Air conditioning won't strain a modern car but is it better to wind down the window?

With the weather so hot it's a good idea to service air-conditioning filters. Image / Supplied

The heatwave bringing record temperatures to New Zealand this week has got motorists turning to their air-conditioning with gusto.

Automobile Association motoring adviser Cade Wilson had his own car air con fixed in early December so that his pregnant wife would be comfortable during their summer holiday journeys.

"I don't think I would have survived the summer without it," Wilson, a mechanic of 22 years, said.

When comparing air con with winding the windows down, Wilson said the latter was a futile effort to save petrol because at best it would only save a few dollars and if the car was travelling at open road speeds wind drag would cost just as much.

"There's two scenarios. Around town where you are driving around slowly, you're car is not that fuel efficient anyway but it would be good to have a window down if you want to save money by not using it.

"But then at 100km/h you can't do the same because the wind drag on your car would be creating that same if not worse load than the air con would be putting on it.

"So apart from being uncomfortable at 100km/h with your windows down it has the reverse effect."

Wilson said the hotter the weather gets, the more stress there is on a vehicle and statistically there were more breakdowns in summer.

"Temperature-related breakdowns, to do with not necessarily air con but to do with cooling systems failing, ramps up over the Christmas summer holiday period.

"With air con, when you're trying to get that little bit of fuel efficiency, what we've found is using the air con does put more load on the engine which does create a strain on the fuel efficiency which goes out the window a little bit."

However Wilson said air con was there to be used and the strain would be less obvious in modern cars.

In the older of his two cars Wilson turns the air con off when driving up a hill because of the strain.

"It's what we used to do in older cars, go up a hill and turn the air con off and then turn it back on as we go over the top."

Overheating cars was such a common summer situation that a resident on the Brynderwyn Hills had kindly filled large bottles of water for motorists who can't make it to the top of the range south of Whangārei, Wilson said.

"You only have to do a road trip and you find cars on the side of the road broken down with their bonnets up and a jug of water beside them."

But be warned, the engine must be sufficiently cooled before water can be added to a thirsty radiator.

"If it's that hot and spurting water out, you have to put the water back in but if you get the [radiator] cap off you risk burning yourself.

"And then if you put cold water in straight away you're going to crack the engine and do more damage internally.

"It is a waiting game but you've got really little choice."

Getting the air con fixed was usually a specialist job, though it could be as inexpensive as just regassing the pressure at a cost of between $100-$200.