Bypass radiator constantly hot + boiler pump constantly running

27 Mar 2021
Reaction score
United Kingdom
Boiler: Potterton Promax 30 SL
S plan/ 1HW/1CH zone valve

Our bypass radiator by the HW tank is constantly permanently hot regardless of the radiator valve position. If I turn the valves off, it causes the boiler to go into overheat lockout.
I've checked and touched the boiler pump and that seems to be constantly on/vibrating.

I've noticed that our boiler also sometimes randomly fires up for a few seconds.

We have had our HW zone valve replaced not too long ago with a wiring mishap which was later rectified. My understanding is, if it was the zone valve issue again, wouldn't that also fire up the boiler and not just have the pump constantly running?

With both HW/CH controls off, what's causing the Bypass radiator to be permanently hot/pump to be constant?
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