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Andrew Smith
Andrew Smith, HVAC Technician
Category: HVAC
Satisfied Customers: 3586
Experience:  Heating and Controls engineer
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Hi, I have got a Vaillant Thermo compact boiler installed in

Customer Question

Hi, I have got a Vaillant Thermo compact boiler installed in about 1996, it provides hot water for 12 rads and the domestic hot water cylinder via a Drayton mid-position valve. I recently replaced this valve and took care to re-wire the new one exactly the same way as the old one. It all seemed fine but I got up in the middle of the night and the boiler and the pump was running despite the programmer being off. I thought it may be a problem with the new valve so I tried another one but the problem persisted,I have also compared the wiring diagrams and it appears to be wired correctly. I have also tried replacing the cylinder stat as it was stiff and jerky to operate to no avail, has anyone got any ideas please?
Submitted: 2 years ago.
Category: HVAC
Expert:  Andrew Smith replied 2 years ago.

Andrew Smith :

Power to your pump and boiler come from the programmer for heating on through your room thermostat, also from the programmer for hot water on through the cylinder stat.

Andrew Smith :

It also comes from the hot water off connection on the programmer though the cylinder thermostat.

Andrew Smith :

See if the boiler still runs with the fused spur to the programmer off.

Andrew Smith :

If that stops it then try with the power on and the programmer off its base plate

Andrew Smith :

If that stops it then try with the programmer on the backplate and everything turned off

Andrew Smith :

Let me know what you find

JACUSTOMER-lasdrnmc- : Thank you
JACUSTOMER-lasdrnmc- : with the fused spur off boiler and pump is off.
JACUSTOMER-lasdrnmc- : With the fused spur off the boiler and pump are off, with the programmer off its base they are still off, with the programmer on its base and with it switched to the off position for both hw and heating the pump and boiler are running. Thanks for helping.
Andrew Smith : Ok, a problem with the cylinder thermostat or motorised valve
Andrew Smith : are you able to check voltage?
JACUSTOMER-lasdrnmc- : No
JACUSTOMER-lasdrnmc- : But I've got an electrical screwdriver that lights up when it touches live
Andrew Smith :

Ok, do you know if it is wired like this:

Full Size Image

JACUSTOMER-lasdrnmc- : No, mine has got 16 terminals in the main block and another smaller block with 6 terminals for L,N & E. If it's any help its a ACL lifestyle bifold system, according to the cover.
JACUSTOMER-lasdrnmc- : Sorry bloomin' autocorrect, should be 'biflo' system
Andrew Smith :

They have 22 designs to choose from!

Andrew Smith :

http://www.draytoncontrols.co.uk/installerzone/trainingcentre/wiringdiagrams/

Andrew Smith :

This is why everybody uses Honeywell....

Andrew Smith :

Like I say, when the programmer is attached and the hot water and heating are off then you will only get power to the permanent live and the cylinder stat through the hot water off connection.

Andrew Smith :

Can you check if that is the only place you have power?

Andrew Smith :

Connection no.2 in the programmer (check the colour of the wire and trace it to the connection box)

Andrew Smith :

Sorry connection no.1

Andrew Smith :

try this diagram

Andrew Smith :

http://www.draytoncontrols.co.uk/uploadedFiles/Drayton/Installer_Zone/Completed/Drayton_BIFLO_LWC1_ED01_1.pdf

Andrew Smith :

so also grey to the motorised valve and terminal 2 on the cylinder stat

JACUSTOMER-lasdrnmc- : Ok, that is the correct wiring diagram, the blue wire from programmer terminal no 1 goes to no 8 in the connection box which is linked to no 7 with the grey wire to the m.valve. The blue wire from no 2 on the cylinder stat goes to no 16 on the block which is linked to 7&8 ( grey to Mv and blue to 1 on prog). Something new I've just noticed, I've just taken the programmer off its base whilst the fused spur is switched on and the boiler stayed on, I then turned the switch back on again and the boiler/pump didn't start back up again
JACUSTOMER-lasdrnmc- : Sorry I didn't say that I turned the switch off after I took the prog off.
JACUSTOMER-lasdrnmc- : With the programmer mounted and both heating and hw turned off, terminals 7,8 & 16 are live
Andrew Smith :

????

Andrew Smith :

Can you tell me what terminals are live at the programmer when the programmer is off?

Andrew Smith :

It should only be the live

JACUSTOMER-lasdrnmc- : Yes it's only the live
Andrew Smith :

Are there any other wires connected to the live, also in the junction box?

Andrew Smith :

The power to the pump must be coming from somewhere - 9 times out of 10 it is the motorised valve

JACUSTOMER-lasdrnmc- : No, there is is only one wire connected to the live terminal at the programmer base and that runs to terminal no 10 at the junction box and that is connected to no 15 which is the common on the cylinder stat
Andrew Smith :

Ok, and only one of the cylinder stat connections should be live too?

Andrew Smith :

You need to be doing this while the boiler is firing when it shouldn't be

JACUSTOMER-lasdrnmc- : Yes Pump is running, programmer is turned off and only one live at terminal 2 on the cylinder stat
Andrew Smith :

Can you follow the power back from the pump?

JACUSTOMER-lasdrnmc- : Where abouts in the boiler is the pump?
JACUSTOMER-lasdrnmc- : Ok I've located the pump but the cable for it disappears into the wall at the back
JACUSTOMER-lasdrnmc- : No cancel that the cable from the pump goes to the back of a box of tricks with a few other cables. I've taken the front cover off the electrical connector box
JACUSTOMER-lasdrnmc- : There is a 12 block connector with a live and neutral, and there are brown, black and blue wires connected to nos3,4&5 respectively
JACUSTOMER-lasdrnmc- : According to the diagram on the cover they go to the room stat
Andrew Smith :

This is turning into a nightmare.

Andrew Smith :

Perhaps if you want to spend a couple of hours tracing the cables you can mark them up and then rewire it?

Andrew Smith :

We would then know it was correct and perhaps it will sort any faults?

JACUSTOMER-lasdrnmc- : The heating did work correctly and the only thing I had touched prior to finding the fault was the valve and like I said I have tried two of these to see if that was the problem. No other wiring has been touched.
Andrew Smith :

Without standing in front of it, the only thing I can suggest is following the live to the pump all the way back to the mains and seeing where it gets its power from

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