The carbon dioxide produced when we heat our buildings, including our homes, is a major target on the path to net zero. The scale of that challenge can be seen in  figures produced by the UK Climate Change Committee.

Its Sixth Carbon Budget Balanced Net Zero Pathway has residential building emissions in Scotland decreasing to just over 3,000 tonnes in 2050; compared to around 6.2million tonnes today. That’s a 99.95% reduction in CO2 emissions from residential heating.

The relative carbon emissions, therefore, of different heating systems matter hugely - which is one of the reasons there is so much promotion of getting off our fossil fuel, frequently gas, systems and onto electrical systems like heat pumps.  

There are various sources of calculations for how much a heat pump can slash the carbon footprint of a home compared to other heating methods. For instance, British Gas has calculated that if a two or three-bedroom UK household with an average annual energy consumption of 12,000 kWh switches from gas boiler to heat pump, its emissions can be reduced by 1,404 kilograms of CO2 annually.

That, it says, is equivalent to the emissions released by driving 4,544 miles in a typical petrol car, or 3.3 transatlantic flights.

Energy Savings Trust has calculated the kind of savings achieved, and we have used their data to produce a comparison graph here. 

The amount a household’s carbon footprint is slashed by shifting to air-source heat pump would depend on how the system is run, and its efficiency, as well as what the emissions of the fuel and efficiency of the system switched from.

An electric boiler or electric storage heater would also run on electricity with the same carbon intensity - what would determine its emissions is how many units it uses.

There are also plenty of interesting comparisons. British Gas for instance states that the annual emission savings from 600,000 UK homes switching to a Heat Pump (which is the UK target for)  could be equivalent to 7,262 Space X rocket launches, driving the earth’s circumference 7,262 times, or 508,875 drivers switching from petrol to electric.

Nesta has calculated that the average UK gas boiler emits more CO2-equivalent emissions in a year than taking seven transatlantic flights. By contrast, the innovation foundation found that running a heat pump in Scotland for a year is currently closer to the CO2-equivalent of driving Glasgow to Edinburgh seven times. (Based on a heat pump being 100kg CO2/per year, a petrol car with 36 MPG and 1 l of petrol being equal to 2.13kgs of CO2)

“And that number,” it says, “is only going to get smaller over time as we get more and more of our electricity in Scotland from renewables.

The carbon intensity of electricity

The electricity that heat pumps run on is not zero carbon, since some gas-fired power stations still provide energy in the UK, though its carbon intensity is on a downwards trajectory. In 2022  the equivalent of 113% of Scotland's gross electricity consumption was generated from renewable sources.

But since the electricity we use is part of a Briain-wide grid, we can't say that just because Scotland is producing its own energ'ys worth of renewables, we are emissions-free.

Nevertheless what is key for decarbonising heat is the direction of travel. The National Grid ESO published in their Future Energy Scenarios report, this graph modelling different paths and their rates of decarbonisation of electricity.

The Herald: Carbon intensity of electricity under different pathwaysCarbon intensity of electricity under different pathways (Image: National Grid ESO, Future Energy Scenarios report)

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What that looks like in a  real-life

We visited Neil Kitching in his Dunblane home, where he has a  heat pump,  solar panels, three 5kWh batteries, and an electric car.

Mr Kitching has written a book Carbon Choices, and writes a regular blog about his journey in reducing his carbon footprint in which he regularly crunches the data on what really does make a difference. 

“It’s the biggest thing you can do to cut your carbon footprint. “The heat pump takes about a tonne and a half off, which is massive, whereas the solar panels take less than half a tonne off.”

Through electrifying his home he believes he has slashed his household emissions by 71%.  He created this graphic explainer. 

 

The Herald: Diagram showing carbon savings by Neil Kitching through electrifying his heating

What about district heating?

One way of driving down emissions still further, and also heating home that might be less suitable for air-source heat pumps is district heating. For instance, the Queens Quay water source heat pump at Clydebank is quoted as producing a 60% lower carbon footprint than burning gas. 

Carbon reduction engineer at Tunley Environmental, Dr Luan Ho, writing in the journal of the Royal Institution of Chartered Surveyors,  has described networks as able to "provide thermal comfort for both residential and commercial buildings”.

He writes: “By using such a system, annual carbon emissions from heating an average house in the UK are reduced roughly 2.5 and 3.2 times compared with using electric heaters and gas boilers, respectively.”


What about the carbon footprint of making and installing a pump? 

But there is also the embodied carbon and wider environmental impacts of the manufacturing and installation of a heat pump, or other energy source - particularly pertinent when it involves ripping out a gas system to replace with a new heat pump, before that system has come to the end of its life. 

Heat pump systems have substantially higher embodied CO2 than gas boilers, but research shows that over the course of their lifetimes the emissions saved far outweigh those in their manufacture and installation. 

A 2018 paper titled 'The embodied CO2e of sustainable energy technologies' reviewed previous research and listed the following figures for embodied carbon dioxide, finding embodied carbon levels far greater in air source heat pumps than gas boilers. However, it said, "Typically the savings made during the use stage overshadow the embodied impact."

What kind of impact do the high embodied carbon levels have over the lifetime of an installation? One calculation, admittedly by Mitsubishi, found that when embodied carbon dioxide was factored in for a particular gas and heat pump comparison, “the total CO2e of running the Heat Pump system… for 15 years is equivalent to running the Gas Boiler system for less than two years.”