Potterton prima overheat problem

[mfgs]

I have got an ongoing issue with a prima 60F. Before I looked at it, someone had replaced the boiler stat and the overheat stat and the system had been cleaned. The problem is it keeps tripping the overheat stat, but not all the time, maybe once every 2-3 days.
I found the inner flue was corroded through, so that got sorted and burner pressure and gas rate checked, all ok. Heat exchanger and burner cleaned. This seemed to solve the problem for about 6 months, then I got a call to say it was playing up again. I found the pump to be noisy so that was replaced and is now working fine. Again the boiler was ok for about 4-6 months.

It is now overheating again. The fan is a little noisy, but could the fan running a bit slow cause an over heat fault on these? All settings are ok, good circulation etc. I have never actually seen it overheat and trip the stat, but its normally right at the end of the night when its been running for a while as they say they don't notice the boiler go off and just notice that is doesn't fire in the morning and the overheat needs to be reset for it to fire.

Any suggestions on what to test or check next?

 

[Gas74]

Is there a bypass valve/rad. ? If not maybe when house hot and trvs shutting down causing overheating issue ???

 

[mfgs]

Yeah its an s plan system with an autobypass fitted.

 

[gassafe]

New pump failing already or the bypass is not functioning causing it to overheat on shut down..?

Blocked cold feed?

 

[mfgs]

The cold feed is clear, I checked that when I changed the pump. The pump seems ok, as that was my initial thought, all rads getting hot and good circulation. I will make sure the bypass is functioning correctly. How does the pump overrun stat work on these? and could this potentially be a problem?

 

[SmokeyJ]

I think they are set to overrun at above 80°c. Basically with a heat demand the overrun stat powers the pump via the switched live regardless of temperature. When heat demand satisfied the overrun stat will then power the pump via the permanent live if the boiler is above 80°c. That's my understanding if it makes sense.
Fire it up on hot water with the stat on max. When it gets up to temp switch hot water off and see if the pump overruns will tell if it's working.