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Is a washer-dryer a good idea?

They do save space and money compared to a separate washer and dryer from the same manufacturer. And the best washer-dryers are as good as washing machines.

But they're not as good at drying as a Best Buy tumble dryer.

The drying capacity doesn't always match the washing capacity. You may have to dry a load of washing in two (or more) separate goes. This is inconvenient, but could also encourage you to use the drying function less often.

The dryer also tends to be far less sophisticated than the average separate tumble dryer, lacking common programmes and extra features.

You can't always trust energy labels

The ratings go from A (the highest rating) to C (the lowest rating). But in the past we've often found energy-label ratings on domestic appliances are inaccurate.

We use a different method from the energy labels to calculate energy efficiency, which we think better reflects the way people actually use their machines.

We look at the annual washing running costs on the most commonly used wash program, 40°C cotton, and drying running costs based on energy consumption per kilogram of washing on four programs.

Bear in mind that washer-dryers have just one energy label, so you can't compare them with those on washing machines and tumble dryers.

Why are my clothes more creased after drying in a washer-dryer than in a tumble dryer?

First, there's about half the space in the drum as you'd find in a tumble dryer with the same capacity. 

This is partly because the drum needs to be thick enough to withstand the stresses of spinning a full wet load at high speed, and also because of reduced space thanks to the extra internal parts washer-dryers need to do both jobs.

Cooling is important too, and washer-dryers are slow to cool down. Our tests show that most loads are still very hot at the end of the cycle.

What's the advantage of a hot-fill machine?

None, in these days of low-temperature washing programs.

A traditional hot water system is more efficient at producing hot water in large quantities, but the difference will only really be significant on high-temperature (60°C or 90°C) programs.

Most machines are now cold fill, which helps the machine control the water temperature and avoid damaging fabrics.

Are washer-dryers a good idea if you have a water meter?

Your machine uses the same amount of water to wash as a washing machine does, but you may not realise that it uses an average of 36 extra litres of water for cooling in the drying cycle.

The amount can vary considerably from model to model. The most efficient machines use around 61 litres to wash and dry on the cotton cycle but the worst can use as much as 194 litres. 

So washer-dryers may not be the best bet if you have a water meter.

Why do washer-dryers have poor reliability?

One in five washer-dryers needs repair in its first six years, compared to around one in ten washing machines and tumble dryers.

It's the price you pay for cramming two sets of technology into one appliance: compromises are made and there's more that could go wrong.

That's why we also tell you which washer-dryers are most reliable.

Is leaving a washer-dryer on overnight or when I go to work a safety risk?

A new washer-dryer is unlikely to break down in the first couple of years, but the older it gets, the less reliable it becomes.

Catastrophic breakdowns, which cause fire or burst pipes, are very rare but there is some small risk.

So it's worth fitting smoke or fire alarms if you want to use the machine overnight or while you're out.

If you are worried about burst pipes then you could look for a machine which is fitted with an Aquastop device, which will cut the water supply to the machine and stop flooding if it breaks down.

How do we measure the size of our washer-dryers?

When Which? measures the size of any product, we ignore the manufacturers' measurements in the manual.

Instead, we measure the machines ourselves, allowing for any space taken up by the pipes at the back of the machine.

Is a washer-dryer cheaper to run than a washing machine and a tumble dryer?

There's not much in it. In our tests, we found some washer-dryers would cost about the same to run as Which? Best Buy washing machines and tumble dryers added together – about £50 a year.