Digistat SCR to Nest Heat Link

[PaulCharlesworth]

Hi.

I've taken a plunge and bought a Nest Learning Thermostat to replace my current Drayton Digistat SCR and RF3.

I'm having some trouble getting my head around the wiring as my set up doesn't seem quite as simple as the instructions would like.

I'll happily call someone out to get it sorted but if it's as easy as someone on here clarifying what wires go where, I'd be grateful.

I've attached a picture of the existing set up, and the thing that is confusing me is that the instructions fir the Nest seem to suggest there should be a wire from L to 2, then the switched live wire connects to 3 and N. But that seems to contradict the existing set up a little?

To clarify as well, it is a Glow Worm 30cxi combi boiler.

Any clarity would be much appreciated.

 

[CBW]

Hello, should be as follows, SCR to Nest:

L to L (both)
N to N (both)
Earth to earth (both)
1 to 2
3 to 3

Don’t put any links in case it’s 24v on Glowworm

 

[PaulCharlesworth]

Hello, should be as follows, SCR to Nest:

L to L (both)
N to N (both)
Earth to earth (both)
1 to 2
3 to 3

Don’t put any links in case it’s 24v on Glowworm

That's what I thought having read other threads on here, and is what I have done, but the thermostat is not able to control the boiler. When I flick the power back on the central heating comes on permenantly and can only be stopped by turning the heating dial off on the boiler?

I did put a link cable in the first time I tried though, so I'm wondering if maybe it's now a boiler issue (that I've caused) rather than a thermostat wiring issue?

The thermostat clicks on and off etc it just doesn't achieve anything.

 

[stem]

As per @CBW a room thermostat can be connected to a CXI boiler using 'voltage free' terminals as per the info from the CXI Installation Manual below. The wire link (E) is removed and the thermostat connected to the terminals in its place.

Or alternatively it can be wired for 230V mains

If your thermostat was wired to the voltage free terminals and you made a link between Nest L and 2, you will have applied mains voltage to the boilers circuit board and have probably damaged it. The manual warns against this:

Sadly I suspect this is probably the case because the switching wires from the SCR (1 & 3) were kept separate from the mains N & L wiring. Where mains voltage is used for switching usually (but not always depending on the installers personal choice) there is a link between L and 1, instead of a separate wire from the boiler to 1.

 

[PaulCharlesworth]

Thank you both for taking the time to reply. I figured it would be something like that.

I just wanted to content myself that it wasn't something that was going to be resolved with wiring on the thermostat, and that it is at least wired correctly now.

I'll get someone out and hope it is possible to replace the likely damaged parts of the boiler.