Grant Insufficient Domestic Hot Water Temperature

The boiler runs, but domestic hot water from the taps is lukewarm or runs hot then quickly cold.

Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Cylinder thermostat set too low or faulty, Plate heat exchanger partially blocked (combi/heat-only with plate), 3-port valve not fully diverting to Domestic Hot Water, Scale build-up in the cylinder coil

How to Fix / Troubleshooting

Safety: Beware of scalding when testing hot water. Cylinder temperatures should be at least 60°C to prevent Legionella.

User checks:

Check the cylinder thermostat setting (typically 60°C) and increase it if it is set too low.
Ensure the programmer is set to provide hot water at the desired times.

Engineer actions:

Test the cylinder thermostat for correct switching and replace it if it is faulty.
Inspect the 3-port or Domestic Hot Water zone valve for full travel to the hot water position.
Check and clean the plate heat exchanger if it is fitted and partially blocked.
Assess the cylinder coil for scale; powerflush or replace the cylinder if it is severely affected.

Repair Difficulty

Medium 3/5

Required Part

Cylinder thermostat

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