| The time-switch may be electromechanical or fully
electronic. Electromechanical means you'll see a timer wheel turning
and pushing against contacts, whereas with fully electronic there'll
be a digital time display. A surprising range of electromechanical
time-switches are still available, and they can be easier to understand/program
for certain elements of the population.
A programmer that has a single switched output
is commonly called a time-switch. A programmer that has two
switched outputs but only allows CH to be selected if HW is
selected is often called a mini-programmer. A programmer that
allows you to turn on HW and CH independently is simply termed
a programmer.
You may see these names in manufactures literature
and merchants catalogues. In conversation you'll find the terms
bandied around in a more casual, non-specific way, just like
we do in the rest of this document!
Note that a programmer may allow you to select
HW and CH independently (i.e. none, any one or both on), but
the timing may be 'common' or 'independent'. With a common time
base if both are set to come on/go off they do so at the same
time. With an independent timebase you really do have full flexibility.
Some systems (because of limitations in the
installation) can't have CH independently from the HW, and this
is usually reflected in the control, so that as you move the
CH control to off then the HW control also moves. This is usually
referred to as "ten" mode, because there are ten different
valid combinations available of "off, twice, once, on"
for heating and water. It may also be described as "gravity"
mode, referring to gravity-operated circulation to the hot water
cylinder (convection). The fully-independent configuration is
thus "16" or "pumped" mode. The actual number
of these modes that you can select is limited by your programmer.
A programmer can have different settings for
each day of the week, and is called a 7-day programmer. A programmer
that allows you to specify one schedule for Mon-Fri and a different
one for Sat-Sun is called a 5/2 programmer. Simple programmers
use the same timings every day and are called 24-hour programmers.
Almost all programmers can have at least two
separate ON periods for each 24 hours. Some have three or more.
You may find an arrangement where there is a
time-switch and an additional, external switch (maybe like a
light switch) in the time-switched circuit that you flick to
get CH when the timer is on. The switch may be in a place pretty
remote from the time-switch. This arrangement is effectively
the same as a mini-programmer.
In some instances you may wish to run different
parts of the building's central heating on different timings.
If you only need three circuits (HW/CH1/CH2) then three channel
programmers are available to do this, although you may prefer
to use programmable thermostats in each zone. If you want even
more zones (say a Granny flat, garage and attic room) then adding
additional, remote time-switches or programmable thermostats
is probably the way to go. Once again, you'll also need an appropriate
plumbing arrangement to be able to isolate the zones with valves.
The programmer produces signals which say 'I
want HW' or 'I want CH'. An ON signal in this context is a wire
which is connected to the 230V mains supply. For fully independent
control using a 3-port valve, an additional 'I don't want HW'
signal is needed.
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