Replacing three wire thermostat

[TangoMan2]

Hi guys could do with some help. We have a Vulcan Continental boiler that is still chugging away. This is connected up to a potterton EP2001 progamable timer and eventually a Satchwell TLX 2259 thermostat.

I want to replace the thermostat with a Siemens RDJ10-GB thermostat. The problem I have got is the old one has three wires, the new has two. There seems to be 240v across the red and blue when not heating and the yellow wire seems to be the same as the blue.

Any ideas in if I can swap these over?

 

[namsag]

Dont use the neutral(snip and make safe) in the new stat just use your existing live in and liveout of stat

 

[bengasman]

quite often you find the neutral wire being used as a live wire.
it must be reconnected to earth at the source and the wire can then be used as a normal 2 wire thermostat connection, with the earth properly terminated at the roomstat.

 

[TangoMan2]

Okay, cheers for the response guys, so I am looking for a live in and live out?

I trace back to the boiler and see what surprises are there too.

 

[gas4you]

I would agree with namsag.

Check to see if you get 240VAC across the yellow and blue when heating is on.

This will prove blue is neutral.

 

[TangoMan2]

Dave, I assume you mean when the programmer is on?

There is 240v at this point when the thermostat contacts are open, when they close it goes to zero. If the timer is switched to off and the contacts are open there is a residual voltage of 14v.

Pretty sure the red wire is live and the yellow is live switched.

Anyway cheers guys for your help, it's appreciated.

 

[TangoMan2]

Okay done, installed last night works as hoped. The CH is now set to be permanently on and the thermostat controls what heat setting it is at depending on time of day. SO for the evening it is set to 21c and at night it switches to 13c.