My bungalow is heated by a Baxi Solo 3 70PF non condensing boiler. It is connected to 11 radiators and controlled by a Honeywell CM927 programmable room thermostat operating a Honeywell two port motorised valve. Hot water is supplied by means of a second motorised valve, separately controlled. This means I can run heating & hot water together or quite separately. My problem is that no matter how I set the system two radiators are failing to warm up properly. So far as I can tell, these are fed by plastic rather than copper tube. I find that the Honeywell stat gives good, close control of the system but the pump as a consequence cycles on and off constantly. The two radiators concerned are of high efficiency but low water capacity, and it occurs to me that they are simply not receiving an adequate water flow to heat them up. The system is fitted with a Fernox TF1 filter for trapping magnetite and is filled with corrosion inhibitor. I am minded to get a plumber in to repipe to these two radiators but would welcome experts' advice as to how to overcome the problem. The other radiators heat up rapidly even with the lock-shield valves almost totally closed. The pump is less than a year old.