Fully pumped system HW 2-way valve closes and pump stop

[tellme_why]

Hi all,
My system is fully pumped with two 2-way valves, open vented, coil cylinder, Boiler Potterton 50e and thermostat potterton EP2002. Page 316 of this link shows my system functional layout:
https://www.pearsonschoolsandfecoll...FreesamplechapterPlumbing/Level2_PLUMB_SB.pdf

The system CH works fine, when I slide the thermostat switch the CH valve opens and the pump starts. The Boiler flame starts and water get hot.

The HW system does not work well, resulting in cold showers.
This is what happens step by step:
1. I switch on HW in manual mode in the thermostat EP2002 which is sited in the airing board.
2. The HW 2-way valve motor runs (typical gear noise) and valves open
3. Pump starts
4. Pipe connected to Cylinder coil start getting hot.
5. However after few second the pump stops and the valve goes to rest position and closes.

I have checked the cylinder thermostat and looks fine.

I think the root cause may be:
1. Some problem with Boiler thermostat stopping the pump
2. Some problem with air in the piping (unlikely)
3. Some problem with the 2-way valve switches
4. Problem with the Potterton thermostat/controller.

Any idea/suggestion?

Many thanks

 

[DP]

Can I suggest you start by looking for voltages at various points that keeps the HW motorised valve open.

 

[Mottie]

Can you manually open the HW motorised valve and see what happens then.

 

[DP]

Can you manually open the HW motorised valve and see what happens then.

Most likely nothing.
Manual opening should not engage auxiliary switch, so no pump or boiler operation.
However, OP will get hot water if CH running.