Loose wire behind Honeywell ST9400C

[Spedley]

My heating is on the blink, hot water is ok and central heating is ok but both together causes Honeywell unit to flick off and on repeatedly.

I took the units of (with the intention of fitting a Hive unit recently on offer at Screwfix) but it looks like the problem is probably just a loose wire. On the downside, the central heating doesn't turn on with the thermostat now but I think the fault will be fixed when I reconnect the loose wire to where it came from.

So, which one it come from?

 

[stem]

Looking at the photos there is no obvious wire missing. You have a connection to every terminal that you should. The fact that the 'loose' wire is wrapped with brown insulation tape indicates a live conductor, so I suspect that it has come from the live terminal because it is very full and so probably was not connected properly. However, before you go connecting cables based on a guess, we should determine what it actually does in order to be sure.

Unfortunately, the photographs show only part of the story. Looking at cables on a photo doesn't show what is connected to the other end. If you could trace it and tell us where it goes, we would be better able to advise.

There is a single four core & earth cable at the SCR, Is this the other end of the four core & earth cable at the Drayton?

The puzzle for me is that there is also a red wire connected to the far right terminal of the Honeywell Programmer to provides the Central Heating 'on' signal. I would have expected that this would to go to the Drayton SCR, but it doesn't seem to. Did it go to a thermostat that isn't used anymore?

Is there another junction box somewhere?

Have you changed any other wires over?

 

[Spedley]

I'll check through it all and draw a diagram.

Out of interest, the 4 terminals for HW on/off and CH on/off. Which are inputs from the thermostat to the Controller and which outputs to the boiler, pump and valve etc. It would make working things out easier if I knew what I was looking at.

 

[steve32]

It's a fault in the programmer it needs replacing

 

[stem]

Out of interest, the 4 terminals for HW on/off and CH on/off. Which are inputs from the thermostat to the Controller and which outputs to the boiler, pump and valve etc. It would make working things out easier if I knew what I was looking at.

1 = Hot water 'off' this goes to a grey wire at the motorised valve. It's there to tell it that hot water is not required and close off the hot water outlet port.
2 = Central heating 'off' this is not used
3 = Hot water 'on'. This goes to the hot water cylinder thermostat, which will switch the boiler on when hot water is required
4 = Central heating "on" this normally goes to the room thermostat which when on then supplies the white wire on the motorised valve to position it in the heating position.

Here's a diagram of the entire system

Untitled

• stem
• 2 Feb 2010
• 2

 

[Spedley]

I've gone through and traced the wires.
I'm pretty sure the 2 pairs of twin & earth that go off where I can't see the sleeve are just power for the pump etc.

 

[Echo the husky]

It's a fault in the programmer it needs replacing

This^ it's a well documented fault.

 

[Spedley]

I agree, a faulty controller but I still need to know where to put the loose wire when I replace it.

 

[Echo the husky]

Is there 2 heating zones with one wired and one wireless stat? If so the loose wire would go into 4, there isn't logically anywhere else it would go.

 

[Spedley]

Only 1 zone as far as I'm aware.

Wireless Thermostat, non-combi boiler, hot water tank (no emersion), pump, 3 way valve. That is it as far as I'm aware.

Hot water still works and I've manually switched the valve to run the heating aswell until I fix it.

 

[Spedley]

Only 1 zone as far as I'm aware.

Wireless Thermostat, non-combi boiler, hot water tank (no emersion), pump, 3 way valve. That is it as far as I'm aware.

Hot water still works and I've manually switched the valve to run the heating aswell until I fix it.

 

[Spedley]

I've traced everything through as best I can (without removing floor boards etc) and this is what it seems to be wired like. It doesn't quite make sense, perhaps the valve is the wrong way around?
I'm not totally sure where the lose wire went but it did work before. (also, water tank thermostat me be wrong way around, difficult to tell)

 

[Steelmasons]

Are the motorised valves Spring return or MOMO.

 

[Spedley]

Spring return HSA3
Pump Wilo RS50
Digistat SCR room thermostat
Tank thermostat (older than me)
3-way valve, think it is 3 position