Through the Repair Cafe project I’ve been running I’ve been asked about all sorts of things which has led to some interesting projects. Just recently, a friend brought me a Honeywell CM927 central heating controller which had been causing trouble.
This appears to be a very commonly installed device, with a battery-powered wireless control unit linking back to a powered relay at the boiler. There do seem to be various issues with them, one of which is a problem with the connection with the screen resulting in a corrupted display. However, this was different – the screen was fine, but some of the keys were poorly responsive and the screen would regularly blank out for a few seconds when trying to set the time or date. This often happened when trying to get the ‘OK’ button to respond which needed several presses.
I found a useful Youtube video which describes a fix for the screen issue (which didn’t apply) but which does give useful instructions for opening the casing and getting the PCB out:
My first thought was that this was something to do with the power supply, so perhaps a problem with the contacts on the batteries. The faulty buttons were probably due to a poor contact between the button.
The internals are very simple – just a screen, a few chips and a relay and the buttons.
The square pads with the interlocking pattern are the contacts for the buttons. The rubber button in the top casing pushes down, and the back of the button has a conductive surface which bridges the two sides of the contact. Several of these – particularly where the ‘OK’ button connected – were dirty and so I cleaned them up with isopropyl alcohol to give the best chance of a good contact.
On the back of the rubber buttons there was a rough surface which is conductive, and on testing them the ‘OK’ button showed almost no conductivity at all. This is a common fault with TV remotes are other device which use this design, and the solution is to renew the surface using conductive paint. I found something suitable on Amazon and painted the back of the OK button with it.
The two large square contacts on the bottom left and right of the board are where spring connectors from the battery make contact. Looking closely at them, I could see two small holes on each pad where it appears that the springs have worn through the pad. I was pretty confident that this was responsible for the cutting out, due to the power momentarily being interrupted. I flowed some solder on to the pads to restore the connection, and also bent out the springs slightly.
On reassembling everything I was pleased to find that the buttons now worked correctly, needing only a gentle push and there was no more cutting out. So overall a great result for a relatively small effort!
Universal Discovery Methodology 
many thanks for this excellent set of instructions, and the link to how to dismantle the unit. I purchased the conductive paint, followed your instructions, and the thermostat now works a treat.
That’s great – thanks so much for letting me know!
I was about to buy a replacement as I could no longer use the ok button, but saw your article.
So I took the plunge, and opened mine up. The contactor looked ok so I just cleaned the contacts with a damp cloth, dried it and closed up. Now works perfectly.
You have inspired me to try and open up my dodgy remote control next!
Thank you for the information and link.
Thanks for letting me know – great to hear. Definitely worth trying with the remote control too, I’ve revived quite a few using a similar method.
super useful page. Had exactly the problem with one of the spring connectors, bit of solder, and bending, and unit behaves again. Great to have the disassembly instructions too. Saved me calling the plumber out and possibly a replacement unit… great. Thanks!
Thanks for letting me know! It’s always great to hear that the posts have been useful to people.
Hi, thank you for the information and the inspiration. Which conductive paint I should buy from Amazon？ Is it the Liquiwire™ — Electrically conductive paint for guitar shielding, PCB repair and DIY applications (100mL) £9.99 ?
Sorry for the delay, yes that’s what I bought. It’s funny stuff though… it dries very powdery and almost feels like a suspension rather than paint. It worked fine for me having said that, but I would see if there are other alternatives which claim to be more sticky as I’m concerned it may just flake off again.
Hi,
My son dropped the thermostat and now it IS all blank. Do you think it can be repaired?
Yes I think so. It is probably a problem with the power connection to the batteries. Definitely worth having a look inside and see if you can see anything wrong