Your boiler will not heat water if the mains water supply is off
A boiler needs a constant supply of cold water to function. When your mains water is shut off — whether by your water company, during maintenance, or because you've closed the stopcock yourself — your boiler cannot draw in fresh water to heat. Most modern boilers will stop operating entirely, and some will display an error code on the control panel.
The exact behavior depends on your boiler type and how long the water has been off. Combi boilers (which heat water on demand) stop working when ready. System boilers and conventional boilers with a separate hot water tank may continue to supply hot water from the stored tank for a short time, but once that reserve is empty, you have no hot water.
If you've turned off the stopcock yourself for repairs or to prevent a leak, your boiler will resume normal operation once water pressure is restored. If the water company has shut off your supply, you'll need to contact them to find out when service will return.
Key Takeaways
- Combi boilers stop heating water when ready when mains water is off, leaving you with no hot water.
- System and conventional boilers may supply stored hot water for a limited time, but will eventually run dry.
- Most boilers will not be damaged by a temporary water outage, but some may show error codes until water pressure returns.
- Once water supply is restored, your boiler should restart automatically, though you may need to reset it if it has locked out.
- If the water has been off for more than a few hours, you may have air in the pipes that needs to be bled from radiators and taps.
Why your boiler needs water pressure to work
A boiler is a closed system that relies on water pressure to circulate heated water through your home. The incoming mains water pressure pushes water into the boiler, where it is heated and then distributed to your taps and radiators. Without that incoming pressure, the boiler has nothing to work with.
Modern boilers have a pressure gauge on the front panel, usually with a green zone between 1 and 2 bar. If pressure drops below 1 bar — which happens when mains water is off — the boiler will not fire up. This is a safety feature: the boiler will not attempt to heat water if it cannot circulate it properly.
Some boilers have a low-pressure lockout that prevents the system from operating until pressure is restored. You may see a red light or an error message like "Low Pressure" or "No Water" on the display. This is normal and does not mean the boiler is broken.
What happens to different boiler types
Combi boilers heat water on demand as you turn on a tap. They have no storage tank. When mains water is off, they cannot draw in cold water to heat, so you get no hot water at all. The boiler will show a low-pressure fault and will not operate until water is restored.
System boilers have a separate hot water cylinder that stores heated water. If the mains water is off, you can still use hot water from the cylinder for a limited time — usually enough for a shower or bath. Once the stored water is used up, the boiler cannot refill the cylinder, and you have no more hot water. The boiler itself will stop firing once it senses low pressure.
Conventional boilers (also called regular or heat-only boilers) work the same way as system boilers: they heat water that is stored in a tank. You can use stored hot water briefly, but once it runs out, the boiler cannot heat more. These boilers are less common in modern homes but operate on the same principle.
How long your boiler can run without mains water
If you have a system or conventional boiler with a hot water tank, you can use the stored hot water for as long as the tank remains full. A typical hot water cylinder holds 100 to 200 litres, which is enough for one or two showers or baths. Once that water is used, the boiler cannot heat more because it cannot draw in fresh cold water from the mains.
A combi boiler has no storage, so you have zero hot water the moment mains water is off. You will only have cold water from the tap if there is any pressure in the pipes at all.
If the water has been off for several hours and then comes back on, you may notice air in the pipes. This air can cause sputtering at taps and radiators. You can bleed air from radiators using the bleed valve (a small square socket at the top of each radiator) and from taps by running them for a minute or two.
Resetting your boiler after water is restored
Once mains water is back on, your boiler should restart automatically in most cases. Check the pressure gauge on the front of the boiler — it should climb back into the green zone (usually 1 to 2 bar). Once pressure is restored, the boiler will resume heating.
If the boiler does not restart or shows an error code, you may need to reset it. Look for a reset button on the control panel — it is usually red or marked "Reset". Press it once and wait a few minutes for the boiler to fire up. Do not press it repeatedly; one press is enough.
If the boiler still will not start after a reset, or if the pressure gauge shows zero even though water is back on, there may be an issue with the filling loop or pressure relief valve. Contact a Gas Safe registered engineer to inspect it. Do not attempt to repair a boiler yourself.
Will a water outage damage your boiler
A short water outage — a few hours or less — will not damage your boiler. The system is designed to handle temporary loss of water pressure. Once water is restored, the boiler will resume normal operation.
A prolonged outage lasting more than a day can cause problems if the boiler has been trying to heat with no water in the system. In rare cases, this can damage the heat exchanger (the part that transfers heat to the water). However, most modern boilers have safety features that prevent them from firing when pressure is too low, so this risk is low.
If you are concerned about damage after a long outage, have a Gas Safe engineer inspect the boiler before you use it again. They can check the pressure, test the heating, and confirm that everything is working safely.
What to do if your water is off for an extended period
If your water company has announced a planned outage lasting more than a few hours, you can prepare by filling bathtubs, buckets, or large containers with water before the supply stops. This gives you water for drinking, washing, and flushing toilets.
Turn off your boiler if the outage will last more than a day. This prevents the boiler from attempting to fire repeatedly when it cannot get water pressure, which can wear out components. Turn it back on once water is restored and pressure has returned to normal.
If you have a system boiler with a hot water tank, you can use the stored hot water during the outage. Do not drain the tank intentionally — just use the hot water as normal, and once it runs out, switch to cold water or use water you have stored in containers.
Frequently Asked Questions
Can I use my boiler if the water pressure is low but not completely off?
Most boilers will not fire if pressure drops below 1 bar, even if there is still some water in the system. You may see a low-pressure warning on the display. Once pressure climbs back above 1 bar, the boiler will resume heating. If pressure stays low even though mains water is on, you may have a leak or a faulty pressure relief valve.
What if I turned off the stopcock and forgot to turn it back on?
Turn the stopcock back on by rotating it counterclockwise (or pulling it out, depending on the type). The stopcock is usually located under the kitchen sink or in a cupboard near the front door. Once it is fully open, water pressure should return to normal and your boiler will restart. If the boiler does not restart, press the reset button on the control panel.
Will my radiators work if the water is off?
No. Radiators rely on hot water circulated by the boiler. If the boiler is not operating because water pressure is off, the radiators will not heat. Once water is restored and the boiler restarts, radiators will warm up again, though it may take 15 to 30 minutes for heat to reach all of them.
Do I need to bleed my boiler after a water outage?
You do not bleed the boiler itself, but you may need to bleed air from radiators and taps. After water has been off for several hours, air can enter the pipes. Use the bleed valve on each radiator (a small square socket at the top) to release trapped air. Run cold taps for a minute or two to clear air from the supply lines.
Should I call an engineer if my boiler won't restart after water comes back on?
Try pressing the reset button first — this solves the problem in most cases. If the boiler still will not start after a reset, or if the pressure gauge shows zero even though water is back on, contact a Gas Safe registered engineer. Do not attempt to repair the boiler yourself, as this can be dangerous and will void your warranty.