A shower that swings between hot and cold is one of the most common calls we get, and the cause is almost entirely determined by what is feeding it.

So the first thing any plumber does is establish what type of system you have, because the same symptom has three completely different explanations.

What a plumber actually checks, and in what order

1. What is feeding the shower?

A combi heats on demand, so it is sensitive to flow rate. A cylinder system has stored hot water, so it is sensitive to running out and to pressure balance. An electric shower heats the cold mains itself, so it is sensitive to incoming flow and to its own element.

Knowing which one you have rules out two thirds of the possibilities immediately.

2. Is the shower head or hose scaled up?

Start here because it is free. In a hard water area, and most of Hertfordshire and Bedfordshire is hard, scale builds in the head and restricts flow.

On a combi, restricted flow is a classic cause of temperature swing: the flow drops below what the boiler needs to keep firing, the burner drops out, you get cold, the flow recovers, it fires again. Descaling or replacing the head fixes a surprising number of these.

3. Does it change when another tap is used?

If flushing the toilet or running the kitchen tap makes the shower go hot or cold, you have a pressure balance problem. On an old gravity system that is normal behaviour and points at the design. On a mains pressure system it points at an undersized pipe run or a restriction.

4. Is the thermostatic cartridge failing?

Thermostatic mixer valves have a cartridge that senses temperature and adjusts the blend. They scale up and they wear. When one starts to fail you typically get a shower that runs fine for a minute then drifts, or one that will not hold a setting.

Cartridges are replaceable and are far cheaper than a new shower valve, provided the make and model can still be sourced.

5. On a combi, is it dropping below minimum flow?

Every combi has a minimum flow rate it needs before it will fire for hot water. If the shower is a low-flow head, or the filter on the inlet is partly blocked, you can sit right on that threshold and the boiler will cycle on and off.

The plate heat exchanger scaling up produces the same effect and is very common on combis over about eight years old in hard water areas.

6. On a cylinder, is it simply running out?

If it goes cold and stays cold after a set number of minutes, that is not a fault, that is the cylinder emptying. Check the reheat time and whether the immersion or the boiler is actually heating it fully.

7. On an electric shower, is the element or stabiliser going?

Electric showers that alternate hot and cold, particularly with a change in flow, are often failing internally. They are generally replaced rather than repaired, and it is electrical work as well as plumbing.

What it usually turns out to be

On combis it is most often scale, either in the shower head or in the plate heat exchanger, tipping the boiler below its minimum flow. On cylinder systems it is usually a failing thermostatic cartridge or a genuine capacity problem. On electric showers it is usually the shower.

When to stop and call someone

Frequently asked questions

Why does my shower go cold when someone flushes the toilet?

Pressure balance. On some systems that is inherent to the design, on others it is a restriction or an undersized pipe that can be improved.

Can a power flush fix a hot and cold shower?

Only if the cause is on the heating side. Domestic hot water problems on a combi are more often the plate heat exchanger, which is descaled or replaced rather than flushed.

Is it worth replacing a thermostatic cartridge?

Usually yes, if the part is still available. It is a fraction of the cost of a new valve and tiling.

How do I know if I have a combi?

If there is no hot water cylinder in an airing cupboard and no tank in the loft, it is almost certainly a combi.

Need a plumber in Herts, Beds or Cambs?

Vantage Plumbing and Heating covers Hertfordshire, Bedfordshire and Cambridgeshire. The line is answered by a person at any hour of any day, and you get a price before anyone starts. Call 01462 506374 or request a callback.

Related: shower installation · no hot water but heating works · boiler repair · water softener installation.