Radiators not always hot

[magrumbles]

Please help... I have a potterton netaheat conventional system, with 2 tanks in the loft and a hot water cylinder. I think I have a one pipe CH system. We had to replace the pump back in November last year (grandfos) but I think this was happening before that as I remember being cold last winter and having to adjust the temperature controls on the radiators. When the central heating is on and the rads are all set to 3 (6 in total in 2bed house), some are just warm The pipework coming out of the floor feeding these rads are hot. If I turn the dial up to 4 the water starts flowing through the rad and it heats up in seconds. Yet other times that same rad can be hot on 3. Why might this be? The boiler does seem to constantly be firing up when the CH is on. I've seen similar posts which talk about the speed of the pump or balancing the system but I'm not sure what that means. Thanks!

 

[Puddle]

Thermostatic valves on the radiator,thermostatically control the temperature of the area that is being heated by that radiator,once the area has reached the temp set through the valve it closes flow down to rad

Once temp falls below setting ,it will open and allow heating again

Rads ill be hot .....as valve open ,heating area,warm......as valve closed as area heated and rad flow been off a while and cool as area temp reached a while ago and flow been off since, allowing to cool,as soon as this area cools,valve will open and rad will get hot again