Gas Boiler Shuts Down When Turning on the Hot Water Tap: Causes and Fixes
You step into the shower, turn on the hot water, hear the boiler start for a few seconds, and then suddenly it cuts out completely, leaving you with freezing cold water. This symptom points to a specific failure in the domestic hot water demand detection, heat transfer, or temperature regulation circuitry.
1. Calcified Plate Heat Exchanger (Overheating Cutout)
The secondary stainless steel plate heat exchanger transfers heat from primary heating water to incoming cold domestic water. If hard water limescale chokes the narrow plate channels, primary water cannot transfer its thermal energy and rapidly spikes past 90°C (194°F).
The boiler's high-limit safety thermostat trips instantly to prevent boiling, cutting the flame.
2. Defective DHW Flow Switch / Turbine Sensor
When you open a tap, a magnetic impeller turbine or reed switch detects water movement and signals the control board to ignite the burner. If this sensor is jammed with grit or magnetized metal shavings, the boiler drops the flame intermittently.
3. Sticking Motorized 3-Way Diverter Valve
The 3-way valve directs hot water either to your radiators or to the plate heat exchanger. If the motorized actuator or internal cartridge sticks midway, hot water is diverted into the heating circuit instead of your shower, triggering a thermal lockout.
4. Faulty Domestic Hot Water NTC Temperature Probe
If the temperature sensor sends erratic resistance readings to the motherboard, the microchip assumes water is scalding and cuts the burner.
FAQ
Why does the boiler work fine on heating but cut out on hot water?
Because central heating uses the primary circuit, while hot water demands immediate rapid heat transfer through the secondary plate heat exchanger and flow sensor.
How much does a plate heat exchanger replacement cost?
A new stainless steel plate heat exchanger with installation and labor typically costs between €130 and €220.