In an earlier contribution to this discussion,
Pete wrote:

Any CH wiring experts out there!

Current heating wiring was put in by someone who practiced creative
wiring - hence no wiring centre

System is semi gravity

There is a switched live on the programmer./thermo stat for CH ON or
HW ON. Output goes live when heat called for. There are output for
live when off (ie. spdt) but these are currently not connected.

When HW ON - there is power to Swl L in the boiler and the boiler
fires up.

When CH ON - there is power to L in the boiler and the pump runs (no
boiler) and Swl L is off. Pump is connected to the boiler

When HW & CH is on, the pump runs and the boiler is fired. (i.e we get
central heating and HW)


We therefore have the following situation:

Both CH & HW On we get Hot Water and Heating - i.e OK
If HW Only is on: We get Hot Water - No Heating i.e OK
If only CH Only is on: We get Pump Only
i.e Problem


I cannot wire the CH On output to Swl as it would work backwards -
i.e HW On we would then get pump.

How can this be rewired to give CH Only. I reaslise that there is
currently no valve on the system, so HW temp control is only provided
when in HW only mode. If CH is running then there is no control of HW
temp.

How should a valve be wired in to given addtional control?

With a conventional semi-gravity system, you *cannot* have CH only - because
when the boiler is on for the CH, it automatically heats the hot water too!
The HW switch controls the boiler amd the CH switch controls the pump -
usually via a room stat - but the boiler has to be forced on for CH to work.
Many programmers have an internal link which needs to be left in place on
such systems - which effectively turns HW on whenever CH is turned on.

If your gravity HW system heats the water fast enough, your best solution -
rather than going fully pumped - is to convert it to a C-Plan. [See
http://content.honeywell.com/uk/homes/systems.htm ]

For this you will need a 2-port zone valve and a tank stat on the hot water
cylinder (if you don't already have one). This gives truly independent
control over CH and HW, and also provides a boiler interlock so that the
boiler only fires when either or both circuits require heat - and shuts down
when both demands are satisfied (i.e. when both stats are open) thus saving
fuel.
--
Cheers,
Set Square
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