Useless Cable on Baxi (w. pics)?

[ukants]

Hello folks,
I'm usually in the electrical section but nipped over here to see if you can help me with something. I've got an old boiler in my house (queue the wife jokes), which is a Baxi FS501OF (it also has FS501SOF on it). Coming from the LP722 control panel there are three white cables going to a junction box and marked from left to right (as we look at it in the picture) 2,3 and 1.

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• ukants
• 6 Jul 2008
• 1

I know the grey twin and earth joining 2 (left) goes to the pump and number 1 (right) goes to the boiler; but what is number 3 (middle) doing? When the central heating is on number 3 is dead but when the heating is off it shows a voltage ranging from 50 to 110 volts. Worryingly it just seems to go straight into a wall about 2 metres away and disappears. There's nothing in the area it's going to that needs this power as far as I can see. The area is a old outhouse that is now the far end of a kitchen extension. If it's doing nothing can I safely disconnect it?

Regards,
Anthony

 

[ChrisR]

First thing to look at is your motorised valve(s) - what have you got?
You could have C, S or Y plan.
Wires could be coming from room thermostat, pump. or cylinder stat apart from the valve(s).
See

http://content.honeywell.com/uk/homes
http://content.honeywell.com/uk/homes/Catalogue/Sundial Plans/3.4 Y Plan.pdf

and thereabouts.

 

[ukants]

Hiya Chris, thanks for replying. Unfortunately I can't open the links. Also I've no idea what system I've got I'm afraid! Electricity I'm okay with; boilers not a scooby! How would I find out what kind of motorised valve I had?

 

[ChrisR]

Play with them then...!
try
http://content.honeywell.com/uk/homes/catalogu.htm#SECTION 2- Sundial Plans

2 port or 3 port.
Here's a 3 port:

 

[superdupergasman]

Room stat??

 

[D_Hailsham]

When the central heating is on number 3 is dead but when the heating is off it shows a voltage ranging from 50 to 110 volts.

This happens when you have a mid-position (3 port) valve in a Y-plan.

Here is the wiring diagram and a link to an explanation how it works and why you get the strange voltage
How a Mid position Valve Works

View media item 70

 

[ukants]

Hey thanks for the help everyone. I'll do some investigating.