Nest Thermostat - Error N253 - explained for me

The other day I gave you my review on the new Nest Thermostat - and as cool as it looked, it had a couple of hiccups for me.

One of which is the N253 error code for me.

Since that review, and I have to hand it to Google for their extremely amazing customer service, I'd been on a webchat and a few emails with their team about what it is and why.

Officially, the N series of codes are 'non-critical', i.e. it's an error detecting something, but it's not stopping it from working.

As you'd seen, I can still use it, it still throws out heat, cool.

But why?

N253 means specifically it's missing the W1 power line.  

With Google we shared a few photos back and forth of my set up and I can see on my furnace.

(the wiring plate is from my previous Nest Thermostat E)

It looks like the CWR lines are on the board, and they seem to be travelling up to the thermostat.

Some sleuthing from the team at Google found that (and you can just barely see it in that photo) I have a 'fast-stat' put in.

When your Nest Thermostat E was installed, the pro added a Fast Stat "add-a-wire" that converts a 2-wire (R&W) heating system to 3-wire (R,W,C) without having to change the wiring through your house (i.e. you probably only have two actual wires running from your furnace to your thermostat).  The Fast Stat multiplexes power (C) and heating control (W) wires down the same conductor.  Behind your thermostat there will be a small device tucked into the wall that converts that one wire into the two that are connected to your thermostat W and C terminals.

The good news is that your thermostat will operate well like this and it is great to have a stable power supply installed.  The Nest Thermostat's wire detection does not currently work with the way the Fast Stat product operates internally but we are looking at improving that detection in a future software update. 

So... great news.  It will work.

But I still have an error code, albeit a non-critical one.  Maybe a future update will remove it for me.  /fingers_crossed

I wonder, if I can remove the fast-stat... will need to find a pro installer friend who might be able to best assist if this is the optimum set up for me.