Boiler fires up even when programmer is not on.

[namsag]

Yep see what your getting at and it does make more sense than how they are actually wired. Cant see any reason for not wiring them like this as it would be far better in a fault condition
Or are we missing something basic staring us in the face.

 

[simond]

I see 3 issues with the 'Hailsham method';

1. The zone valve with a stuck end switch would backfeed the other valve AND open it if either HW or CH was invoked. The more zones, the more exciting this becomes.

2. Some boilers eg: Vaillant 6 series should have the 240v wired in series through the pin switches to avoid backfeeding the PCB when the boiler mains switch is 'off'. The Hailsham method would not facilitate this.

3. Some boilers use a low volt connection to energise; the pin switches give the necessary isolation.

Having just rewired some controls (and fitted a new ecoTEC) in a flashy apartment in Battersea I must tell DH that persons were wiring with this method 25years ago, and I found fault (1) was evident. British Gas had been 3 times and could not understand it.... no surprises there!

I consider the S plan is superior for the 3 reasons above.

 

[nickso]

By connecting the grey wire for the HW valve switch to terminal 6 and the grey wire for the CH valve switch to terminal 4, the grey wire will only be live when the relevant timer is switched on; i.e if HW is off at the timer, the HW grey wire will be dead so the boiler cannot run through the HW microswitch.

View media item 72

both ways can cause problems. we have all seen the greys connected to the switched live side of either stat causing backfeed. its also a problem if the grey is connected to the feed side of the cyl stat and the ch brown if there is no room stat.

i suppose doing it hailshams way would be handy for working on the valves without isolating them.

 

[D_Hailsham]

1. The zone valve with a stuck end switch would backfeed the other valve AND open it if either HW or CH was invoked. The more zones, the more exciting this becomes.

Please explain what you mean by "backfeed".

On the standard S-Plan both CH and HW grey wires are connected to the same terminal. How does connecting them to different terminals make it worse?

2. Some boilers eg: Vaillant 6 series should have the 240v wired in series through the pin switches to avoid backfeeding the PCB when the boiler mains switch is 'off'.

What do you mean by "pin switches"?

3. Some boilers use a low volt connection to energise; the pin switches give the necessary isolation.

If you are talking about boilers which use "volt free switching", then the grey wires would not be connected to terminal 1 in any case! They would be connected to an unused terminal on the block, e.g. 9, which would be connected to one of the volt-free terminals on the boiler. The standard S-Plan diagram also assumes that the pump and boiler are supplied from the same terminal. This will not be true of boilers with pump overrun or with volt-free switching, so the wiring would need modifying to suit.