HW comes on with CH - Wiring?

[jcwacky]

I've just moved into a new house and something doesn't seem quite right with the heating system: if I turn the hot water off, and the central heating on, the hot water will turn on as well.

I have a regular boiler (Potterton Prima F) with a vented cylinder. There is a mid-position valve by the cylinder. By the boiler is a programmer (British Gas DCP - like a dumb Hive), thermostat in the hall and thermostat for the cylinder.

My guess is it's something to do with the programmer or wiring, as when explicitly setting the HW to OFF and CH to ON, on the programmer, it shows the status for both as ON:

This is the same if the CH is set to come on using the schedule, the HW will also turn on, despite it being set to Always Off, or set to come on at a different time.

Here is the wiring behind the programmer:

And a few more pics, in case they help:

Behind hall thermostat:

Behind cylinder thermostat:

Can anyone see anything wrong here?

 

[Echo the husky]

Yes, your Y plan system is missing a connection from HW off on the programmer to the grey wire on the 3 port valve.

 

[jackthom]

Plus the programmer is set up for single channel so hot water is showing on when heating is on. I can't find the installation instructions for this one but there must be a jumper or a menu option to change it over to 2 channel operation.
As Echo says, the wiring will also need fixing to make this work.

 

[Echo the husky]

It has to be set to have the water on at the same time as the heating or the valve will not get a HW off signal from the cyl stat and wont be able to move to CH only, and the heating would go off once the cyl reaches temperature.

Post a photo of the inside of the wiring inside the wiring centre that is just below the cylinder temperature controller.
There looks to be loads of 3 core and earth wires, which is a good sign, but the HW off wire may have been stolen to be used for pump overrun.

 

[jackthom]

Yes it does at the moment but once the hw off is properly wired in, that will provide the signal which would otherwise have to come from the cylinder thermostat satisfied wire.

 

[Echo the husky]

Ah, I missed your last sentence!

 

[jackthom]

I though we were most likely in agreement.

 

[jcwacky]

Thanks Jack and Echo.

Here's a photo of inside the grey box:

And there is a jumper on the back of the programmer, it's marked G P, so presume this is how to change it to dual channel as nothing in the menus unless it's hidden.

A few further points:

• It is currently possible to turn ON the HW and keep the CH OFF. (Just not the other way around)
• I don't believe I have pump overrun. When the thermostat reaches temp, the boiler and pump turn off instantly.
• Planning to get the boiler replaced soon!

Any further advice much appreciated!

 

[Echo the husky]

It does indeed look like the pump is wired separately back, assuming it is the brown wire joined to the yellow wire in the 6th terminal from the top. You might be lucky if that unused cable goes to the programmer position, or if the pump overrun isn't used, the wiring could be converted back to a standard Y plan configuration.

 

[jcwacky]

Thanks Echo. So am I effectively hoping that the blue wire (in 9th terminal from top, joined to grey wire) goes all the way to where the programmer is, and if I can locate it I need to connect it to terminal 1 (HW OFF) on the programmer? (And change the jumper over)

 

[Steelmasons]

I don't believe I have pump overrun. When the thermostat reaches temp, the boiler and pump turn off instantly!

This boiler utilises a pump over-run.
Pump over-run is active when boiler temp exceeds 80c (pump over-run stat situated within boiler).
Voltage to pump is fed via boiler switched live unless block temp exceeds 80c whereby voltage to pump is fed via boiler perm live..

 

[Steelmasons]

BTW...I'd be altering the position of pump if I were you...

 

[Agile]

By which he means, rotate the pump head by 180 degrees so the connection box is at the top and rotate the axis of the pump so the rotor shaft becomes horizontal.

Tony

 

[andytw]

Don't think you had an answer re the link. Yes you'll also need to move the link to P for Pumped from G for Gravity.

 

[jcwacky]

Success!! Thanks guys!

Located 3 unused wires behind the programmer and confirmed they were same 3 unused ones that terminated in the wiring centre. Connected one of them to terminal 1 of the programmer, and the other end of it to where the blue wire joins the grey wire in the wiring centre. Then just changed the programmer jumper from G to P (thanks for confirming what they stand for), and now I can have the HW on a different schedule than the CH!

I'll get the pump position looked at when I get the boiler replaced, thanks for the advice there.