Nest thermostat installation and review

Out here in the sticks we haven't got mains gas, so the heating and hot water is supplied by an oil boiler. As this is a finite resource in the sense you have to keep some oil in the tank, I wanted to keep it as efficient as possible.  I researched a number of smart thermostats, and eventually settled with Nest. I liked the Honeywell Evo setup with smart TRVs but the cost was prohibitive in my opinion.  The original setup was a programmer and room thermostat and I was struggling to get it to work properly.

My system is a Y-plan, with a wiring junction box in the airing cupboard. It was wired pretty much exactly the same as the below diagram. Although I have a lot of electrical experience I'd never worked on heating systems and always had a combi boiler so it took a bit of head scratching!

Danfoss Y plan wiring diagram

First things first, I turned off the supply to the heating system. As the room thermostat in the nest system connects wirelessly to the heat link, rather than being hard wired in as before, I needed to modify the wiring to bypass the room stat. This meant removing the feed to the thermostat from pins 2, 4 and 5 and then moving the feed to the mid position valve from pin 5 to pin 4.  Unfortunately I didn't have enough spare cores (only 1) between the heat link and junction box, so couldn't make use of the facility to supply the nest thermostats power from the heat link. This wasn't a problem as it comes with a USB Power supply.

Once this was done, I connected the heat link up. I originally intended to fit it in the same place as the previous programmer, but as it was on a tiled kitchen splash back, I instead joined the cables through and fitted it next to the boiler. I found the terminals quite small and fiddly in the heat link, and struggled to get them in, probably because I was using the existing 1mm solid cables. As I had the kit, I used ferrules on the ends to make things easier as I struggled to get a proper connection on the ones with 2 cables in.  Pictures below show my first attempt followed by the second attempt which was much neater.

First wiring attempt

Second attempt, with ferrules, much better

With all the wiring completed, I turned the power on and plugged the Nest display in.  There were a few simple settings to sort out on the display, and it asked for the WiFi password, and that was it, all up and running.  I got the app downloaded as it was easier to set up the schedules this way, it defaults to having hot water on for 0600-0800 and 1700-2000, I thought this was a bit excessive so we've got it set for 0600-0700 and 1900-2000, with a view to using the 30 minute boost function if required.  This seems to work well for us.  At the moment we have the heating side of things turned off, as it's not cold enough yet.

Nest app overview

Hot water schedule

I did have a small problem with the Nest system, as I noticed my boiler randomly starting up in the middle of the day, when there was no call for heating or hot water.  My initial thoughts were the frost stat in the boiler itself, but that checked out OK.  After much head scratching and testing, it turned out to be a faulty relay on the hot water side of the heat link.  It was leaking around 30v to the normally open contact, and creeping up to around 50v which was enough to start the boiler.  This was a major concern as it could have caused the hot water cylinder to explode!

A quick call to Nest support sorted everything out, I explained the symptoms and they agreed it sounded like a relay fault, and sent me a replacement unit which was dispatched the same day (from Germany!) and arrived at my door the day after.  It was a simple case of swapping the heat link over and then the display, and registering to my Nest account again.  I was very impressed with how easy it was to get this problem sorted and get back up and running again.

Even with the minor issue, I am very impressed with the Nest thermostat.  It's incredibly easy to control and adjust, and of course there are lots of other products that work alongside Nest.  The aftersales support was excellent, and the returns process was very fast. 

One thing this did highlight though, was that my old thermostat was not entirely to blame.  My boiler (a HRM Wallstar) has a tendency to lock out first thing in the morning, which seems to be a common problem with this model.  It always fires up straight away on pressing the reset, which is inconveniently located behind the cover outside.  I've been reading up on troubleshooting the boiler, and there will be further posts on this.