Do Washing Machines and Dishwashers Need Hot Water?

Why washing machines went cold fill

Older machines often had two hoses, one for hot and one for cold, and many kitchens still have the red and blue valves under the sink to prove it. Manufacturers moved to cold fill for a few practical reasons.

A machine only uses a small amount of water for each fill. When it opens its valve, the first thing out of a hot supply is the cold water that has been sitting in the pipe from the cylinder or boiler. On a typical kitchen run, the machine can finish filling before any genuinely hot water arrives, so a hot connection often delivers lukewarm water at best. Heating a small, precise amount with its own element is more predictable.

Modern detergents are also designed to work well at low temperatures, and many everyday programmes now wash at 30 or 40 degrees, where a hot feed would be too hot rather than helpful. Some stains, particularly protein ones, set if the first water they meet is hot. A cold fill lets the machine control that.

Does a dishwasher need hot water?

Not usually. Dishwashers also heat their own water, and most are connected to the cold supply. Some models state in their manual that they can be connected to hot water up to a set maximum temperature. If your manual allows it, a hot feed can shorten the heating stage of a cycle. If it does not say so, keep it on cold.

The same pipe length problem applies. A dishwasher draws only a few litres at a time, so on a long run from the cylinder it may fill with the cold water already sitting in the pipe. A hot connection makes most sense when the dishwasher sits close to the hot water source.

Which valve under the sink is which

The small lever or tap valves under the sink or behind the machine are appliance valves. The convention is blue for cold and red for hot, but colours are not always reliable in older kitchens, so follow the pipe if you are unsure. A cold fill appliance goes on the cold valve. A spare hot valve with nothing on it can be left closed, or capped off.

If you are fitting a new machine where the old one had two hoses, the unused valve should be turned off. Leave it in place if you like, but check it does not drip. A slowly weeping valve behind an appliance can go unnoticed for months and rot a floor.

Never connect the single hose of a cold fill machine to the hot supply. It can overheat the wash, damage delicate fabrics and the machine itself, and may void the warranty.

When a hot fill can make sense

There are a few situations where hot water to a dishwasher, or to one of the few dual fill washing machines still made, is worth considering. The common thread is cheap or free hot water, close to the appliance.

• Solar hot water, or solar panels with a diverter heating the cylinder, where daytime hot water is effectively free. Our guide to a solar diverter for an immersion heater explains how that works.
• A heat pump cylinder, where the heat pump makes hot water more efficiently than an appliance element. Our guide to the heat pump hot water cylinder covers the setup.
• A kitchen where the cylinder or boiler is only a short pipe run away, so hot water arrives quickly.

What fill problems tell you about your plumbing

Appliance fill trouble is often a symptom of something in the water supply rather than the appliance. These are the ones that come up most with our engineers.

• A loud bang in the pipes each time the machine stops filling: the machine's valve closes almost instantly, and the pressure shock rattles loose pipework. Our guide to water hammer and banging pipes explains the fix, usually better clipping or a small arrestor near the appliance.
• A fill or water supply error on the display: often a kinked hose, a half closed valve, a blocked filter screen where the hose connects, or low incoming pressure. If the hot taps are weak as well, low hot water pressure explains where to look.
• Water under the machine: usually a failed hose or a loose connection at the valve. Turn the valve off, and if water has spread, our guide to water escaping in the house covers what to do in the first five minutes.
• Warm water coming out of a cold tap after the machine has run: that points at the plumbing, not the machine. See warm water from cold tap for the usual causes.

Should you be worried about your hot water instead?

If the reason you are asking is that the hot water in the house has stopped or is not getting hot, your washing machine and dishwasher will carry on working on cold, and the problem is at the cylinder or boiler. Our no hot water checklist before you call goes through the quick checks, and if you are on a cylinder, hot water runs out quickly covers the common reasons a tank does not last as long as it should.

If you need an appliance valve replaced, a hammer cured, a leak behind an appliance traced, or your hot water system looked at, call us on 020 4652 5111 or send an enquiry with a photo of the valves and the pipework. We have 20 engineers across London, and our rate is on the pricing page and agreed before we attend.