On Sun, 10 Oct 2004 09:20:36 +0100, "Brownie"
wrote:

I did Google prior to this post but was a bit confused by a variety of
answers.

I have a gravity fed CH system with standard boiler with a programmable
timer. Temperature for heating is set by those commonly found Honeywell
mechanical large-dial room thermostats. I work it so the CH comes on and
off when I want and when it is on, the room thermostat controls the
temperature. i.e. boiler runs until room temperature matches set
temperature on thermostat. If CH is not in an 'on' program, room thermostat
does nothing.

It all works fine except it would be great to have more temperature control
so the heating could be on low overnight or when the house was unoccupied
and rise just before waking up or coming home. I am hoping that replacement
of the mechanical thermostst with something like the electronically
programmable Drayton Digistat 2 or 3 is the answer?

Is this just a simple swop over for a novice? I am seeing references to 2
or 3 wire systems and wires having various controls of the boiler. Surely
the mechanical system I have now simply sends to the boiler 'below
temperature = on (makes circuit)', 'at or above temperature=off (breaks
circuit)'? Doesn't the end result of the pragrammable version just do the
same?

I was thinking it would be simply an easy case of swopping over the boxes?


It can be.

In the case of bimetal strip thermostats, some have an accelerator
heater to improve operational behaviour and so a three wire connection
is used - live from the programmer, neutral and live out to (normally)
a/the motorised valve.

You can see the standard hookups at

http://content.honeywell.com/uk/homes/Sundial.htm

Sometimes where a simple electromechanical thermostat is used, only
two wires are used and connected.

There is a wiring scheme used with some types of motorised valve where
three wires are used, but consist of the switch common (coming from
the programmer) and then the outputs are normally on and normally off,
i.e. live out on one when heat is required (opens valve etc.) and the
other giving live out when it is not (closes valve).
These are not very common, because most installations use spring
return motorised valves needing a live to move them one way and the
spring moves them the other.

If you have a two wire hookup, then it is fairly obvious how to
replace. If it's three wires, then it's important to check what the
wiring arrangement is.

From the perspective of an electronic thermostat or room programmer,
the electronics has to be powered. Some require a permanent live and
neutral as well as the switch contacts.
Others like the Digistat 3 that you mentioned, have batteries inside
to power the electronics and can thus work on a two wire arrangement.

To have night set back and differing temperatures during the day, you
could use the Digistat 3 or 3i - the latter has an optimised start to
delay starting the heating on warmer days.

Others to look at are the Honeywell CM67 series and Danfoss Randall
TP75 series (now superceded).

Another approach is to use wireless (RF) models of these. In this
case, you can have a receiver close to the boiler or wiring centre,
where it is usually easy to pick up the wiring connections needed, and
then the room unit is battery powered.





..andy

To email, substitute .nospam with .gl