A friend has an issue with his heating and i said i would give him a hand. His Pump was only changed last year and it is working fine but only about 3 of the 6 radiators get hot. the rest get luke warm and one is just cold. The pipes seem to be 12mm coming out of the floor and some of the radiators are bottom entry and central (ie not either end). The system hasn't been flushed or drained in ages so i was thinking it could be one of 2 issues.
Motorised valve is stuck...
Gunk built up in system.....
The radiators have all be bleed and there is no air in the system. The boiler on the other hand has not been serviced in about 8 years. It is a Potterton 16-20 or something. Any advice would be appreciated. :)