Potterton boiler radiator problem

[June]

Potterton promax he boiler...
5 years installed with problems them out of blue 2 downstairs radiators not heating up.
If I turn off the valves of the upstairs 2 then the downstairs 2 get hot.
Hot water works fine.

Plumber put new diveter valve on and worked ok for 2 weeks now same again.
He's tried new thermistor and pump head, no different.

I note that...

When the heating is on and boiler reaches 85ish the heating knocks off like it would if the room had come up to temperature in the room thermostat. It drops to about 40 then does the same, rinse and repeat. All the while the room isn't coming up to temperature.
The return flow pipe is cold.
Plumber says he's no idea now and can do no more.
Any ideas?????

 

[HCT]

There's a filter/gauze in the return isolation valve. Sounds to me like it's blocked given what you're describing. I'm assuming the hot water works ok?

 

[June]

Yes the hot water works fine, they led me to believe there's a blockage in the pipes and don't want to get involved! It seems strange how the divertor valve cured it temporary.

Where is the filter located please

 

[HCT]

Return isolation valve. It's not something I'd recommend doing yourself though, and it's just an educated guess given what you're describing, but it's the next thing i'd be looking at.

 

[June]

Thank you. I may contact a different plumber and suggest this to them.