Will Turning The Water Supply Off Affect My Boiler?

People worry that turning off the water supply will affect their boiler, perhaps by overheating, like boiling an empty kettle. In reality, it’s not likely to cause any damage.

Is water leaking?

First, if you have discovered a water leak, whether in your boiler, storage tanks or elsewhere, turning off the water at the stop cock should be the first priority.

Your stop cock is usually underneath your kitchen sink. If you can’t find a stop cock, there should be a valve outside, possibly on the pavement. You will need to contact your water authority to turn it off, because it might be shared with a neighbours’ supply, and you may need a special key to open the cover.

Put a bucket under the leak, switch off your boiler and isolate the electricity at the fuse box.

Turning off the water may not stop the leak immediately. If a tank is leaking, it may need to empty itself. A leaking tank will drain down to the level of the leak and then stop.

What kind of system do you have?

There are three types of boiler, and they all provide hot water in different ways:

• A combi boiler heats water on demand.
• A system boiler supplies water to a cylinder, which is heated by hot water circulated around it, like a radiator.
• A heat only boiler is similar to a system boiler, but the water pressure comes from gravity, so the cold-water supply tank is often in the loft.

In all three systems, , the water that comes out of taps is not the same that passes through the heat exchangers. The water is heated in a closed loop, and circulated continually. The water from your hot taps will be either from the mains, a cylinder or a loft tank. Isolating the water supply has no effect on the loop that warms up radiators and exchanges heat to your tap water.

Is it safe to turn the water supply off?

As long as your boiler is in working order, it is safe. A combi boiler would shut down automatically if it detected a drop in mains pressure or the temperature got too high.

System and heat only boilers will still heat up the radiators, and you may even get a bit of hot water from the cylinder which will dry up quickly when there’s no new water coming in.

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