2 and 3 wire room stats...?

[ohreally]

I have a WB 28cdi rsf to which i intend connecting a room stat. WB stipulate a 3 wire stat for this boiler. I've noticed the majority of digital stats are 2 wire connections, my question is can i fit a two wire stat and if so which wires do i connect into the 3 wire terminal and if i make a mistake with these two wires what are the likely consequences?

What is the difference (apart from the obvious) between two and three wire stats?

 

[Mr Smyth]

wait for it...................................................





................................................. anticipation. (an anticipator)

Improves accuracy, (3 wire that is)

 

[D_Hailsham]

You can use a two wire digital stat with your boiler. Older mechanical stats needed the third connection to Neutral in order to improve the accuracy of the stat.

The Live connection is Ls and the Switched Live connection is Lr.

 

[AlanE]

Because of the inherent delay in a mechanical thermostat operating a small heater (resistor) is fixed to the bimetal strip to simulate the room warming up (hence its name anticipator as it anticipates the warming up of the room). This has the effect of smoothing out the operation of the device.

There is no delay in a thermistor, the normal sensing device used, so there is no need for this extra heating device henco only the need for two wires.

 

[ohreally]

Would anyone care to recommend a suitable device? I don't need anything elaborate, just straightforward to install and operate.

 

[mikely]

The Honeywell DT90E or CM907 depending if you want programmable control or not. Both thermostats use Time Proportional + Integral control which gives a better temperature stability than a simple ON/ OFF stat.

 

[ohreally]

Thanks, i'll chew them over.

 

[ohreally]

Ordered a CM907, thanks.

Can i have a quick run through on wiring it to the boiler please - do i retain or discard the existing WB mechanical timer?

 

[mikely]

You will not need the mechanical timer because the heating will be programmed by the stat. Just connect the stat terminals A and B to connections Ls and Lr respectively on the boiler.

 

[ohreally]

Ignore.

 

[ohreally]

Just connect the stat terminals A and B to connections Ls and Lr respectively on the boiler.

I found this on another thread, is this saying the same or is it slightly different? //www.diynot.com/forums/viewtopic.php?p=823347#823347

discon existing integral timer wires.
use boiler terminals r1 & rl wire these 2 wires to A & B.in cm907.
rl=A.....r1=B.
use cable suitable for 230v.ensure 3amp fuse in spur/plug.
cm907 has no neutral wire,it is just a switch.
===========================================
DO NOT connect any wire to boiler terminals rn or cn.these are neutrals.an incorrect wire here will give you grief.
===========================================
boiler terminals link ci & cl.

 

[mikely]

I can't see any such connections on the wiring diagram for your boiler. The external thermostat connections are to terminal block ST8. To where is your existing programmer connected?

 

[ohreally]

Existing WB device has 4 terminals...

1. Brown (pair linking to 5).
2. White

3.Grey (3 appears to switch to terminal 4)

5. Brown (Linked with Brown from terminal 1).

 

[mikely]

Fig 13 on page 12 of the Installation manual, http://www.worcester-bosch.co.uk/ca...structions-for-24cdi-rsf--discontinued-07.pdf shows how to connect an external programmer and/ or thermostat. Unless you have existing wiring, you will have to run a two core cable from the boiler (ST8) to your CM907 thermostat. I assume that your existing programmer is external to the boiler and if so it should be connected to the ST8 terminals. You need to diconnect the programmer and connect the CM907 in its place. You could put the CM907 where the programmer now is but that is unlikely to be where you want the thermostat. As pointed out before, you only need a two wire connection to the thermostat. If wiring is not possible then you need to use the wireless version, CM927.

 

[ohreally]

I assume that your existing programmer is external to the boiler

The existing device is integral - fitted on the boiler control pannel, similar to that shown in fig.15 on the link except it's mechanical.