Response from Google AI
Based on your description, the core issue is that your
microswitch inside the 3-port valve actuator is failing, or there is a wiring fault preventing the switched live signal from reaching the boiler.
Here is exactly what is happening under the hood to explain why your system is behaving this way:
Why it stays on "H" (Heating Only)
When your room thermostat calls for heat, a full 230V is sent to the valve motor, driving it all the way to the
H position. Once it gets there, a mechanical lever inside clicks a microswitch. This microswitch is supposed to "hold" the valve there electronically.
When you turn the heating off, the valve
should spring back. However, because it is receiving constant power or because the internal mechanism is physically sticking (or the internal switch is welded closed), it stays stuck at H.
Why it moves to "W" (Water Only) but won't fire
When you cut the mains power by turning the boiler off and on, you interrupt the electrical hold on the motor. The internal return spring successfully pulls the valve back to the
W position.
However, for the boiler to actually fire up for Hot Water, the system relies on the
Cylinder Thermostat to pass 230V directly to the boiler's switched live wire (usually the grey or orange wire from the valve, depending on configuration). Because the boiler doesn't fire, it means:
- The Cylinder Thermostat is satisfied (it thinks the water is already hot).
- The Valve's internal microswitch is broken and failing to pass the live signal out to the boiler.
- The Valve motor is burnt out/stuck, meaning even though the indicator says W, the internal gate hasn't fully cleared to allow proper operation.
️ Recommended Next Steps
To narrow down the exact culprit, you can try these quick diagnostic checks:
- Check the Cylinder Thermostat: Go to your hot water cylinder and turn the temperature dial all the way up. If the boiler suddenly fires up, your valve is fine, and the cylinder thermostat was simply telling the boiler it didn't need to heat the water. (Remember to turn it back down to ~60°C afterward).
- The "Tap" Test: Safely isolate the power, remove the plastic cover of the 3-port valve (if it's a model like a Honeywell V4043/V4073), and see if the mechanical lever moves freely. Sometimes the synchronized motor or the brass spindle itself gets stiff and requires a gentle tap or replacement.
- Replace the Actuator Head: On modern 3-port valves (like the Honeywell V4073H), the plastic box (actuator) can be unclipped from the brass valve body without draining the system. If the valve spindle turns smoothly with a pair of pliers but the electrical logic is failing, you simply need to replace the actuator head.
To help narrow this down, could you tell me:
- What is the make and model of your 3-port valve (e.g., Honeywell, Drayton)?
- Does the boiler fire up for heating only when the hot water is completely turned off?
- Have you tried turning up the cylinder thermostat on the side of the water tank to see if it forces a click?
