People often worry that turning off their water supply will adversely affect their boiler. The worry is usually caused by a fear of overheating, like boiling a kettle with no water in it, but in reality, it’s unlikely to cause any damage or risk. We’ll have a look at running your boiler without a water supply here, so if you’re worried, read on.

Is water leaking?

First up, if you have discovered a water leak anywhere in your home, whether that’s in your boiler, your storage tanks or anywhere else, then turning off your water at the stop cock should be your first priority.

Your stop cock is usually found underneath your kitchen sink, and is a tap or valve. Because they are rarely turned off, they can sometimes be hard work, but you or a strong neighbour should be able to do it. Note that if you’ve changed the location or arrangement of your kitchen, the stop cock could be somewhere else completely – now’s a good time to find it so you know where it is in future. If you can’t find your stop cock, there should be another valve outside, possibly outside your property’s boundary. You’ll probably need to contact your local water authority to turn it off, partly because it might be shared with neighbours’ supply, and partly because you might need a special key to lift the cover.

Put a bucket or washing up bowl under the leak, and if you want to be extra safe, switch off your boiler and isolate your electricity at the main fuse box.

Stopping the supply of water might not stop the leak immediately. If a tank or cylinder is leaking, it may be that it needs to empty itself first, but at least it won’t be constantly re-filling itself. A leaking tank or cylinder will drain itself down to the level of the leak and then stop once the water is turned off.

What kind of system do you have?

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

  • a combi boiler heats up water on demand, by passing mains water over a heat exchanger containing freshly boiled water circulating the other way.
  • a system boiler supplies mains water to a cylinder, which is heated up by hot water being circulated around it, rather like a radiator. The cylinder is under pressure from the mains, however. It draws hot water from the top and supplies pressurised cold water to the bottom, which is how pressure at the taps is maintained.
  • a heat only (conventional) boiler is similar to a system boiler, but the cylinder is not under mains water pressure – the water pressure comes from gravity, which is why the cold water supply tank is usually in the loft, or at least at a high point in the home.

In all three systems, however, the water that comes out of your tap is not the same water as that which is passing through the heat exchangers. The water that is heated by the boiler is in a closed loop, and is heated, cooled and circulated continually. The water that comes from your hot taps will be either direct from your mains, from a cylinder or from a loft tank. Isolating your water supply will have no effect on the enclosed loop warming up radiators and exchanging heat to your tap water. You will only cut off the supply of water to the tanks, cylinders and taps.

Is it safe to turn the water off at the stop cock?

If boilers didn’t come with a host of sensors and controls, it might indeed damage your boiler if you turned it on with no water to heat up. Your combi boiler would be the worst affected, as the heat exchanger could suffer, but your cylinder and radiators would probably be fine.

But as long as your boiler is in good working order, you shouldn’t even have to worry about this small risk. A combi boiler should shut down automatically if it detects a drop in mains pressure or if the temperature inside the system gets too high, as would happen without a supply of cold water.

System and heat only boilers will continue to heat up the radiators, and you might even get a bit of hot water out of the cylinder, but that will dry up pretty quickly when there’s no pressure or new water entering the system.

Unless you turn on a hot tap, the system will carry on running as if nothing has happened. Turning on a tap will either have no effect or the boiler will shut down. All things considered, the risk of damage is extremely small.

Is there a likelihood of freezing?

We sometimes hear of people turning off their water supply before leaving the home empty for a few weeks in winter. Their reason is that they wouldn’t want to come home to a burst water main caused by frozen pipes.

It can be a sensible safety measure to isolate the mains water before going away in winter, although remember that if the pipes freeze and burst before the stop cock, it will still start to pour out once it thaws. That may or may not be inside your home.

If you are leaving the house empty for more than a few days, it should be safe to leave your heating on, however, even if you do turn off the water. It can be useful to have your central heating ticking over on a low temperature while you’re not there. As well as keeping the inside of the house from getting too cold and causing condensation, in extreme cold, you could stop your central heating from freezing, especially if you have pipes running near external walls or in a conservatory.

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