Puma 80 - Red LED on modulator board shuts down boiler

[nap]

My Puma 80 (perm pilot) pilot light is extinguised when the red LED lights up on the Modulator Board. I have to wait for this light to go out before the boiler can function properly again. This occurs when:

I turn off the DHW tap and the light stays on.

Room Thermostat is calling for CHW and stays on when thermostat is turned down. If the CHW is running and I turn on the DHW tap the red LED light stays on when I turn off the DHW tap.

When the boiler shuts down there is a rummbling sound followed by
the Air Vent hissing and dribbling a little water and steam.

Can anyone help?

 

[kevplumb]

If the CHW is running and I turn on the DHW tap the red LED light stays on when I turn off the DHW tap.

H/W flow switch is stuck

 

[nap]

Thanks for your reply.
I have replace the DHW Flow switch and the boiler is now able to switch from DHW to CHW without sticking.

There is one more thing, (there always is) After closing the DHW tap the boiler pauses for approx. 1 minute then fires again to continue with CHW.
Is this normal or symptomatic of another fault?

 

[croydoncorgi]

Short delay is entirely normal.