Dodgy neutral to thermostat.

[Hendersonm66]

I'm looking for a 2nd opinion.

A friend of mine asked me how easy it would be to upgrade his thermostat to something a bit more modern, it's connected to an Alpha 280 boiler. I suggested we have a check of the wiring to see what's there and then we would choose a new thermostat.

I've attached the wiring diagram of what's fitted, but on checking voltages on terminals I see something that seems to suggest a fault somewhere, although it all seems to be working fine.

With the Thermostat switch OPEN I measure 244 Volts between Pin 4(neutral) and Pin 1(live), but nothing on Pin 2(load) because the switch is open. Which was what I expected to see.

If I then turn the thermostat up so that the switch is closed, then I see 209Volts between Pin4(neutral) and Pin 1(Live), also 209Volts on Pin2(load) as the switch is closed.
So in this state the 240V signal to fire the boiler is going back to the boiler via Pin 2(Load), and also turned on the thermostat Accel. Heater. But the voltage has dropped.

If I do the same checks but instead check the supply voltage using the Earth pin instead of Neutral, it always reads 245V regardless of the thermostat switch position.

It appears that the Live, Feed to Load and Earth at the thermostat go directly to the boiler, but the neutral is coming from somewhere else in the building. The suspicion is a nearby lighting circuit.

Could this be caused by a loose or high resistance join wherever this neutral is being sourced? this is shown up because of the load of the accel. Heater in the thermostat.

I know the wee heater in the thermostat is a really small load, so I imagine that it would need a really bad joint elsewhere to show this up.

Or could it be some other wiring problem?

I am convinced that the wires on pins 1 and 2 are directly connected to the pins on the boiler that are looking for a volt-free contact.


Mike

 

[Johntheo5]

What do you get between pin1 and pin 2 with switch closed.

 

[Hendersonm66]

What do you get between pin1 and pin 2 with switch closed.

Interesting thought. Now I never measured that directly, and contact resistance did cross my mind though.

I think I never measured it because in relation to Neutral and earth, both pins 1&2 were at the same voltage.

Maybe I'm wrong then to assume I wouldn't measure a voltage across the switch. I can check this later in the week.

However, I was thinking, that regardless of switch position, I would always measure 245 V at pin1 to both Neutral and Earth.

MIke

 

[trojanhawrs]

What do you get between pin1 and pin 2 with switch closed.

209 on each he mentioned.

Think you're on the right track but it could also be a loose connection on the live, won't drop until there's a load on it. It's obviously not ideal robbing a neutral from somewhere either, if you're replacing with a nest though you can install the heat link next to the boiler and you only need 2 cores from heat link-stat. Ideally you can find the neutral and disconnect though.

 

[Johntheo5]

Is L a switched live from the programmer?
But you are getting 240v to earth.

 

[trojanhawrs]

Is L a switched live from the programmer?
But you are getting 240v to earth.

Right enough, it has to be the neutral. Either that or the heater load is too much for the cable (unlikely) or the heater is ****ed (more likely).