And it also seems to be overheating the rads
I am not a plumber, however the TRV and lock shield should stop radiators over heating, what ever the motorised valve does. Specially with a modulating boiler.
I remember my mothers house, a thermostat failed to switch off, and when I arrived the room was roasting, and it was pointed out to me, two things would have needed to have failed, the thermostat which sat on her tea trolley and the TRV control, the thermostat needed fixing yes, but room should not have over heated.
Once TRV set together with lock shield valve, the radiator would get hot on change of room temp in morning but then only get warm to maintain the room.
The three port actuator has a load of V3 micro switches,
and is as seen more complex inside, and when those micro switches stick it can do some odd things, although it fires the boiler when the central heating demands, for the DHW it does not switch on the boiler, the tank thermostat does that directly and also grey wire is made line when DHW is NOT required, moving the valve fully over if CH (white) is already energised, there also needs to be a connection to N/C contact at the programmer.
The Y Plan was favoured with boilers without a over run to cool down, since default is HW open, once every thing turned off the boiler can cool by heating the DHW. Not quite as good as the old C Plan, but C Plan got DHW too hot and wasted energy, it relies on the pump allowing some water to still flow with thermo syphon with Y Plan to cool the boiler on switch off, and I am told reduces the stress on the boiler as a result.
However I am an electrical engineer not a heating and ventilation engineer, so I have only been involved when I have found wiring faults, all too often I have found 3 core instead of 4 core cables to the tank thermostat, and the green/yellow used as a line wire instead of earth, this has always been against the rules, as an earth shall be run to all fixed equipment even if the equipment is class II, different rules for portable equipment. but the rule goes at least back to 1955 with 13th edition, I have not seen older copies of the regulations, today it is not permitted to use green/yellow for anything but earth, and over sleeving green/yellow is not permitted, even in 2008 there was some ambiguity for green/yellow cores in a multi-core.
I say this as I have seen electricians disconnect green/yellow from a line supply, and as a result cause Y Plan central heating not to work correctly. A case of two wrongs not making a right.
I personally hated working on Y Plan, all those resistors and diodes make use of a multi-meter to diagnose faults hard, and since there is a diode where an RCD is required it must be type A
you have never been permitted to use type AC
which is a pain when you find type AC fitted. Today we no longer used type AC so no longer a problem, but I can see why people did not like the Y Plan even if it does allow the boiler to cool down better when no run on built into the boiler.
