@richw and
@muggles have given the answer, a condensing boiler, is designed to capture the latent heat, to do that is needs to modulate (turn down) to match what you require, so if set up correctly it should be running all the time, but not flat out. Unless using a modulating thermostat like with OpenTherm, the standard set up is this.
The boiler fires up at full output, the water goes through all radiators, at a rate set with the lock shield valve, so all get a fair crack of the whip. As each room heats up, the TRV starts to close, and reduces the flow in that radiator, which increase the flow in the rest, and when enough have started to close, the by-pass valve starts to open, so hot water returns to the boiler, the boiler senses this and reduces the output to suit, but there is one small problem, it can't totally turn the boiler off, it will first reduce output to the minium and then start turning on/off/on etc, called the mark/space ratio, but it needs some device so on a warm day, it will not run.
The old books said a wall thermostat should be put in the room kept coolest (so it is unlikely to fire up in the summer) with no alternative heating, this includes sun through windows, on the ground floor, as heat raises, and no outside doors, and that room has no TRV, however most homes there is no such room, so we look for an alternative, this could be used the hall, which does have an outside door, but also fit a TRV, so the room can recover fast after the door is opened, but the TRV and wall thermostat needs carefully matching.
Today, we tend to use multi-thermostats, I have three in the main house, one in the living room on the wall, one in the hall on the wall, and one in wife's bedroom built into the TRV head. One could use all linked TRV heads and have no wall thermostat, but it depends on where the radiator is. Ideal would be an internal wall, so the TRV is not cooled by the outside temperature, but in the days before double glazing, we tended to put the radiator under the window, and we still see the pictures in the text books.
View attachment 387318and the air does move,
View attachment 387319 so the TRV head is in the best place when the heating is running, but not always the best place before the heating switches on. My wife's bedroom TRV head will turn on heating because the wall is cold, even when the room is still warm, today, clearly don't need central heating, but wife's TRV is showing 20.5ºC and living room where I am sitting shows 23.5ºC wall thermostat, one TRV shows 23ºC the other 21.5ºC and the room is a bit on the warm side, so one has to consider where to place thermostats which can fire the central heating.
With the likes of Drayton Wiser, I think it allows one to have up to 9 thermostats connected to one hub, these can be wall mounted, or combined with the TRV, my first attempt I got it wrong, the hall cools down too slowly, you can adjust heat uptime with the lock shield valve, but not the cool downtime. So had to put a second thermostat in the living room, but still need one in the hall, as there is an open fire in the living room, which would if lit, cool the rest of the house, and wife's bedroom likely coldest room in the house, but only used at night, so the TRV is programmed only to heat room at night, and Living room TRV programmed only to heat in the day.
Mine is a large house, and to heat all rooms 24/7 would cost a fortune, so the 10 programmable TRV heads means rooms only heated when required, so only need one tank of oil per year.
