I've just replaced my Randall CH programmer with a Horstmann C27 (from
With the programmer comes a conversion table and a wiring diagram.
I have wired my system as per the Honeywell C Plan.
(See http://content.honeywell.com/uk/homes/systems.htm )
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?
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!
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?!
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
[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
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
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.
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