Everything you need to know about air conditioning in your car




The UK is undergoing a bit of a rare heatwave at the moment so if you’ve got into the car this week and cranked up the air con for the first time in a while only to find nothing but hot air coming out, you could be in for an uncomfortable journey. Never fear, here’s our quick guide to everything you need to know about recharging your car’s air conditioning system.

Why recharge your

car air con?

There are many reasons to get your air conditioning recharged besides the most obvious one being to keep you and your passengers cool on journeys in hot weather. If you like getting the most out of your tank of fuel then you might be interested to know that recharging your air con increases fuel efficiency. This is because when your air con refrigerant is low, the air con system struggles to cool the air and so works harder to try and produce cold air. This extra work uses more fuel.

Similarly, it pays to recharge regularly if you don’t want to have to pay out for expensive air con parts and repairs later on. The refrigerant used in your air con system is extremely cold and has the added benefit of cooling down the air con compressor. Without refrigerant, or if it were to run too low, the compressor would get dangerously hot and fail. 

Air conditioning even has its uses on cold winter days. If you are struggling to demist your windscreen, pop the air con on to produce dry warm air to rapidly clear your view. Turning your air con on during the colder months of the year also ensures the system doesn’t seize up and will continue to work effectively when the temperature starts to warm up.


How does air

conditioning work?

Ok so here’s the science bit. Inside the air conditioning system is a compressor which contains a refrigerant – this is the gas which needs refilling regularly. When the air conditioning system is switched on, the gas travels to a condenser. Fresh air from outside the vehicle passes over the condenser and the temperature of the gas drops quickly and causes the refrigerant to liquefy. Now in liquid form, a drier removes impurities before reaching a thermal expansion valve which essentially allows the driver to control the temperature in the car by limiting the liquid flow. Lastly the liquid is turned into vapour as it travels through evaporation coils and is then blown into the cabin as cool air. Meanwhile, the refrigerant, now back in gas form, returns to the compressor to start the process all over again.


Is air con part of my

MOT or Service?

No, your annual MOT ensures the vehicle is safe to drive as per strict DVSA guidelines. Since air conditioning is considered a ‘nice-to-have’ feature and is not critical to the safe running of the vehicle, your MOT tester will not check to see if your air con is working.


Similarly, air con is not included in the standard service schedule set out by your vehicle manufacturer. Regular servicing will help keep your car running for longer in a safe and reliable condition and includes some replacement parts like the oil and air filters. But those drivers expecting their air con to be recharged as part of their service will be disappointed. Many manufacturers provide a separate air con service schedule which will recommend a regas, on average, every 2 years.

Air con on or

window down?

If you’re not sure whether to use the air conditioning or simply wind the window down on a hot day you probably need to be asking yourself “how important is fuel economy to me?” Air con will certainly provide the most comfort on a hot day and the driver has the option to control the temperature and the blowers. Having the window down provides just one uncontrollable setting and it can get quite loud and blustery in the cabin – the faster you go, the worse it gets. Also, when travelling at speed, the vehicle becomes less aerodynamic and experiences ‘drag’ so you’ll be using more fuel to get to your destination. That said, using the air con comes at a price too. The compressor at the heart of your air conditioning system requires energy in order to circulate cold air around the vehicle. It gets the energy from the engine so when the air conditioning system is turned on you will also be using more fuel. So for optimum fuel efficiency, it’s windows down at low speeds, air con on at high speeds.


Why does air con

stop working?

The refrigerant, or gas, found in your air conditioning unit is critical to the air con system working properly. It is this refrigerant that flows around the system and provides the cold air in your cabin. However, as a result of this process which involves continually shifting states from gas, to liquid and back to gas again, some of this air con gas permeates through the system each year. Less gas means less cool air so refilling every few years will keep your air con working in tip top conditionOf course there are other reasons why air con stops working but low refrigerant is the most common and, thankfully, the easiest to fix. If after a regas your air con is still blowing warm, it probably means there is a crack somewhere in the pipes of the air con system and the refrigerant is escaping.

no ac

How do you recharge

a car’s air con unit?

Recharging the air con system involves removing all of the existing refrigerant and any traces of moisture from the system and replacing with fresh refrigerant.

Using an automatic air con servicing machine, two hoses are attached to the air con system under the bonnet – one to the high pressure port, the other to the low pressure port. Firstly, a pressure test is carried out to find out just how much gas has been lost. If the test result shows that little or no gas remains in the system, this indicates a more serious issue such as a potential leak and the recharge is stopped. If no faults are found, the machine recovers the old refrigerant and any impurities from the air con unit using a specialist vacuum pump. Next, fresh gas and lubricant is injected back into the air con system as per the volume and type recommended by the vehicle manufacturer.


How often should you

recharge your air con?

Opinion varies but most vehicle manufacturer’s recommendation is every two years. Whether you use your car’s air con a lot or hardly ever at all, air con gas will continue to permeate through the system over time so an air con service every two years will keep the gas topped up and the system lubricated so you always have a cool blast ready when you need it most.

Closeup view of pages of tear-off calendar

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