Combi Boiler Hot Water Goes Cold Mid-Shower

What's happening?

A combi boiler has no tank. It heats water as it flows through a small plate heat exchanger, so the temperature you get is the boiler's output divided by the flow. Turn the flow up or have the cold supply arrive colder and the same boiler gives you less heat. Two things make a shower go cold part way through. The boiler can fail to hold the flame, usually because the flow dropped below its minimum or a sensor misread it, so it switches off until the next call. Or it keeps firing but cannot keep up, because the plate is scaled, the temperature is set low, the shower mixer is taking more hot water than the boiler supplies, or someone started a second tap. The cause shows in what you hear: a boiler that clicks off is cutting out, one that keeps running is running out of heat.

What usually causes it?

• A thermostatic mixer shower that drops the hot flow, so the boiler sees too little flow and switches off
• Limescale on the plate heat exchanger, which cuts heat transfer in hard-water areas
• A hot water temperature setting on the boiler that has been turned down
• A second hot tap, dishwasher or washing machine drawing at the same time
• A failing flow sensor, hot water temperature sensor or diverter valve
• A shower flow rate higher than the boiler can heat, especially in cold weather
• Low or fluctuating mains pressure that makes the boiler lose the flow signal

Is it dangerous?

No. The main risk is a thermostatic shower that swings from cold to scalding as the boiler comes back on, so stand clear of the water until it settles.

Can I fix it myself?

Turn the boiler's hot water temperature up to its normal setting and run the shower at a lower flow for a test. Try with nothing else running. If it only goes cold when a second tap runs, the boiler is undersized for that use and no repair will change it. Do not descale or touch the flow sensor yourself: the plate and sensors are part of the gas appliance.

When should you call a heating engineer?

Call a Gas Safe registered engineer if the boiler switches itself off during the shower, if it has done this suddenly after working fine, or if it shows a fault code. Ask them to check the flow rate and the heat exchanger, since the answer may be a descale rather than a new boiler.

What will the heating engineer actually do?

• Run hot water at a measured flow and watch the boiler and temperature to see where it drops
• Check the hot water temperature setting and minimum flow switching point
• Test the flow sensor, temperature sensors and diverter valve
• Descale or replace the plate heat exchanger where it is fouled
• Check the gas rate at full output so the boiler can deliver its rated heat
• Talk through flow limiters or a mixer shower valve if the boiler is matched to the wrong demand

Who fixes this

• Boiler repair

Questions people ask

Sources & References

1. Vaillant: what is a combi boiler?
2. Vaillant: what temperature should my combi boiler be set at?
3. Vaillant: what is a heat exchanger?
4. Vaillant: most common boiler problems