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  3. No hot water: what to check before you call a plumber

No hot water: what to check before you call a plumber

By Declan Barry, plumber, Stockport. Updated 2026-09-10.

If you have no hot water but the heating works, or no hot water and no heating, check the boiler display for an error code, the pressure gauge, the programmer and thermostat settings, the immersion switch if you have a cylinder, and whether a tap is stuck open somewhere. Half the time one of those is the answer.

What to do

  1. Look at the boiler display

    An error code or a flashing light tells you what the boiler thinks is wrong. Write it down. If the pressure gauge is at zero, the boiler has shut down for low pressure and topping up via the filling loop will bring it back.

  2. Check the programmer and the thermostat

    Hot water on, at the right time, and the cylinder thermostat, if you have one, turned up. Clocks that have lost their settings after a power cut are a common cause.

  3. If you have a cylinder, try the immersion

    The switch is usually in the airing cupboard. Turn it on for an hour. If you get hot water, the cylinder is fine and the fault is in the boiler side or the motorised valve. If you do not, the immersion or its thermostat has failed.

  4. Check the motorised valve

    A small box on the pipework near the cylinder with a lever. If it hums but the lever does not move, the motor has failed. Moving the lever to manual gets you hot water until it is replaced.

  5. Run the hot tap for a minute

    A long pipe run from a cylinder in the loft can take a minute to come through. If it is warm but not hot, the cylinder thermostat may be set low or the cylinder may be short-cycling.

When to ring a plumber

  • The boiler shows an error code you cannot clear, or locks out repeatedly. If it is a gas fault it needs a Gas Safe engineer; if it is the hot water side, I can help.
  • The immersion does not work either. That is a cylinder fault.
  • The motorised valve has failed. A quick replacement.

Questions

Hot water works but heating does not. Same problem?

Usually the motorised valve or the programmer, on a cylinder system. On a combi it is usually the diverter valve inside the boiler, which is a gas engineer job.

Hot water runs out fast. Is that a fault?

It may just be an undersized cylinder for the house. An unvented cylinder sized properly fixes it for good.

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