Shower Goes Cold or Hot When the Toilet Flushes

What's happening?

A mixer shower blends a cold and a hot supply to the temperature you set. If both are at the same pressure, the mix is steady. When the toilet cistern refills, or a tap is opened elsewhere, the cold supply loses pressure for a moment, and the shower gets more hot than you set. In a house with a combi boiler or mains-fed hot water, the pressures are similar and the swing is small. In a house with a loft tank on the hot side and the mains on the cold, they are not, and a flush can turn the shower sharply hot. WaterSafe lists fluctuating pressure, a faulty thermostatic valve, or an issue with the hot water system as the main causes.

What usually causes it?

• A shower fed by mains cold and tank-fed hot, so the two pressures are unbalanced
• A manual mixer with no thermostatic or pressure-balancing valve
• A thermostatic cartridge that is scaled or worn and reacts slowly
• Undersized pipework to the shower, so any other use takes its flow
• A shared supply branch that also feeds a toilet or washing machine
• A failed non-return valve in the mixer letting hot and cold cross over
• A hot water system that cannot keep up and cools between draws

Is it dangerous?

Yes, in a limited way: the swing can go to hot with no warning, which is a scald risk, especially for children and older people. WaterSafe advises ways to reduce the risk of burns and scalding. A thermostatic shower valve that stays within a safe limit is the fix, not a quicker hand on the dial.

Can I fix it myself?

Work out the pattern: does the shower go cold or hot, and when? Ask the household to avoid using the toilet or taps while someone is in the shower, which confirms the cause. Check whether the shower is thermostatic by looking for a temperature stop at about the mid-point of the dial. Descaling or replacing a thermostatic cartridge can restore the response; the maker's guide shows the part. Fitting a pressure-balanced or thermostatic mixer, or a pump on the weaker side, is a plumber's job. Keep the temperature control set to a comfortable mid-point rather than the maximum.

When should you call a plumber?

Call a plumber when the shower swings to scalding, when it happens with no other use, when it is a manual mixer, when a new cartridge does not cure it, or when the hot and cold supplies are on different systems.

What will the plumber actually do?

• Establish the supply type for hot and cold, mains or tank, and measure pressure and flow on each
• Test and replace the thermostatic cartridge, or the mixer where the cartridge is not the cause
• Fit a thermostatic or pressure-balancing valve to even the two supplies
• Check the non-return valves and the pipe sizes to the shower
• Add a pump or a separate branch where the supply cannot support the shower and the rest of the house

Who fixes this

• Tap and shower repair
• Low water pressure fix

What it usually costs

Low Water Pressure Pump Cost

£400–£600

Questions people ask

Sources & References

1. WaterSafe: why does my shower keep going hot and cold
2. WaterSafe: reducing the risk of burns and scalding
3. Aqualisa: shower goes hot and cold, solutions