In an earlier contribution to this discussion,
Tony Lennard wrote:

I am trying to replace my central heating timer. It is an old model I
guess from the 1960s, it branded Potterton, but inside it says
manufactured by Hortmann, model Q mk 3.

Does anyone know what the connections are? Horstamnn could not help.
Otherwise, I am upgrading to a Horstmann C27 Programmer, is there an
easy way to know what wire to connect where? - I have a puped central
heating sytem, with gravity hot water. There is no cylander
thermostat, so I have 2 wires to the boiler, and one to the room
thermostat.

Thanks for any info,

Tony Lennard

With your existing programmer, do the 2 boiler wires connect to the same
terminal at the boiler end? If so, it sounds like a complicated way of
achieving a link in the programmer!

The boiler needs to run whenever *either* HW or CH is selected. Bringing the
boiler on with the CH is usually achieved with an internal link which brings
HW on whenever CH is selected.

You should only need one live wire to the boiler (plus neutral & earth of
course) and this should be connected to the HW ON terminal. The CH ON
terminal should have a wire which goes to the room stat and onwards to the
pump. The programmer should have an internal link or switch which has to be
set in the Gravity mode.

If you are updating your control system, you should seriously consider doing
it in a way which provides a boiler interlock - so that the boiler doesn't
waste energy keeping itself hot when it's not needed, and so that the DHW
doesn't get dangerously hot. The least painful way to achieve this is to
convert it to a C-Plan system - for which you will only need a 2-port zone
valve and a cylinder stat - which need to be connected up exactly as per
C-Plan in http://content.honeywell.com/uk/homes/systems.htm
--
Cheers,
Set Square
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