Thermostat incorrectly wired

[jjbrown84]

Hi all,

I've recently moved into a flat that has a Baxi 105 HE connected to a Sunvic 2259 TLX thermostat.
The stat seemed to be very laggy so I popped it open to check it's wiring and the resistor.

The resistor isn't wired up at all, there's just Twin & Earth with black appearing to be Live and red Switched.

The trouble the Earth is connected to the metal back plate and was initially giving strange readings. Sure enough when the boiler is powered on the entire back plate is 120VAC referenced to a nearby water pipe (and later checked with an earth contact in a socket.

There is also another Twin and Earth running to behind the stat which is completely disconnected (cut)

I was initially planning to use the L/SL and install a battery operated digital Stat but the fact theres an earth wire at 120V when the system is on is a bit concerning (centre tapped to the boiler?).
The landlord is actually a charity shop so I was hoping to avoid any unnecessary call outs but I may not have a choice.
I haven't installed a thermostat before but I know my way around circuits
Bit of a long shot, but has anyone seen anything like this before and have a suggestion on how to proceed? Or have an idea where the 120V is coming from? It disappears when the boiler is off - also, although it's black and red by the stat, it's blue and brown into the boiler so clearly connected elsewhere.

 

[stem]

I find it frequently, a mechanical thermostat without the accelerator wired up.

Normally, as you suggest, I would simply swap the existing thermostat for a battery operated one that doesn't need a neutral. The batteries last several years and you have the advantage of a permanent visual display of the room temperature. For example a Drayton Digistat or Danfoss RET. If there is an existing programmer controlling the on & off times, the a basic thermostat (ie not a programmable version) would be a good replacement. They don't lose accuracy over time as the mechanical thermostats do because of wear of their moving parts, and are far more accurate to start with even when the mechanical thermostat has its accelerator is wired up.

What do the other wires do? well that all depends on what is connected to the other end, and making suggestions over a forum can only be guess work. Not ideal for an electric scenario. There certainly shouldn't be any voltage on the earth, but much will depend on the voltage of what is connected to the other probe on the multimeter as it will measure the voltage between the two points. A nearby water pipe isn't really a solid reference point, but I see why you might use it, I've done so my self in the past. A known neutral such as a nearby socket, would be better. Alternatively, can you not trace the thermostat cable back to its origin and check the connection there? It simply might not be connected to anything, so is picking up stray voltages by inductance.

 

[jjbrown84]

Thanks Stem,

I had later tried a socket for a true reference with the same result.
I agree I need to get eyes on the cable to find out what's really going on, I was just hoping there was a reason for it all (though I wasn't holding out hope) before I started crawling.
I was almost hoping it had been bodged to an extent where the E wire was supposed to be used as the N but the installer forgot halfway through
I think a trip to the loft is on the cards after work.
I'd rather be up there in the winter than the summer anyway.

Edit

I hadn't realised the battery life was so good for an electronic one. No brainer replacement once I've checked this Earth wire