Upgrading to an OpenTherm thermostat

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I have an Intergas boiler that supports OpenTherm and currently a 7-day digital thermostat (Honeywell CM67). The thermostat is getting long in the tooth so I'm thinking about replacing it with a T4M which is OpenTherm. I've had a look at the wiring on the control panel and discovered that at present the thermostat is switching 240v.

I did think it was just a matter of moving the two thermostat wires off those terminals and onto the OpenTherm terminals then swapping out the thermostat but a heating engineer (who admitted they weren't a sparky) was sceptical when they saw the wiring. They were of the opinion that because my house is Y-plan what I wanted to do would stop the hot water working. That seems unlikely to me because from what I can read online the hot water will fire the boiler when timer requires it and the cylinder 'stat requires it - with the boiler switching the pump on itself. Surely this is no connection between the thermostat and the hot water otherwise I wouldn't be able to get hot water in the summer when the room 'stat is almost always off.

My only concern is that that the thermostat is sharing the same wiring cable as everything else going to the boiler and there might be a risk of the OpenTherm signals being corrupted because they are in close proximity to mains voltage (seems unlikely given that O/T is 1,000b/s.

Actually as an aside would anyone care to guess what efficiency improvement I might see? I'm assuming not very much because the Intergas is already modulating to an extent but would be curious if anyone has any experience of doing this.
 
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I think OT would still possibly work? It would be wired into the boiler as opposed to activating the motorised valve?
 
I think OT would still possibly work? It would be wired into the boiler as opposed to activating the motorised valve?
So you think that's what he might have been getting at?

I assume that since OT is low voltage and digital it must rely on the boiler to start the pump (which it clearly can do since it can force an overrun) but you've made me wonder now what causes the valve to change position. Normally the default is for water. Hmmm.
 
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Hmmm. This is interesting:


It's a series of articles. They seem to imply that I will need some rewiring in order for the boiler to be able to operate the valve. Also it's best advised to upgrade the cylinder 'stat. There's also an issue around what happens if the HW has just finished and H is called for because the water that first enters the H circuit could be way hotter than what is wanted.

So I think the answer to my question is 'no'. There's more work required and since it involves running more cables up to the airing cupboard from my garage I doubt if the efficiency gains will offset the cost. Worth doing as/when I change the boiler but my boiler should last many years yet (possibly to the point where they can no longer be bought).
 
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I wouldn’t dismiss it right away, there’s some on here who know this boiler and controls better than me. Also, that link from heating academy, he used to post on here. @Razor900 or @muggles might be able to assist you further.
 
If you're worried about running power and data cables together, and you should be, use RF.

It might be possible to use DHW stat and relay in the airing cupboard to switch the MV and link the DHW receiver to the heating receiver by the boiler by RF.

The OT info is RF from cylinder cupb and room stat then wired from the heating receiver next to the boiler.

One of the cables used at the moment to switch the boiler could be used from the heating receiver to the Y plan MV.

Signal strength would be decisive. EPH controls can I recall be configured this way and provide PDHW.

Just an idea.
 

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