Fluctuating hot water on a combi has four common causes, and they behave differently. Two are repairs, one is a sizing mismatch, and one is a plumbing-design issue elsewhere in the house. Working out which you have saves paying for the wrong one.
Start by noting when it happens: on demand, mid-shower, or only when another outlet is used.
The cold water sandwich
Turn the tap off and back on quickly and you get hot, then a slug of cold, then hot again. That is stored water in the pipe run between the boiler and the outlet, not a fault. It is more pronounced on long pipe runs and in properties where the boiler is far from the bathroom.
Scaling on the plate heat exchanger
- Symptom: gradual decline in achievable hot temperature over months.
- Often paired with reduced flow at the hot tap specifically.
- Fix: descale or replace the plate exchanger, then protect with a filter and, in hard water, a softener.
Flow rate versus boiler output
A combi heats water as it passes. If the flow rate through the boiler exceeds what its kW output can raise to temperature, the water arrives lukewarm — and swings whenever demand changes elsewhere in the house. A 24kW combi feeding a high-flow shower and a running tap simply cannot keep up.
This is a specification problem, not a breakdown. The answers are flow restriction at the outlet, a higher-output appliance, or moving to a stored hot water system.
Diverter valve wear
A partially sticking diverter produces temperature that wanders unpredictably, often with heating demand active at the same time. Unlike scaling it appears suddenly rather than gradually, and unlike the cold-water sandwich it does not settle after a few seconds.
Frequently asked questions
- Is the cold water sandwich fixable?
- It can be reduced with shorter or better-insulated pipe runs, and some appliances have a pre-heat or comfort mode that helps. It is not a fault to be repaired out entirely.
- How do I know if my combi is simply too small?
- Compare the boiler's hot water output in kW against the flow rate your outlets actually draw. If two outlets running together always fails, it is sizing rather than wear.
- Would a water softener help?
- In hard-water areas it slows the scaling that degrades a plate exchanger, which protects the appliance long term. It will not fix a sizing mismatch or a diverter fault.
- Is switching to a cylinder a big change?
- It is a bigger job than a like-for-like swap — space for the cylinder, mains flow for an unvented option, and cylinder-side controls. For households where several outlets run at once it is often the right answer.
Get the right fix for a swinging shower temperature
Tell us the boiler model and whether it happens with one outlet or two — that separates a repair from a sizing problem.