What is the environmental impact of replacing a functioning furnace with a heat pump?

Question

It's generally accepted these days that electric vehicles are better for the environment than similar gas-powered models. Despite that, I have heard (and this very site confirms) that the most environmentally friendly option of all is to keep an existing car on the road, regardless of its power source, because a few years of emissions benefit are outweighed by the environmental impact of manufacturing any new car.

Does similar logic apply to home heating systems? I currently have a furnace that burns natural gas, and have been planning to switch to a heat pump. I want to know if I should do it ASAP or wait until the furnace fails. I'm asking purely from an environmental perspective, not taking into account things like money (e.g. the cost of gas) or comfort (e.g. the potential of not having heat for a few wintry weeks). The furnace is currently in good condition, if that makes a difference.

My searching only turned up one web page about this question, which leans towards not waiting, but it's by an HVAC contractor, not a neutral source.

Answer 1

To use as similar logic to that of whether to replace a car, we need to know the current operational emissions of your gas boiler, the embedded emissions of a new heat pump, and the operational emissions of the new heat pump.

In this meta-analysis of embodied emissions in low-carbon technologies they give a figure of around 1.5 tonnes CO2e for a domestic air source heat pump from the UK.

Nesta reckon an average UK gas boiler emits 2.2 tonnes CO2e per year (this will obviously be less if you're more eco-conscious than average, which presumably you are if you're on this site).

Ovo seems to think 10,000 kWh a year of heat energy per household is a reasonable guess so we'll go with it. Assuming a slightly pessimistic COP of 2.8 for your heat pump, you'll need 3,570 kWh of electricity for your heating. The average emissions intensity of the UK national grid is about 250 gCO2e/kWh, so that's less than a tonne of CO2e per year. (And obviously the hope is the grid gets cleaner every year.)

These are rough numbers, but it means the "payback" time for investing in a heat pump is about 1 or 2 years, much less than the payback for switching to an EV.

Part of the difference comes from just how massive and complicated cars are, and on average getting heavier all the time. So they need a huge amount of materials to make, which leads to such a massive embodied footprint.

Obviously you'll make the rough calculations much better if you can fill in the numbers I've guessed, or use better sources.

---

One thing to note though, if you do buy a heat pump be sure to dispose of it properly at the end of its life. Otherwise if the refrigerant gases are allowed to leak their potent greenhouse gas effects will more than cancel out any savings you will have made during its life.

Answer 2

Despite that, I have heard (and this very site confirms) that the most environmentally friendly option of all is to keep an existing car on the road, regardless of its power source, because a few years of emissions benefit are outweighed by the environmental impact of manufacturing any new car.

Indeed. Few years of emissions on the road corresponds to the emissions of manufacturing a new car. But cars aren't on the road for just "few" years. In the country where I live (Finland), cars are generally scrapped at the age of about 22 years old. So scrapping an old internal combustion engine car and replacing it with a new electric car makes a lot of sense from the emissions standpoint. Unfortunately, it generally doesn't make sense from the economic standpoint, so that's why everyone keeps on driving on old cars burning fossil fuels.

Does similar logic apply to home heating systems? I currently have a furnace that burns natural gas, and have been planning to switch to a heat pump. I want to know if I should do it ASAP or wait until the furnace fails.

Similar to cars: ASAP, since the emissions here are almost entirely usage and not manufacturing. The other answer gives the exact numbers. Also, natural gas isn't such a clean fuel as we are led to believe; while its carbon dioxide emissions per unit of energy are lower than for example coal, the infrastructure leaks in a major manner and the methane that natural gas is, is a very potent greenhouse gas.

Something you might want to consider: while ozone-depleting refrigerants are banned, some of the hydrofluorocarbons that replaced them still are very potent greenhouse gases. You might want to select a heat pump that uses a refrigerant with very low global warning potential. Such refrigerants include propane, butane and isobutane -- and carbon dioxide too.