Nest E & Baxi 228 OpenTherm issue

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Hi all,

I've searched online and can't find anyone with the same issue, so thought I'd post here.

At the weekend I installed a Nest Thermostat E to my Baxi 228 using OpenTherm, replacing an on/off thermostat. Initially, I removed the old thermostat's wire from terminal X4 and did not reinstate the link wire (following advice from Baxi, as the manual isn't clear), however the Nest didn't seem to work. Having put the link wire back in to terminal X4, the Nest seems to be controlling the heating as expected and modulates the central heating temperature.

However, the boiler now fires up periodically (roughly every hour or so) and heats the domestic hot water to 60c (even though it's set to 50c on the Nest and on the dial on front of the boiler). This didn't happen with the on/off thermostat. I thought it might be the Bacteria Prevention feature on the Nest, but this option does not appear in the Nest menu (probably because it's set to combi boiler).

Any ideas what could be causing this? Could the link wire be the issue here?
 
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Anyone got experience of wiring the Baxi 200/400 series with OpenTherm? Nest is controlling the central heating and domestic hot water temperature as expected, so I guess the wiring is okay. If the link wire in the 230v switch terminal was causing a problem then the boiler would be firing for central heating and the Nest wouldn't work, right?

Still not sure why the boiler is firing every hour to heat the domestic hot water to 60c - you can see the difference between the on/off control and the Nest in the attached images.
 

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Just wondered how you conclude that it's heating the domestic hot water? Does the boiler or Nest display say that it is? As you have a combi, unless there is a pre-heat function the hot water would only be heated when a tap was running. (I seem to think your model doesn't have pre-heat, but can't be 100% sure)

Also, the thermostat wiring options in the diagram you posted don't show any facility to control the hot water heating. So I wonder if it's all part of one of the Nest's central heating algorithm's. Nest is notorious for doing things that appear strange especially when it's in it's 'learning mode'. Changes in the weather will also have an effect. For example, it notices how long your home takes to heat up and cool down, and the low heat might be it simply running the heating before the temperature gets too low.

You could try disabling the 'True Radiant' setting and see what happens then. But, it may be that Nest might just be 'doing its thing'. Provided that your home isn't being overheated then I would tend to leave it be, and see if it settles. Part of the Nest concept is that one efficiency principle is a bit like a car, in that it uses less fuel when running gently rather than under acceleration.
 
Hi both,

Many thanks for your replies! I'm confident that the Nest heat link is wired up correctly — OT1 and OT2 to terminal X8 on the boiler. The old thermostat controller had four wires — two for power and two for the 230v switch. I terminated the power and removed the 230v switch cable from terminal X4, then reinstalled the link wire to that terminal.

I think it's heating DHW because when the boiler fires up, the little tap icon on the front of the boiler flashes. Nothing appears on the Nest thermostat display. I've turned True Radiant off for now, so let's see if that makes a difference.

It's a bit frustrating because I switched to OpenTherm to stop the boiler from cycling so much...
 
This is a known issue with Baxi as they've not implemented OpenTherm in the way you might expect. The Nest 3rd Gen works, Nest e doesn't. Honeywell T6R-HW works, the old T6 Lyric single channel (now discontinued) didn't. Their boilers require a certain hot water control ID to be transmitted to the boiler by the thermostat and not all controls do this.
 
Right, thanks. So I guess the options are live with it or switch to a different thermostat, unless there's a workaround?

I don't think Baxi or Nest are aware of this, or at least they didn't mention it when I spoke to them
 
Thanks, I think I'll try to return the Nest E and replace it with the 3rd Gen then. I got the E because installation seemed to be easier, but in the end I had to install a new wire anyway. Luckily the terminals on the boiler are accessible without removing the main cover...

Nest reckon their heat links aren't compatible with stranded wires, but the 230v supply from the boiler is stranded. Should I use a couple of Wago connectors or similar to switch to solid core, or will stranded be ok in practice?

Thanks for all your help. This is turning into a real headache. I'd live with the boiler firing up all the time, but it'll cost at least £30 a year, and more once prices go up.

So much for OpenTherm being a standard protocol...
 
Stranded wire is fine. I use ferrules on the end but you probably don't want to buy a pack of 100 ferrules to do one job!

OpenTherm is standard, and all manufacturers who are members of the OpenTherm Association need to demonstrate interoperability between controls and appliances. The problems come when non-members like Baxi try to use the OpenTherm communication protocol to control their boilers but deviate from the standard specification.
 
DHW is set to 50c at the moment. The Nest does seem to control the temperature as it should, as I tried turning it down to 40c and it worked.

However, once the boiler cools down, it fires up and heats the water to 60c (as shown on the boiler display). It's odd.

EDIT: The only thing that seems to stop it is turning DHW off via the dial on the boiler, but then of course there's no hot water. It does suggest some kind of conflict between the boiler's DHW control and the thermostat's (or as muggles says, the lack of a control in the nest e).
 
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So, this evening I returned the Nest E and bought the 3rd Gen. Successfully wired it up (a bit fiddly but I got there in the end) and... the boiler's doing exactly the same thing.

The Nest equipment settings are: combi boiler, OpenTherm, central heating temp 65c, DHW 50c.

Any ideas? I have a video of what the boiler's doing but I think it's too big to upload.

It's a shame the uSense isn't wireless or I'd be tempted to try that.
 
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