Central Heating Thermostat Questions

[theaxe]

I'm attempting to replace the thermostat in my brother's house (to a NEST). Looking at the existing setup I see:

1. A timer/controller next to the boiler. This just seems to turn the boiler on / off at set times. I assume the hot water then flows into the tank and that it turns off if a) the timer turns it off or b) the return temperature hits some limit.
2. A timer/controller next to the tank. It seems to be wired so that if the controller is ON then power goes to both of a) a thermostat on the tank and b) a room thermostat. Either of these then send power to the pump (the wall thermostat if the room is too cold, the tank thermostat if the tank is above 60 degrees).

This seems unusual to me, I can't see how the upstairs setup can trigger the boiler. I also don't understand why the tank being above 60 degrees would trigger the radiator pump separate to the room thermostat. Maybe they should be in series but they're in parallel.

If anyone is able to explain what's happening here I'd really appreciate it. My current thinking is that I should be able to use the NEST to replace the wall thermostat (to drive the pump) and leave the rest as-is.

[Jambo85]

Haven't seen that kind of set up before, some pictures would be helpful perhaps.

Does the timer beside the boiler only do central heating? If so then perhaps the one at the boiler is only for hot water.

Is there an immersion heater and are you certain that the timer at the tank isn't for it?

[theaxe]

Thanks for the reply. As far as I can tell the timer by the boiler is for hot water and the one by the tank is for the heating. There is an immersion heater but that's just on a switched spur and is generally off.

[Rickyy]

A modern conventional system works as such. Timer sends power to the room/cylinder stat, which if calling for heat will power the motor on the two port valve, when the valve is fully open, the mechanism inside physically closes a microswitch. The microswitch has a permanent live supply going into it, which powers the boiler when closed. It can also power the pump on older boilers,most modern boilers control the pump via their PCB.

Similar process for a 3 port valve, just not as straightforward.

Wiring for two port valve is as follows:

Blue = Neutral
Green/yellow = Earth
Brown = Motor live (this is powered from the 'call for heat' from respective stats)
Grey = Permanent live (power in to microswitch)
Orange = Switched live (power out to boiler)

Three port:
Orange = Switched live/'call for heat' from cylinder stat
Grey = HW off from timer/'satisfied' from cylinder stat
White = 'call for heat' from room stat
Blue and Green/yellow same as two port

Hope this helps.