Why does my shower go hot or cold when someone uses a tap?

What a thermostatic valve is supposed to do about pressure changes elsewhere in the house, why it stops doing it, and the descaling that usually restores it.

Difficulty: beginner
Time: 30 min
Read: 3 min

Short answer

A manual mixer has no defence against someone flushing a toilet — the cold pressure drops and the shower goes hot. A thermostatic valve should compensate automatically, and when it stops doing so it is nearly always scale or debris jamming the thermostatic cartridge. Descaling or replacing the cartridge is the fix.

A shower mixes hot and cold. If either supply pressure changes, the mix changes with it. The whole point of a thermostatic valve is to sense the outlet temperature and shift the mix to hold it steady — so a thermostatic shower that swings is a faulty shower, not a plumbing quirk.

What you'll need

Descaler or white vinegar
An Allen key set and a screwdriver
A replacement cartridge if needed

Step by step

Work out which type of valve you have.Two controls, one marked with temperatures and usually with a red safety button at 38 degrees, means thermostatic. A single lever or two simple taps means manual mixing.
Accept what a manual mixer can and cannot do.It has no temperature sensing at all. If the house pressure changes, so does the mix. The answer is a thermostatic valve, not an adjustment.
Test the pattern deliberately.Run the shower and have someone flush the toilet, then run the kitchen cold tap, then the hot. Note which one causes the swing and in which direction — it tells you which supply is dropping.
Isolate and open a thermostatic cartridge.Turn off the shower's isolating valves, remove the temperature handle and the retaining clip or nut, and withdraw the cartridge. Photograph the orientation first.
Descale the cartridge.Soak it in descaler or white vinegar for an hour, work the mechanism by hand, then rinse thoroughly. Scale on the wax element is what stops it responding.
Check the inlet filters.There are usually small gauze filters and non-return valves in each inlet. Debris there restricts one side and unbalances the mix permanently.
Replace the cartridge if descaling does not restore it.Cartridges are model-specific. Take the old one to a merchant or match the manufacturer's part number; there is no universal replacement.
Recalibrate the handle afterwards.Most thermostatic valves let you reset which temperature the 38-degree stop corresponds to, usually by repositioning the handle on its splines. Set it with a thermometer under the flow.
Look at the wider system if it is house-wide.A shower fed from a cold mains and a gravity hot cylinder will always be sensitive. Balanced supplies — both from the cylinder, or both from the mains — behave far better.

Tips

Test with a cheap thermometer in the flow rather than by feel. Skin is a poor judge of a two-degree change.
In a hard water area, descaling the cartridge once a year keeps a thermostatic valve responsive for its whole life.
If it only swings when the washing machine fills, the machine's inrush is the trigger — it is still the valve's job to compensate.

Common mistakes

Assuming a thermostatic shower cannot be at fault — The wax element and the sliding shuttle inside a thermostatic cartridge scale up like anything else. When they seize, the valve behaves exactly like a manual mixer.
Replacing the whole shower valve first — The cartridge is a fraction of the cost and is the part that fails. Swapping the valve means retiling and replumbing for a problem a cartridge solves.

If it doesn't work

Goes scalding when a cold tap runs

Cause: Cold pressure dropping, valve not compensating — Fix: Descale or replace the thermostatic cartridge.

Goes cold when hot water is used elsewhere

Cause: Hot supply starved, or shared with a demanding outlet — Fix: Same cartridge check, then consider whether the shower shares a small-bore hot supply.

Never gets hot enough even on maximum

Cause: Scale on the wax element, or hot supply too cool — Fix: Descale the cartridge; check the cylinder or boiler hot water temperature.

Fixed at one temperature whatever the handle does

Cause: Seized cartridge — Fix: Replace it — a fully seized wax element does not recover.

Questions people ask

Is a thermostatic shower worth it?

For anyone sharing a house, yes. It is the only kind that reacts to pressure changes, and the safety stop at 38 degrees matters a great deal with children and older people.

Why does my shower only do it in the morning?

That is when other outlets are being used. The valve is not behaving differently; it is simply being tested more. It should still compensate, and if it does not, the cartridge needs attention.