain’s homes.

The scale of the issue

The 2020 figure for CO2 emissions from houses, which is predominantly gas for heating and cooking, amounted to 67.7 million tonnes, over a fifth of the overall total emissions for the UK. The figure increased slightly from 2019 to 2020 as more people stayed at home due to the pandemic.

There are an estimated 26 million gas boilers in homes in the UK. The government has announced that all new homes will not be allowed to have gas boilers from 2025, but that doesn’t deal with any of the current 26 million. On the other hand the first ‘key principle’ of the Net Zero Strategy is ‘no one will be required to rip out their existing boiler or scrap their current car’ (or, as the Prime Minster put it more colourfully ‘the Greenshirts of the Boiler Police are not going to kick in your door with their sandal-clad feet and seize, at carrot-point, your trusty old combi’). How will that circle be squared?

Before that, what are the alternatives to a domestic gas heating system? There are four main options.

One is simply to heat from electrical appliances directly, eg bar heaters, oil-filled plug-in radiators, and the like, just as if you were reversing the installation of central heating. Installation costs are fairly low, but running costs are the highest (9-16p per kilowatt hour).

Secondly, a heat pump, which is like a reverse fridge that, like a fridge, runs on electricity, but moves the heat from the colder to warmer area rather than the other way round. For an air source heat pump, the most common, there is a unit that goes outside that is about the size of a large air conditioning unit, and piping runs into the house to heat radiators and the water supply. There is also a more expensive ground source version that runs coils underground to extract the heat. Because heat pumps aren’t as powerful as burning gas, you may need bigger radiators and a bigger hot water tank, and they work better if your house is well insulated so that may need to be improved too (no bad thing). Also, it gets harder for the system to extract heat the colder it is outside, so it is less efficient just when you need it more. Running costs are about the same on average as gas central heating (4.6p per kWh), but installation costs are high, between £5k and £15k. Plus, you need outside space for the kit, which millions of people simply won’t have.

Installation costs are currently not subsidised by the government, although they will be from April 2022 by up to £5,000 each – so there will still be a shortfall but it will be supposedly cheaper than installing a new gas boiler. Until then, payments are currently available for the running of the system under the Renewable Heat Incentive (RHI), where you get up to seven years’ payments for the renewable heat you have generated (this will close on 31 March). Some companies will pay for the installation if they get your RHI, if installation costs are an issue.
Those first two options are only zero carbon when the electricity being used is zero carbon, of course, so you will need to be on a zero carbon tariff, have solar panels, or something equivalent.

The third option is connecting to a district heating system. This is a multi-property central heating system usually run by a local authority and powered by various means such as a power station, a combined heat and power (CHP) plant or a biomass plant. Instead of a boiler, a property would have a heat exchanger, which is about the same size. As an individual home-owner you would be dependent on whether there was a district heating system in your area, but you could apply pressure to your local authority to provide one, especially if there was an obvious source. There are government webpages about this but they don’t look very recent.

This method will only be zero carbon if the generator of the heat is zero carbon – the examples will need to have their carbon captured if they are not directly renewable methods such as solar or wind.

Finally, there is hydrogen. Theoretically this could replace natural gas for heating and cooking, but actually doing so is some way off. Existing gas boilers can mostly already cope with 20% hydrogen in the mix, which may be described as ‘hydrogen blend-ready’, and this is expected to be introduced in 2028 at the earliest. 100% hydrogen (or truly ‘hydrogen-ready’) boilers and cookers do not exist yet but are in development. Hydrogen is likely to need a smell added, like natural gas, but also a colourant, because its flame is nearly invisible.

The other issue with hydrogen being supplied to homes is that existing gas pipelines cannot take 100% hydrogen, because they become brittle and crack (so-called embrittlement) and the surrounding infrastructure needs to change – although a 20% hydrogen mix would be OK. The law would need changing even for that, though, because the Gas Safety (Management) Regulations 1996 currently forbid more than 0.1% hydrogen at the moment. A 20% blend was successfully trialled in September – see here.

The gas transmission and distribution networks can be repurposed by converting the pipelines (which is cheaper than installing new ones), but only where there are alternative supply routes, so supplies don’t get cut off; where there aren’t, new pipelines will have to be provided. Hydrogen could be supplied by lorries, but that won’t really work at scale.

Those are the scenarios. Will they achieve net zero? Gas boilers are supposedly replaced every 10-15 years, so if new gas-only boilers will be forbidden from 2025 for new-build properties and 2035 for existing properties, these should get replaced by heat pumps and hydrogen boilers by 2050 – just in time. In reality, though, I suspect boilers are like mattresses and toothbrushes, we don’t replace them as often as manufacturers recommend (who after all, have a vested interest, we tell ourselves) and there will be many very old ones. The incentives will have to get more and more powerful (compare and contrast with the Ultra Low Emission Zone in London, discouraging older diesel use in particular) or the Boiler Police may yet be called upon.