| Wall thermostats, when fitted, are in series
with the CH signal from the time-switch. Some have a built in
resistor ("accelerator heater") to reduce the hysteresis,
i.e. the "backlash" in temperature between switching
on and switching off. When the thermostat is on, the resistor
supplies a minute amount of heat, which makes the thermostat think
that the room is slightly warmer than it is, and therefore makes
it switch off again earlier. The room temperature will thus fluctuate
less. Such thermostats require 3 cores and earth: live, switched
output and neutral for the resistor. If live and switched output
are interchanged, the resistor will be powered constantly and
the advantage will be lost. Simpler/older thermostats only need
2 cores and earth, so if you're replacing a thermostat you might
want to check the wiring beforehand to see if you can use this
feature.
Wireless thermostats are available, which use
radio waves to communicate with the rest of the system. These
can be convenient if you wouldn't otherwise be able to site
the thermostat in a sensible place because of the wiring. Such
devices are battery powered, and this will need replacing periodically.
One potential advantage of wireless thermostats is that you
can move it around the house with you, ensuring the room you're
occupying is the one you're using to determine whether heat
is needed.
Some room thermostats are available with timers
in that allow different target temperatures to be set for different
times of the day or week. 6 different temperatures over a 24-hour
period is not unusual, so these give a much greater degree of
control over the heating system. An 'optimisation' version is
usually available. This will predict when the heating needs
to be turned on in order to achieve the target temperature.
For example, if you program a desired temperature of 21deg for
7am the thermostat might decide it needs to turn the heating
on at 6.30am on one day, but at 6am on another (colder) day.
Unfortunately, room thermostats effectively
control the whole CH system by monitoring a single point in
the house, and so TRVs (Thermostatic Radiator Valves) are an
advantage since they control each radiator. Earlier models tend
to have the habit of sticking on or off and have given TRVs
a bad name. For some valves it is important that they are installed
with regard to the direction of water flow. Some valves don't
have this restriction. Some angled valves can be fitted with
the head vertical or horizontal, so you can always get the correct
flow direction through them.
If all radiators have TRVs, some sort of bypass
arrangement must be made, to prevent circulation through the
boiler ceasing if all valves close down. The bathroom radiator/heated
towel rail may not have a TRV (reasoning being that the heat
is needed to dry towels even if the room is warm enough), or
there may be a separate bypass loop, controlled by a gate valve,
and some distance from the boiler so that there is a reasonable
volume of water (and hence a reasonable heat sink) circulating.
You may also find a 'pressure detecting' valve that will open
up as the TRVs close down.
The cylinder thermostat (tank-stat) is an effective
device which switches off the HW when the storage cylinder is
fully heated. They can only be used on systems which can CH
independently from the HW. (On other systems, a thermostatic
valve can be fitted, taking note of the bypass arrangements
detailed above - in this case the bathroom radiator may be plumbed
in to operate all the time.)
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