Natural air bubbles form in the heated water and can collect in some of the radiators - only the bottom part gets warm - there's a bleed screw in the top at one end or the other to let the air out.
Thermostatic Radiator Valves (TRV) sometimes get stuck in the closed position if they've been turned off for any length of time.
The boiler will have a user control thermostat to set the temperature of the heated water - if it's on a low setting the radiators will be cooler. They usually get set to the mid position which is fine in most situations. Too high and the tap water can sometimes scald and the rads be too hot to touch.
The circulating pump may be struggling to pump the water around the system - they often have a simple speed control you can adjust.
Sometimes the radiators and pipes get sludged up and the flow of water is seriously restricted. The system may need flushing out.
Sometimes the first few radiators on the system take most of the heat leaving little left for the rest of the house - radiator balancing usually solves this.
Try turning off all the radiators except one and see what happens to it - can you get it good and hot but not too hot to touch?
Good luck...
Peccavi
January 2010
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