C Plan electrics

[Kev Parkin]

I've just replaced my Randall CH programmer with a Horstmann C27 (from Screwfix), With the programmer comes a conversion table and a wiring diagram.

I have wired my system as per the Honeywell C Plan.

(See

)

However, the diagram from Horstmann is slightly different:

Brown on zone valve to call on cyl stat instead of to common and no wire to Sat on Room stat

Although not a great deal of difference does anyone know the pro's and con's of each version, and if one is better than the other?

Tia

Kev

[BigWallop]

The boiler and pump should come on together and the zone valves should be activated by their respective thermostats. Any other wiring configuration would cause problems.

[BillP]

Hi BW,

He's got C plan.

HW (gravity feed) Boiler only.

CH Boiler + pump on CH.

Only one 2 port zone valve. This is in the HW circuit.

[Set Square]

In the C-plan, both stats are just simple switches - with nothing connected to the NO (satisfied) terminal. If you reverse the two wires going to the cylinder stat, it makes no difference - it will still work the same. [If your room stat has a little heater in it as per the Honeywell diagram, the live and neutral wires must be connected to the specified terminals].

The most important bit is the wiring to the changeover switch which operates when the zone valve opens. This must be exactly as per the Honeywell diagram.

[C-plan is actually a very cunning system for updating old gravity fed hot water systems. When only hot water is required, it opens the zone valve and operates the boiler. When only heating is required, it closes the zone valve and operates the boiler and pump. When both are required, it opens the zone valve and operates the boiler and pump. When both are satisfied, it shuts everything down].

[Kev Parkin]

The boiler and pump do come on together and the zone valve (only 1 on GF HW)is activated via the Cyl stat, the only differences are as stated:- "Brown on zone valve to call on cyl stat instead of to common and no wire to Sat on Room stat"

As I said not a deal of difference - I was just curious to know why!

Cheers,

Kev

[Set Square]

But like I said earlier, the Honeywell diagram *doesn't* have a wire connected to Sat on the Room stat - so where's the difference?

Also, the cyl stat - being effectively an on-off switch - it doesn't matter which way you connect the wires. Honeywell do it one way, and Horstmann the other. So what?!

[ARWadsworth]

configuration

Hopefully they only come on together when you are calling for central heating.

As Set Square says, there is no difference electrically and neither plan has a wire to the sat on the room stat.

-- Adam

adamwadsworth@(REMOVETHIS)blueyonder.co.uk

[Kev Parkin]

I know, however, as I wasn't going to change the electrics (i was just replacing the programmer)I just glanced at the C-plan wiring diagram and assumed the connections to the room stat marked 1,2 & 3 related to Common, Demand and Satisfied.

Just for peace of mind I checked the wiring and everything is as it should be.

Kev

[Set Square]

No! On the C-plan diagram, 1 and 3 on the room stat are the actual switch. No 2 is a neutral connection for the benefit of the built in accelerator heater. You won't need this on your programmable stat. Your new stat may have a dummy terminal for "parking" the unused neutral connection. Otherwise, tape it off and tuck it out of the way.

[ARWadsworth]

To be fair, the way I read Kev's reply meant that he had already spotted this.

-- Adam

adamwadsworth@(REMOVETHIS)blueyonder.co.uk

[Set Square]

I think you're right - but to be on the safe side, I didn't want him connecting the NO terminal to neutral - which would result in a nasty bang when the set termperature is reached!

[Kev Parkin]

Yep, I did spot this - Anyway, thanks for thinking of my well being

Cheers,

Kev