Thermostat wiring help needed

[JimmyJamJerusalem]

Hi guys, I appreciate this is a common theme but I have searched and am just growing more and more confused.

I have a Worcester Greenstar 35CDi Combi boiler and am trying to replace a mechanical type honywell thermostat with a Seimens RDJ10RF/SET wireless thermostat.

Problem I have is the cables to the stat are very confusing.

Three cores

Blue
Brown
Yellow and Green

Simple I hear you cry.....

However looking in the manual for the boiler the three cables to the stat are marked up as

Lr
Ls
Ns

So which is which....?

Looking at the old Honeywell stat (pictured below)

Again it looks very simple.

However it contradicts the boiler schematic which says there is no earth.

Also

When I put a meter on them I get strange results.

Honeywell stat wound to off position -

yellow/green to brown = 240v
yellow/green to blue = 100v

I'm assuming the 100v is nothing to worry about, but when I test the earth against another earth (on a nearby light switch) I read 170v

Really quite confused.

When the honeywell switches it opens brown to blue and sends 240v back through the blue cable.

I'm guessing I've not got any neutral in the thermostat.

Are there any wireless thermostats that require no neutral to power the reciever as my current mechanical stat is in a poor poistion and the house bakes before the stat gets warm enough to switch off.....

Any wise advice please!???

 

[COLIN LITTLER]

I suggest you need a a length of 3 core plus earth cable (6243Y). Connections Earth to Earth, using green-yellow sleeving.
1 to Ls, using the brown core.
2 to Ns, using the black core with blue sleeving.
3 to Lr, using the grey core with brown sleeving.

 

[JohnKay]

No doubt this problem is now solved as the original post is 2 months old.
If not - I totally agree with COLIN LITTLER, that is the correct way that thermostat should have been wired.
Had the neutral been conneced in the first place, the existing thermostat would have been quite accurate.
On the boiler diagram, Ls, Lr, Ns - the s means supply, the r means return.

This thread highlights some of the problems of fault finding with a multimeter on the voltage scale. Readings will be misleading without a greater understanding of the expected results. the 100 volts is a reading between line and the boiler control panel.
The reading between the earth of the thermostat and a true earth would indicate the earth is not connected to the boiler, and is simply induced current, but should be checked.
As that thermostat is double insulated, the terminal being provided for convenience, it's not vital to be connected, but good practice to do so.

I feel I should say that I'm a bit concerned on the safety of the use of a multimeter in DIY hands. Some of those cheap meters have such poor leads and probes. The thought of someone inexperienced holding a probe in each hand, and testing at 230 volts make me cringe ! A spare set of leads for my meters may cost up to £50, something's wrong when a complete meter can be obtained for less than £10.

So if anyone is thinking of buying a meter, please don't go for the cheapest, and take the trouble to learn how to use it, especialy on the ohms scale. It may be confusing at first,
but with a safely disconneced item, the meter can be your friend.

One last thing on multimeters, they are for fault finding only, and CAN NOT be used to test a circuit for safety.

OK I'm going on a bit, but this is a DIY forum, and as I'm sure Colin will agree, electrical safety has to be stressed.