Almost every week a customer tells us the same story: the shower starts warm, turns icy, blasts hot for thirty seconds, then goes cold again. Most assume the heater is dying. Sometimes it is — but in our experience the heater is innocent more than half the time.
Cause one: someone opened another tap
Instant heaters heat water as it flows through. When a toilet flushes or the washing machine fills, cold-water pressure to your shower drops, less water flows through the heater, and the same heating power concentrates in less water — suddenly hot. When the other tap closes, flow surges back and the water runs cooler. If your temperature swings line up with other people using water, the fix is pressure management, not a new heater: a small booster pump or a pressure-balancing valve.
Cause two: a heater on its way out
If the temperature see-saws even when nobody else is using water, suspect the unit. Scale builds up on the heating element over the years, making it cycle on its thermal cut-out: heat, overheat, cut, cool, heat again. You feel that cycle as hot-cold waves roughly twenty to forty seconds apart. A service can buy time, but an element scaled badly enough to cycle is usually within a year of failing outright.
Cause three: crossed hot and cold lines
Rarer, but we see it after bathroom renovations: a mixer installed with hot and cold reversed, or a single-check mixer letting cold push back into the hot line. The telltale is temperature that changes when you adjust a different tap's mixer. This one needs a plumber — the fix is at the mixer body, not the heater.
A quick self-diagnosis
- Swings only when others use water → pressure problem. Booster pump or balancing valve.
- Swings on a steady rhythm with nobody else home → scaling element. Service or replace the heater.
- Started right after a renovation → crossed lines. Get the mixer checked.
Still not sure which pattern is yours? Time the swings, note who else was home, and tell us when you book — those two details usually let us arrive with the right part on the first trip.