This is a very long shot - does anyone have an old/unused Honeywell CM927 control unit like this?
I don't care if the unit is broken or otherwise unserviceable, as long as the 3 buttons on the right of the screen are intact.
Ours has a faulty button, which could be easily fixed if just one component is changed. I can't get just the control unit, just the whole set of
control unit and boiler receiver, at around £100. If I fit another unit I have to get the boiler re-wired. 
quite a few on ebay, auctions for used units and new units on buy-it-now.
quote:
Originally posted by craigdiver
quite a few on ebay, auctions for used units and new units on buy-it-now.
Is it not just a switch on the PCB?
Could you unsolder it and solder a new on on, or if is broken just solder onto the terminals some fly wires and add a floating switch.
I have this exact thermostat and it works very well. I would also be gutted if it packed up!
It's more complicated than a simple switch - I'd have fixed it easily if it was.
On the PCB there are 2 interlocking 'combs' for each switch position, with each trace close together, but not connected. When you press the
button a rubber membrane underneath goes down; inside the membrane is a tiny metal plate (probably iron or steel) which, presumably, changes the
capacitance across these traces and triggers the change. In my controller, it looks like the metal plate has corroded - its little 'cell'
is brown and mucky, where all the others are clean.
This is what's frustrating - the bit I want consists of 5g worth of bendy plastic that's easy to replace if I could just get the right part.
This is why I'm looking for a knackered old controller to strip for spares.
I don't what would happen if I mounted a switch over those pads, as I guess that I'd either trash the processor or it simply wouldn't
work. I don't want to take the risk while it's sort-of working, especially while it's keeping my central heating going in this chilly
weather.
I've put an ebay bid on a controller with a dodgy LCD, in the hope that other people won't be tempted; working ones are going for silly
money. I still expect that I'll end up getting outbid as there's only so much I want to pay for something I'm going to tear to pieces!
quote:
Originally posted by David Jenkins
It's more complicated than a simple switch - I'd have fixed it easily if it was.
On the PCB there are 2 interlocking 'combs' for each switch position, with each trace close together, but not connected. When you press the button a rubber membrane underneath goes down; inside the membrane is a tiny metal plate (probably iron or steel) which, presumably, changes the capacitance across these traces and triggers the change. In my controller, it looks like the metal plate has corroded - its little 'cell' is brown and mucky, where all the others are clean.
This is what's frustrating - the bit I want consists of 5g worth of bendy plastic that's easy to replace if I could just get the right part. This is why I'm looking for a knackered old controller to strip for spares.
I don't what would happen if I mounted a switch over those pads, as I guess that I'd either trash the processor or it simply wouldn't work. I don't want to take the risk while it's sort-of working, especially while it's keeping my central heating going in this chilly weather.
I've put an ebay bid on a controller with a dodgy LCD, in the hope that other people won't be tempted; working ones are going for silly money. I still expect that I'll end up getting outbid as there's only so much I want to pay for something I'm going to tear to pieces!
David if I read this right it the Concutor under the Rubber cup that the problem and the 2 track it connects together when you push it are ok.
Have you tried cleaning it with some Alcohol / thinners / brake cleaner and a Cotton bud ? done this with Mother in laws TV remote after on of the
grand kids decided it needed to drink some lemonade
Could you replace the 'Contact' with one out of an old tv remote control, Keyboard ect swapping the gunky contact with another one or
sticking it over the top ?
Regards
Agriv8
That wouldn't work - it's different to TV remotes. The metal bit in the bendy plastic never touches the PCB track, it just goes very close
when the button is pushed. Even when a working button is pushed, there's a piece of non-conducting bendy plastic between the track and the metal
bit.
I did try cleaning the pads and the back of the button when the problem first started - no effect.
quote:
Originally posted by Agriv8
David if I read this right it the Concutor under the Rubber cup that the problem and the 2 track it connects together when you push it are ok.
Have you tried cleaning it with some Alcohol / thinners / brake cleaner and a Cotton bud ? done this with Mother in laws TV remote after on of the grand kids decided it needed to drink some lemonade
Could you replace the 'Contact' with one out of an old tv remote control, Keyboard ect swapping the gunky contact with another one or sticking it over the top ?
Regards
Agriv8
As I said above - nothing is meant to make contact when the button is pushed. It's something to do with capacitance or inductance, or similar
magic.
...and I don't think I want to order anything from Maplins! Seeing that they've just called in the receivers...
quote:
Originally posted by peter030371
Quite a common design switch. You could try a tiny blob of conductive paint on the rubber membrane, something like this https://www.maplin.co.uk/p/electrically-conductive-silver-paint-n36ba
It may not last forever but if it goes every 6 months just clean off and repaint.
Ok David I cant picture how it works.
When my old controller stating giving me issues I could set it to on 24/7 and replaced the old bywire thermostat with one of these cheaper than a
replacement control.
https://www.tado.com/gb/
Saved ££ on gas and the fact we can switch on heading up the M6 after visiting the Mother in law priceless.
We can Switch it on from bed anywhere you have a signal even hooks up to Google Alexa ( but Grandma wont talk to her !!! )
Good luck
Agriv8
Ah well, I'm not often right, and I was wrong once again.. 
I took another look inside the box and realised that it WAS the same sort of system as found in a remote - a conductive pad on a bendy membrane that
gets pushed into contacts on the PCB. I didn't recognise it because the pad and membrane were almost transparent, instead of the black rubber
and pad in a remote.
I used
this kit off ebay, and it seems to have done the job. With any luck it'll last a good while...