Zone valve electrics

[cully]

I have a Siemens zone valve in use on a DHW cyl that is priority demand to a four pipe gas boiler the wiring is configured as follows;
When brown is live (240V) the aux switch creates a circuit between Grey and Orange.
On my boiler Gr and Or connects to the boiler's 24V DC terminal strip.
I am about to change boilers to a Viessmann 100-W and they advise I need a demand box.
Any reasons why?
Thanks

 

[ianmcd]

when you have low voltage switching, the 240Vac that powers the motor to open, can cause false switching if you are using 4 or 5 core flex, especially over long runs, I havent come across it on Viessmann boilers but I dont see a lot of them , some Worcesters and some Vokeras are prone to it but using a relay stops it

 

[cully]

Thanks Ian,
It probably explains why Viessmann want a demand box (240V in 24V out) as that box must house the relay you describe.
Back in 2002 when I had my boiler installed they didn't bother and it has worked ok ever since.

 

[picasso]

 

[cully]

Picasso can you send your message again, I read it whilst out (alert on my mobile) now I am back home I cannot retrieve it, your name is there but not the content which did make interesting reading.
Thanks

 

[picasso]

Hi Cully what type of cylinder do you have ie vented or unvented?

 

[cully]

Unvented

 

[picasso]

Not much use to you but you can do away with the zone valve on vented cylinders, you can also omit the zone valve if you have a Viessmann unvented cylinder and boiler (I am guessing you haven't) the instructions for a 4 pipe system with third party cylinder say you "may" use a zone valve, not must or should so not compulsory ?

 

[ianmcd]

Thanks Ian,
It probably explains why Viessmann want a demand box (240V in 24V out) as that box must house the relay you describe.
Back in 2002 when I had my boiler installed they didn't bother and it has worked ok ever since.

I have seen it a few times and it is very hard to, measure as the problem is the cable, running two separate cables can cure it, rather than running one 5 core cable, but using a separate relay is the proper way to do it in my humble opinion

 

[picasso]

This is a 4 pipe boiler with internal diverter valve, I think the 240vac signal from the zone valve has to go into the demand box to tell the software to run boiler at higher temp and move diverter, don't think a relay will work in this case.

 

[cully]

My existing boiler is a four pipe with internal diverter valve. The cyl.zone valve (240V) opens it's auxiliary switch and the grey and orange wires from the zone valve connect to the boiler's 24V rail to fire the boiler.
The connection between the boiler and the cyl zone valve, 9 mts away, is via a two core cable.

 

[ianmcd]

This is a 4 pipe boiler with internal diverter valve, I think the 240vac signal from the zone valve has to go into the demand box to tell the software to run boiler at higher temp and move diverter, don't think a relay will work in this case.

OP is moving to a Viessmann, he doesnt say what his existing boiler is, only that it has a Siemens 2 port zone valve fitted

 

[picasso]

I was talking about the Viessmann.