Air conditioning and carbon emissions

Air conditioning doesn't burn anything in your home, but it still has a carbon footprint. It comes from the electricity the system runs on, and from its refrigerant if that ever escapes. Understanding both helps you judge a system's effect on the climate and what you can do about it.

The two sources of emissions

Engineers sometimes call these indirect and direct emissions. A full picture would also include making the equipment and dealing with it at the end of its life, but electricity and refrigerant are the parts you can influence.

Emissions from electricity

The carbon from electricity depends on two things: how much you use, measured in kilowatt-hours, and how much carbon went into generating each kilowatt-hour. That second figure, the carbon intensity, isn't fixed. It changes through the day and across the year, depending on how much of the electricity is coming from wind, solar, nuclear, gas and other sources.

To estimate emissions:

• energy used (kWh) × carbon intensity (grams of CO2 per kWh) = emissions (grams of CO2)

Because carbon intensity varies, any single figure is an average. Some energy suppliers and apps show the grid's carbon intensity as it changes, which lets you see when electricity is cleaner.

Most ways to reduce emissions from electricity also cut running costs:

• choose an efficient system, sized correctly for the room;
• use moderate settings rather than extreme ones;
• keep doors and windows closed while it runs, and shade sunny windows;
• use timers so it isn't cooling or heating empty rooms;
• where your tariff and routine allow, run it more when electricity is cleaner.

How to cut running costs goes into the practical side.

Emissions from refrigerant

A sealed system in good condition should keep its refrigerant for its whole life. Emissions happen when it leaks, or when refrigerant is released during a repair or removal instead of being recovered.

How much that matters depends on the refrigerant's global warming potential and how much escapes.

That's why the details of an installation matter: well-made joints, pressure testing, and a vacuum before the refrigerant is let into the pipes. It's also why refrigerant must never be vented, and why a qualified engineer recovers it when a system is removed. Regular servicing gives an engineer the chance to spot a problem early.

A system that gradually cools or heats less well, or ice forming where it normally doesn't, can point to a leak. Both can have other causes, so an engineer should check.

Heating compared with a boiler

When an air conditioner is used for heating, the fair comparison is with whatever would otherwise heat the room, often a gas boiler. A boiler's emissions come from burning gas in your home. An air-to-air heat pump's come from the electricity it uses, and in mild weather it moves several units of heat for each unit of electricity.

Which produces less carbon for the same heat depends on how efficiently the heat pump runs in the conditions it meets, how efficient the boiler is, and the carbon intensity of the electricity when it runs, so there isn't one answer for every home. The method mirrors the cost comparison: work out the heat delivered, then compare the emissions behind it. Heating with air conditioning compared with gas sets out that comparison for costs.

What you can do

• Choose an efficient, correctly sized system and have it installed by a certified engineer.
• Ask which refrigerant it uses and how much it holds.
• Keep it serviced, and have a suspected leak checked promptly.
• Use it thoughtfully: moderate settings, doors and windows closed, timers.
• When it's eventually replaced, make sure the refrigerant is recovered.