Three port valve Green/yellow = earth. Blue = neutral. White = Line supply from room thermostat when calling for heat. Grey = Line supply from tank thermostat when satisfied. Orange = Supply to boiler.
The valve only supplies boiler (Orange) when White + Grey are both line, when the valve is at rest or half way boiler gets supply direct from tank thermostat. SW2 only switches when valve is all the way across and to get valve all the way grey must be line.
Apologies for the lack of details, I'm not sure what is necessarily relevant. To be clear, the system worked as expected before the Nest install and no wiring changes have been made in the junction box (old thermostat just left on full for now to 'bypass' it).
The valve is a Drayton HSA3, so doesn't have an earth connection. Does this change the recommendations above, especially as it works fine with previous programmer?
Just a thought, you have put the link from L to 2 (also 5) in heat link? I would have thought you would have complained the motorised valve was not moving if that was the case.
Test the three wires of the motorised valve, orange must be line for boiler to run, and for orange to be line, white and grey must also be line.
It could be the tank thermostat, when NOT calling for heat the grey wire to motorised valve becomes line, it is a long time ago when my daughter had a Y plan, and I went with a meter to get it going, putting the meter leads on the connections I found there were so many back feeds it seemed impossible to work out what was going on.
It seems my son-in-law had tidied the airing cupboard, and previous owners had a fault with the central heating, and had got around the problem by latching the bleeding lever on the motorised valve to bleed (centre) position, while tiding he caught the lever (un-latching it) so then central heating would not work, fault was a micro switch in the valve head, I to be frank was not sure what the fault was, but thought it could be micro switch simple V3 as common as muck, about £4, but Sunday so could not buy one, so got whole valve around £50 only shop open was Screwfix, and it worked, after spending that much, it was a relief that it worked. Some makes you can get replacement heads. Would not recommend trying to replace micro switch, likely springs will go everywhere.
Do say how you get on, likely others will read this, and it may help them.
I also had to replace the motorised control recently, the valve had rusted stuck, so I presume the motor had burned out trying to turn it. Cost £35 from Amazon, so not too bad. I replaced this before the Nest install, and everything worked fine. I'll check the line from when the tank is satisfied and see if that leads me down the right path.
It's a confusing mess in the junction box - I'm yet to discover where the thermostat is wired in; cables in the thermostat are blue, red and yellow - but that doesn't exist anywhere in the junction box, and no signs of a join anywhere! Joys of a new home I guess...
I hope better than mine was, lucky sorted in the summer, there was a programmer with 6 options however it turned out really only 3 as there were only two of the three wires still connected.
Boiler was in the granny flat and the flat worked OK, but to get central heating in the main house, I had to walk outside down a set of steps, and plug in the pump.
I just could not believe the last owners had lived with such a heath robinson setup. And some where behind the wall panelling or false ceiling the three and earth changes colours from old red, yellow, blue to brown, black, grey but grey not connected.
Looked at the reasonable new consumer unit, and tried to switch off main house, did not work, found an old fuse box between false ceiling and original garage ceiling before garage made into a granny flat.
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