Help please! Central heating/thermostat problem

[AMT]

My (aged) CH system doesn't work properly despite despite having had everything except the boiler (which works just fine) replaced at various times. The electronic Honeywell timer/control panel, the CH two port valve, and the pump all seem to function as they should. But....

There is a room thermostat (very basic Honeywell manual dial) which I am beginning to suspect is wired up wrongly. It is set at 21°. When the heating has been off all night and then the timer calls for heat in the morning, everything works fine. When the house reaches temperature, the heating goes off - again, as it should. The problem is that then the heating doesn't come on again by itself, however cold it gets. The only thing that works is to turn the room thermostat right down - say to 10° - and leave it for at least half an hour. Then turning it back up to 21° will usually result in the boiler firing up and everything working normally until the house reaches temperature once more. Then, rinse and repeat...

I've uploaded a photo showing how it's wired (and another showing the wiring diagram inside it.

I've also uploaded a photo I found via Google of someone else's similar thermostat where the yellow and blue wires are transposed.

I want to know if this is likely to be my problem, and whether or not I'm safe to change them around myself?

It should be said that countless tradespeople have looked at our system and scratched their heads (hence my reluctance to throw more money at it). My worry is that the other bits of the system may also be wired 'unconventionally', and I obviously don't want to make an expensive (or worse!) mistake...

Can anyone help?

 

[doitmyself]

It's impossible to diagnose the problem from the photos as wire colours don't mean a thing.

Do you have a multimeter (and know how to use it 😉 )?

 

[AMT]

No, sadly not. I have an electrician's test screwdriver but that's it. (I was a bit surprised to find that when the heating is off at the timer, and the room stat is turned to zero, but the room stat is otherwise all connected up as shown, the indicator light on the screwdriver lit up when touched on the earth terminal and, also on terminals 1, 2 and 3.)

 

[Harvest Fields]

No, sadly not. I have an electrician's test screwdriver but that's it. (I was a bit surprised to find that when the heating is off at the timer, and the room stat is turned to zero, but the room stat is otherwise all connected up as shown, the indicator light on the screwdriver lit up when touched on the earth terminal and, also on terminals 1, 2 and 3.)

Please do not use an electricians screwdriver to test for voltage. It will just tell you there is power there not how much or how little. Throw it in the bin and buy a cheap multimeter, one like this for £8.00 would do

 

[doitmyself]

As Harvest Fields.

 

[AMT]

Ok. So first up: does the problem described sound as if the likely problem is with the room stat rather than with a different part of the system? Or, at least, does it sound as if it's likely to be one that I need an electrician for, rather than a CH specialist (given that these mostly seem to be plumbers)? And second, if I were to get a multimeter (I've watched the videos on how to use it and it all seems pretty straightforward) what would I be looking for in relation to my system to help diagnose the problem?

 

[Harvest Fields]

Firstly do not take this the wrong way. But plumber are not heating engineers. You need an Heating engineer. Not a plumber and not a sparky. And you need to test the thermostat to see if it is that or something else.

 

[AMT]

Don't worry - I won't take it the wrong way, but I hope you'll forgive me for being confused! We've had people round who call themselves heating engineers, and advertise as such, on more than one occasion, who then say we need an electrician because they are trained as plumbers and don't do the electrics! Some have then brought in their own electrician. I come in peace - believe me, but as a customer it can be really hard to know who to go to.

I'd still like to be able to test the stat, and its wiring, myself though if that's possible!

 

[Harvest Fields]

If you are being told that then in my opinion they are not heating engineers. A heating engineer will do everything from the spur inclusive of timer and stat. To test the thermostat properly you will need a multimeter.

 

[ShaunCorbs]

2 neutral
3 switch live eg call for heating
1 common eg live from prog

so prog to call for heating with stat all the way down to 10/off

you should have live on 1 eg 240 to earth
2 to 1 you should have 240
3 there shouldnt be anything until you turn the dial up etc

 

[Harvest Fields]

2 neutral
3 switch live eg call for heating
1 common eg live from prog

so prog to call for heating with stat all the way down to 10/off

you should have live on 1 eg 240 to earth
2 to 1 you should have 240
3 there shouldnt be anything until you turn the dial up etc

Without a multimeter mate how is he going to know if green and yellow has 240v or blue or red?

 

[ShaunCorbs]

Without a multimeter mate how is he going to know if green and yellow has 240v or blue or red?

hes going to need to get one eg your link

 

[Harvest Fields]

hes going to need to get one eg your link

Just checking 😉😉

 

[AMT]

Thanks all - that's really helpful. Think I'm off to get me a multimeter tomorrow Always wanted one, and now I have a proper excuse! Cheaper than just replacing the stat (which someone suggested as being cheaper than getting it checked, but that wouldn't tell me if it was wired up wrong as I'd just have to copy the existing connections).
It's 'she', actually, but no worries

 

[Ric2013]

Those are pre-2005 wiring colours.
Red = live
Yellow=live
Blue=live

Of course, if the installer was using the yellow wire as a neutral, he or she would have wrapped black tape around it or used black sheathing to show it was neutral. In theory...

In practice, all we know for sure is that there is a switch built in (i.e. the thermostat) between 1 and 3, that 3 would usually be wired to the zone valve, and that 2 would usually be wired back to neutral and that the thermostat connects 2 and 3 together via a resister built into the thermostat.

A reasonable assumption is that a lightswitch connected to blue and red (yellow safely isolated) would allow you to turn the heating on and off and that, if the system works correctly when wired to a lightswitch, then the system is okay and something is wrong with that thermostat.