Myson MEP2c Wiring Question

[stem]

@stem Presumably the brown that was disconnected was the live feed to the grey on the 2 port valves, so by disconnecting that the boiler wouldn't get a feed when the valve opened.

It's a possibility that the 2 port valve microswitches also pick up their live suplies from the thermostat connection, but they could also get then from elsewhere. However, that was what I was thinking when I said it would be worth checking the hot water function again. However, it is irrelevant now if jinglieD goes for another programmable thermostat upstairs, as the programmer wiring will remain as is.

 

[Johnmdc]

Fair enough.

 

[JunglieD]

@Johnmdc Digging behind the MEP2c I couldn't find a spare brown wire as such but did observe a block connector joining two together as pictured:

An overview of the Joint Box area is below:

Close-up inside the joint box below:

Note wires lead to the following components (starting left-to-right with main wires from bottom of joint box)
Black 1 > Myson Power Extra set to Auto (pen labelled 'Up)
Black 2 > Myson Power Extra set to Auto (pen labelled 'D')
White 1 > Thermostat on Hot Water Tank
White 2 > The Red Grundfos component in the overview photo
Black 3 (on right hand side) > Honeywell Device sitting onto of pipe into tank

@stem it's good that Option 3 of fitting a programmable stat should work. You are right, the MPRT is battery powered downstairs and I can live with that arrangement for the upstairs too (I do have a black, grey and brown wire - 3 in total - to the fixed stat though, clearly no blue).

 

[Johnmdc]

Thank you for the pictures. So that explains the missing brown, but now there is a missing blue (unless one of the cables used is just a single brown). What I was trying to determine was if there was a spare wire not being used that could be used to feed the upstairs heating.
It is hard to tell from the picture of the joint box.
But if you are going to change the upstairs thermostat for a programmable one all of this is irrelevant anyway.

 

[JunglieD]

@Johnmdc Thank you - I think I've reached a satisfactory outcome following the advice from stem

@stem - I will be ordering a 'new' battery powered MPRTto replace the fixed thermostat. When it comes, which wires should I be putting where to get the desired outcome for the upstairs zone? I have 3-wires to my Fixed stat and seemingly only use 2 on my downstairs programmable one

Current Fixed thermostat:

Back of the batter MPRT I have downstairs (likely to be the same for the one I purchase):

 

[stem]

@stem - I will be ordering a 'new' battery powered MPRTto replace the fixed thermostat. When it comes, which wires should I be putting where to get the desired outcome for the upstairs zone? I have 3-wires to my Fixed stat and seemingly only use 2 on my downstairs programmable one

It's difficult to see in the photo, but are you sure there are 3 wires, and that one is not part of the thermostat? There may be three but if there are can you confirm their colour and what terminals each goes to. If there are 3 one is probably a neutral and you won't need it for the new programmer.

That was a bit of good thinking by johnmdc. If there had been an obvious unused wire, or wires that could have been joined to create a link between the thermostat and the existing programmer, then it would have allowed it to be connected to the upstairs stat. However what you propose to do is also a good solution.

 

[JunglieD]

I think you may be correct about the third (Grey) wire being part of the fixed thermostat. There is a Brown > 1 and a Black > 3 but also a Grey to 3 (though this could be part of the thermostat). Based on this, when my MPRT arrives I’ll replicate the positions of the Brown and Black wires to what is used in the downstairs one pictured and see how that runs. If the Grey turns out to be a proper wire and not part of the stat then I’ll aim to put that in with the Black wire because it seems to work in that config right now....

 

[stem]

I have found the wiring information for the MRT1, and according to the diagram below, in reality, it should have a neutral connection connected to terminal 4, to operate a small 'heater' known as an 'accelerator' shown as the 'zig zag' in the drawing. This is a feature designed to make the old technology of a mechanical thermostat that little bit more accurate. It will work without it, but not at its best.

The full instructions for it and where I got the diagram from can be found on the Myson website here.

The wires connected to terminals 1 and 3 are the Live switching wires that you would need to connect to the new programmable thermostat. They are the only ones I can see in the photo, I think the grey wire will go to the 'accelerator' and will be an integral part of the thermostat.

I can't see a neutral wire connected to terminal 4 (more poor installation! ) but if there was one present, being a Neutral and should be isolated from the live wires and tucked out of the way when the new battery operated MPRT was installed.

 

[JunglieD]

@stem thank you - seems the new installation should be simple. I went ahead and bought a 'new old stock' item through an eBay seller which arrived promptly and I've tried to install today. However, I didn't get past fitting the batteries because it's arrived broken. The seller issued a refund immediately and (weirdly) doesn't want it sent back. Oh well.

The problem is that with fresh batteries fitted the screen bursts into life for 5-10 seconds max before fading to the blank screen again. I tried this with 2 x sets of new batteries and the same diagnosis. I even attempted fitting it to my existing MPRT 'fitting' and no joy there either. When comparing the backs of both (new-broken and old-working) there appears to be some kind of diode-type thing missing from the new one. See below:

Existing (working) MPRT:

New (broken) MPRT - missing diode toward the top where I'm pointing:

Before I go and order a new MPRT I thought it would be at least worth me attempting to replace the missing part by soldering a replacement in. Does anyone have an idea of what type and spec I would require?