Hot water runs hot then quickly goes cold during a shower on a Gledhill mains-pressure unit.

Safety: Water may suddenly become very hot. Take care when testing. Isolate power and water before removing any valves or heat exchangers.

Steps:

• Check other taps: See if the problem affects all outlets or just one. If only one, the local mixer/shower valve may be faulty.
• Clean inlet filters: Many Gledhill units have strainers on the cold inlet and on the plate heat exchanger connections. Isolate water, remove filters, and clean debris.
• Inspect thermostatic mixing valve: Locate the TMV on the hot outlet from the cylinder. Check for scale build-up and ensure the temperature setting has not been altered. Replace if the valve does not maintain a stable mixed temperature.
• Plate heat exchanger: On thermal store models, a scaled or partially blocked plate heat exchanger can cause rapid temperature swings. A heating engineer can remove and chemically descale or replace it.

Note: Always restore any insulation and set the TMV to a safe outlet temperature after work.