Hot Water Lukewarm Or Runs Out Too Fast

What's happening?

This is hot water that works but disappoints. On a cylinder, it can be set too low, or the thermostat can be wrong, so the water never reaches the temperature you expect. A coil furred with limescale passes less heat to the water, so the cylinder takes much longer to recover, and one that was fine ten years ago now runs out. A cylinder that is simply too small for a larger household or a bigger bath will run out however well it works. On a combi, the output is limited, so lower mains pressure, a set temperature that is low, a scaled plate or a high-flow shower all leave the water lukewarm. Stored hot water should be kept hot enough to stop bacteria growing, which is why a cylinder thermostat is normally set to about 60 degrees.

What usually causes it?

• A cylinder thermostat set too low or reading wrongly
• A coil or heat exchanger furred with limescale in a hard-water area
• A cylinder too small for the household, or recovery too slow after heavy use
• A mixing valve on the supply set cooler than intended
• A combi hot water temperature turned down or the plate heat exchanger scaled
• Cold incoming water in winter reducing the temperature a combi can reach
• A failed immersion element or a motorised valve only partly opening

Is it dangerous?

Not dangerous in itself. Do not turn the cylinder thermostat below about 60 degrees to save energy: stored warm water is where bacteria grow, and HSE guidance for stored hot water is to keep it hot enough to prevent that. Water from the tap can still scald, which is what a mixing valve is for.

Can I fix it myself?

Check the cylinder thermostat is set to about 60 degrees, and turn a combi's hot water temperature up to its normal setting. Time how long the cylinder takes to heat from cold. Check that nothing else is drawing hot water at the same time. Note how long the water lasts, as this helps an engineer tell a recovery problem from a size problem.

When should you call a heating engineer?

Call a heating engineer if the thermostat is set right and the water is still lukewarm, if the cylinder takes many hours to heat, or if it has got worse over months. A descale, a new thermostat or a larger cylinder may be the answer.

What will the heating engineer actually do?

• Measure the cylinder or hot water temperature at the cylinder and at a tap
• Test the thermostat, the motorised valve and the heat source
• Descale or replace the coil or plate heat exchanger where it is scaled
• Check or reset any thermostatic mixing valve to the required temperature
• Size the cylinder or boiler to the household and recommend a larger one if needed

Who fixes this

• Hot water cylinder repair and replacement
• Boiler repair

Questions people ask

Sources & References

1. HSE: legionnaires' disease
2. Vaillant: what temperature should my combi boiler be set at?
3. Vaillant: what is a heat exchanger?
4. Viessmann: my boiler is working but I have no hot water