Why does my combi shower run hot, then cold, then hot again?

The cold water sandwich explained — what causes the slug of cold water a few seconds into a combi shower, and what genuinely reduces it.

Difficulty: beginner
Time: 20 min
Read: 3 min

Short answer

It is the cold water sandwich. The combi delivers residual hot water from the heat exchanger, then a slug of cold that was sitting in the pipe while the burner lights, then proper hot. Reducing the flow slightly, or fitting a thermostatic shower valve, smooths it out; nothing removes it entirely from a plain combi.

A combi heats on demand. When you open the shower it delivers whatever hot water is standing in the pipework, then has to detect the flow, fire the burner and bring the exchanger up to temperature. The gap between those two is the cold slug people notice thirty seconds in.

What you'll need

A thermometer or a hand
Access to the shower's flow control

Step by step

Confirm the pattern.Warm for a few seconds, then distinctly cold for five to twenty seconds, then hot and stable. That specific sequence is the cold water sandwich and not a fault.
Rule out an actual fault.Repeated cycling hot and cold throughout the shower is different — that is the boiler cycling on a flow rate near its minimum, or a failing diverter valve, and it needs an engineer.
Check the shower is drawing enough flow.A combi has a minimum flow rate before its burner lights, usually around 2.5 litres a minute. A very restricted shower head can sit right at that threshold and make the boiler hunt.
Try a slightly higher flow.Opening the shower a little more often gets the boiler firing sooner and more decisively, which shortens the cold slug.
Run a hot tap for ten seconds first.Firing the boiler on a nearby tap before stepping in clears the standing cold water down the pipe. Crude, but it works and costs nothing.
Fit a thermostatic shower valve if you have not.A thermostatic valve reacts to the falling outlet temperature by shutting the cold side, which blunts the sandwich considerably. It is the single most effective change.
Shorten or insulate the pipe run if you can.Most of the cold slug is water standing in the pipe between boiler and shower. Insulating that run keeps it warmer for longer between uses.
Check the boiler's preheat setting.Many combis have a preheat or comfort mode that keeps the exchanger warm. It largely removes the sandwich at the cost of a little standing gas use. It is a setting on the boiler's own controls, not a repair.
Stop at the boiler casing.Anything beyond the user controls — the diverter valve, the flow sensor, the exchanger — is Gas Safe work. Do not open the boiler.

Tips

Preheat mode trades a small standing gas cost for a much better shower. Worth trying on and off for a week each.
A thermostatic valve helps far more than any plumbing change to the pipe run.
If it happens on every hot outlet, not just the shower, it is normal combi behaviour rather than a shower problem.

Common mistakes

Turning the shower flow down to make it hotter — Below the boiler's minimum flow rate the burner will not stay lit, so the shower cycles cold repeatedly instead of once. Slightly more flow usually gives a more stable temperature.
Assuming it means the boiler is failing — The sandwich is inherent to instantaneous water heating. A boiler that produces it once at the start and then holds steady is working as designed.

If it doesn't work

One cold slug then stable

Cause: Normal cold water sandwich — Fix: Run a hot tap first, or enable preheat if the boiler has it.

Repeated cycling all the way through

Cause: Flow rate near the boiler's minimum, or a faulty diverter — Fix: Increase the shower flow; if it persists, call a Gas Safe engineer.

Worse in winter

Cause: Colder incoming mains, so the exchanger takes longer — Fix: Expected. Preheat and a thermostatic valve both help most in winter.

Cold slug is very long

Cause: Long uninsulated pipe run between boiler and shower — Fix: Insulate the run; consider the boiler's preheat setting.

Questions people ask

Is the cold water sandwich a fault?

No. It is inherent to how a combi works — it heats water on demand, and there is an unavoidable delay between opening the tap and the burner delivering heat. A single cold slug is normal.

Does preheat waste much gas?

It keeps a small heat exchanger warm rather than a whole cylinder, so the standing loss is modest. Whether it is worth it depends on how often you draw hot water; try it for a week and compare.