At last, one I can answer and one which caused me mucho grief in the finding out of.
Two main types of radiator systems, the older system which is used in large buildings - on this there is one flow pipe and all the radiators are connected to this one pipe.
Each radiator is teed off at either end so that it is in parallel to the flow, but the entry tee also has a baffle inside it which causes a small pressure drop across it, as you open your radiator valve hot some hot water tries to flow around the restriction(ie through the radiator) thus warming it up, the more you open the valve the greater the flow.The radiators are in series with each other.
On the other type of heating system, mostly domestic, there is a supply pipe from the boiler and a return pipe but the two are never directly joined.
Radiators are connected between the two and all the water flow passes through the radiators which are effectively in parallel with each other.
The problems of one part of the system being hot and the rest cold when the boiler and water pump are running are usually down to folk not understanding what the second valve on the radiator does - the lockshield.
There are a couple of things you do to correctly balance a system when it is new but once done it is rare to have to go through it all again,but one that does is in the case of people trying to get more heat out of their radiators by opening the lockshield valve fully.
Each radiators presents a resistance to the flow of fluid through it and it follows that the hot water will take the easiest route.The idea of the lockshield is to make sure that this resistance is equalised.If you open all of them up fully then all that happens is that you create a low resistance loop and all your hot water goes down one,maybe two radiators - a short circuit.
People often think that by opening both the main valve and lockshield valve fully they will get more heat but this is not true, one radiator will get hot, the others stay stone cold and, because the water does not give up enough heat on its passage through this truncated system, the boiler overheats the water(you can oftenh ear it just start to boil) and keeps cutting out on the boiler thermostat.
This is what you do - take a thermometer and go to the hottest radiator, measure the temperature at the inlet end and the outlet,(just hold the bulb to the pipe) there should be a drop of around 10degrees C, if it is less than that then the water is not giving it’s heat up properly - the flow through that radiator is too great.Close the lockshield down say halway and wait maybe ten minutes and compare the inlet and outlet temperatures, if the temperature drop is greater than the 10 or so degrees then open it a little.
Eventually you will get this radiator to its correct setting, now you go round all the other radiators and do the same.You actually end up having to go round several times because altering one radiator affects all the others a little but eventually you will come to a point of balance.
Ideally what you should do is to replace all the inlet valves with the thermostatic ones, that way each area will be maintained at the desired temperature, but this will only work on a balanced system.
It may be that someone else in the building, probably the ones closest to the boiler, is too hot and they turn it off or turn the electric thermostat down instead of turning their own radiators down.As I say it’s simply a case of a badly balanced system, in fact you should be able to run your system all year with the correct settings without turning it off as many folk do for the summer months, after all it should only come on when the temperature fall below comfort levels.