I'm about to replace my thermostat, but was surprised to find no neutral connection. It seems to function ok though - when the thermostat is on, I see 230V AC across T1 and T3, and the boiler turns on. It looks like the blue wire was in the right position, but is unscrewed and the metal end of the wire removed. I can measure 190V AC from T1 to the expose tip of the blue wire. Should it have been connected? Is it just a mistake?
If you can verify that the other end of the blue wire is connected to a neutral then it should really be connected. It is something to do with temeperature gradient on this type of stat that required the neutral to operate it. It makes the room stat more accurate I believe. However, if you are changing it there may be no requirement for it in some types of thermostat. https://www.honeywelluk.com/product...perature-onoff-control/T6360-Room-Thermostat/
Marcos, if you are only measuring 190V between the bare blue wire end and the Live, then I wouldn't connect it as something is amiss and that may be why it was removed in the first place. Also, you should not be detecting 230V between T1 and T3 when the thermostat is turned up beyond its 'click', only when it's set to below room temp (ie the themo is 'off'). Are you planning to replace it? If so, will it be a battery type (ie '2-wire')? If so, you can ignore that blue, folding over the end and taping it for complete safety. If, however, you want to keep that stat or replace it with a new 'mains'-powered type, then we'll need to investigate where exactly that blue is going to at the other end! Do you have a wiring centre (a rectangular box with a lid that all the cables are going to)? If so, could you turn off the power, open the lid and take a photo? Can you work out which cable is the 'stat one?
Yes it should be connected, there is a small heating device in the stat which senses the ambient temperature (accelerator I have seen it called) and makes the stat more sensitive to temp changes than just relying on the bi-metal strip. Many plumbers don't understand this because to them a thermostat "is only a switch"!
Thanks all for the responses - nice to know for the future, but as it turned out the new one didn't need neutral (only COM and NO), and so I just stripped the end and put it in one of the parking terminals. There was no box, just a cable coming out of the wall (annoyingly to the side, meaning on the new one I could only drill a screw hole for one side).
By 'wiring centre', I mean at the other end of that cable, usually near where the boiler is (or hot cylinder). Anyhoo, with a 2-wire replacement stat, you certainly don't need that Neutral.
Well done for resolving it - Happy New 'Stat (You are 100% sure that these are 'parking' terminals, and the top unit doesn't have any pins coming out of it that engages with them?)
Yes as @Risteard says, NC means 'Normally Closed', which means the 'normally the relay is closed, completing the circuit, and turning the device on', which is what you'd have for aircon. NO is 'Normally Open', and does the reverse (only closes the relay to turn on boiler when room below temp).
Yep - it's from the instructions (as an amateur DIYer I'd have probably just called the 'spare terminals', but I like the sound of 'parking slots'. You were right too about the box near the boiler, there's a surface mounted box, but it's behind a washing machine. Glad I didn't have to go poking around in there.
It's nice to see the Tado provide these terminals - some 2-wire stats don't have them which is a nutty omission.
Yes, I agree, it is a useful addition to allow redundant wires to be parked. The usual trait is finding them cut which helps nobody f they are required again in future.