Another Central Heating Question

IANAHE but I expect each thermostat will be connected to a motorised valve, so when the thermostat calls for heat the valve is opened and then inside the valve gubbins there is a microswitch that will turn the boiler on once the valve is open. The system is probably wired so it needs the timer to be "on" to give power to the valves, then the thermostats control the valves which control the boiler.

Time clock calling? Yes

sends signal to thermostat, room temperature colder than set point, yes

signal sent to motorised valve, valve motors open, makes a switch, boiler and pump run.

water will only flow through the valve that is open, of course both could be open so both zones of heating will supplied.

It will run for as long as the time clock is on and there is demand from a thermostat. How long that is depends on the heat loss of your building, how hot you are asking the room to be, the outside air temperature, how many times you open doors etc and also if you introduce other heat sources, wood burners, cooking, showering, lights, tv’s, even us all make a difference

OK, so it makes sense that the thermostats are wired to the appropriate valve to open as required. There's a load of wires going between the valves and a couple of black boxes which must be doing something.

On the pump, it seems to run almost continuously, even when there is no demand from the thermostats, which doesn't seem right to me.

Thanks for the replies.

The pump will usually run on a bit afterwards once the thermostat no longer demands heat.  This is normally only 10mins or so though, it would be unusual to be continuous

There is also usually a frost setting.  That shouldn't be on quite yet I wouldn't think but you never know if someone has set it wrongly

We had that controller downstairs (until it was swapped for a Hive one) and have similar two zone heating. There's a white box that all of the cables go into on the side of our hot water tank. That's where everything comes together, pump, valves thermostats.

Thanks. One of the valves has a small leak that we're getting replaced later this week so I'll ask the heating guy what everything is.

Swapping to a smart system is a possibility, but there are also 2 UFH systems! Electric in the kitchen and wet in the newish extension. (Not sure why it was done like this.)

I get called to such things regularly. It is very common to find wiring faults on heating control systems. A recent one was 2 different plumbers had been to a house to see why the hot water wasnt getting very hot. Both said there was nothing wrong with it. The house owner asked me to have a look at it when I was doing another job, the moment I took the wiring centre cover off I saw it had been wired wrongly. There were 3 valves, so should have been 4 wires in terminal 10/12. There were 3, with the 4th one in another terminal. The 4th one was the HW valve utput to boiler/pump. The idiot had wired it up wrongly, so when the valve was open, calling for heat, it only opened, but did not send that signal to the boiler, so the only time the HW was being heated was when the HW and CH were both on, which was for half an hour or so when they overlapped in the morning, hence thye were only getting around 5o litres of warm water each day.

Such a simple fault caused so much trouble.

Yes, there are two overlapping disciplines, plumbing and electrical. The plumber we use is very upfront and says he doesn't do the wiring side, but has a mate who does, and so far both have been fine. Part of the problem is me not understanding the system, so hard to know what's a fault and what's normal. Thanks to the answers above I understand a bit more.