We have a 3 bed semi & had to have a new boiler installed approx Jan 2008. We now have a room stat situated in the hall (usually set to 19 degrees to keep costs down a bit), which we didn't have before but the rads don't all seem to heat up at the same time nor to the same temp & can take well over an hour or more to get hot. The living room has two smallish rads & does warm up pretty well if we keep the door closed, but none of the other rooms seem get "warm" if you know what I mean. The main bedroom has a small radiator situated just inside the door, room size about 13 x 12' app. Only one radiator was replaced in another bedroom when we got the new boiler & is the only one with a thermostatic valve. The pump is situated in the airing cupboard upstairs & when the central heating is on can be heard going all the time.
It sounds as if the system has not been balanced to spread the heat evenly though all the radiators, from cold all rads should start to heat at the same rate. Or it may be the pump is not delivering sufficient movement of water to satisfy all the rads.
Modern central heating does not just switch on and off but changes the flame height according to return water temperature.
If a radiator has not had the lock shield valve set then it can return hot water to boiler and turn the boiler down. However if that radiator also has a TRV fitted as the room heats up then the TRV will stop the flow and the next room will heat up. Again if lock shield valve is wide open it will return hot water and turn boiler down until that TRV starts to work.
Once all TRV's are working the whole system will then work A1 until turned off. Then all rooms cool and the process starts all over again.
Cure is to set lock shield valves I think from memory about 10 deg C between incoming and outgoing temperature with TRV wide open.
What I do is turn radiator off then once pipe is cool turn it back on slowly until just feel heat once done with all then next time heating turns on I check them and any cool radiator just open it a little touch more.
Takes some time but once done that's it you never touch again.
Does that make sense to you? So many people open the valves without realising it will then turn boiler down. Did not matter with old boilers they only measured output water heat.