Why does hot water temperature fluctuate in the shower?

First question: does it fluctuate only in the shower

Check the same water at the kitchen tap. This distinguishes two completely different problems.

• Fluctuates throughout the whole house — the cause is the hot water source (cylinder, boiler, or heat exchanger)
• Fluctuates only in the shower — the cause is the shower mixer or its connection
• Fluctuates when someone else opens a tap or the toilet cistern fills — a pressure balance issue, not temperature
• Fluctuates only in the mornings — peak household demand or a circulation setting

If the source is a hot water cylinder

Temperature fluctuation in a cylinder is usually linked to limescale, the thermostat, or its capacity.

A heating element coated in limescale heats slowly and unevenly, so the cylinder can't keep up with demand.

If the cylinder is too small for the size of the household, it runs out during a shower and the water gradually cools — that's not fluctuation, it's depletion.

If the source is district heating or an instantaneous heater

A flow-based system reacts to changes in flow rate. If someone else in the house opens another tap, the flow rate changes and the unit can't compensate instantly.

In an apartment building, the cause can also be the heat substation's regulator or a circulation imbalance affecting the whole riser. In that case, neighbours usually have the same problem too — and that's important information for the management company.

If the problem is the mixer itself

The inside of a thermostatic mixer scales up over time. Once the thermostatic element can no longer move freely, the mixer starts overcorrecting temperature, and the result is exactly this kind of jumping water.

In a single-lever mixer, the cartridge can wear so much that a small movement of the lever changes the ratio disproportionately.

If the problem is limited to one mixer and the water elsewhere in the house stays stable, the answer has been found — see mixer repair services.

What you can check yourself

Measure the water temperature at the kitchen tap with a thermometer and write it down. Repeat the measurement when the problem occurs.

Check the cylinder's set temperature — too low a setting, for example below 55°C, means less cold water gets mixed in and every fluctuation is felt immediately.

Clean the shower aerator or showerhead. A partly blocked showerhead changes the flow rate and makes fluctuation worse.

Frequently asked questions