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Individual Controls
This page explains the range of controls that
are commonly available, what they do and why they are important. Controls for
wet, warm air and electric storage systems are described. These controls are
normally fitted away from the heating appliance, although some controls are
integral with the appliance.
Time Switch
Simple time control of a system that will
only switch one circuit. Most suitable for combi boiler systems.
Programmer
A programmer can switch two circuits
separately (usually heating & hot water) There are three basic types
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A mini-programmer allows space heating and
hot water to be on together, or hot water alone but not heating alone.
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A standard programmer uses the same time
settings for space heating and hot water.
Room Thermostat
Simple room temperature control. Mechanical
thermostats include an accelerator (or anticipator) which has the effect of
smoothing out the temperature cycle, so that on and off periods are not too
long. Wireless units are now available that provide increased flexibility in
positioning and eliminating visible wiring.
Programmable Thermostat
Allows different temperatures to be set for
different periods in the day or week. Provides a better match to the
householder living pattern. Important if occupancy is varied over the day or
week. Provides a night setback where a minimum temperature can be maintained
at night. Many are battery operated and can replace a conventional thermostat
without the need for additional cabling. Some also allow time control for hot
water.
Cylinder Thermostat
Simple control of stored hot water
temperature, usually strapped to the side of the hot water cylinder. It is
commonly used with a a motorised valve to provide close control of water
temperature.
Frost Thermostat
Simple override control used to avoid frost
damage to the dwelling and/or boiler and systems. A frost air thermostat
should be fitted in a suitable place inside the dwelling so that a minimum
temperature is always maintained.
Pipe Thermostat
Where the boiler is fitted in an unheated
area, for example a garage, a pipe thermostat should also be fitted (to the
exposed pipework) This pipe thermostat is in addition to a frost air
thermostat to prevent the boiler firing unnecessarily in cold weather and
wasting fuel.
Thermostatic Radiator Valve
Used to limit temperatures in individual
rooms and so reduce energy consumption. Provides an upper limit to room
temperatures and so can usefully prevent overheating due to solar and
incidental gain.
Thermostatic Hot Water
Temperature Limit Valve
Self acting valves without motors that are
used to limit hot water temperatures in domestic hot water cylinders. Units
are available that sense the primary water (boiler) temperature and also with
a separate remote sensor that can sense stored water temperature. Cylinder
controls should not be used unless they also operate an electrical switch to
provide boiler interlock, otherwise the boiler will cycle unnecessarily.
Motorised Valve
Used to control water flow from boiler to
heating and hot water circuits. Two-port valves can also be used to provide
zone control, eg lower temperatures in sleeping area or different heating
times.
Boiler Interlock
This is not a control device but a wiring
arrangement to prevent the boiler firing when there is no demand for heat. The
boiler can be said to be interlocked when the boiler is switched on and off by
the operation of a room thermostat or cylinder thermostat. In many cases the
interlock will also apply to the pump operation but any requirement for pump
overrun must be observed. Without an interlock, the boiler is likely to cycle
on and off regularly and waste energy by keeping the boiler hot when it is not
necessary.
For regular boiler systems, the interlock is
usually arranged so that the room thermostat or cylinder thermostat switches
the power to the boiler (and sometimes the pump) through the motorised valve
end switches. For combi boilers interlock is usually achieved by using a room
thermostat.
Automatic Bypass Valve
This device controls water flow in accordance
with the water pressure across it, and is used to maintain a minimum flow rate
through the boiler and to limit circulation pressure when alternative water
paths are closed. A bypass circuit must be installed if the boiler
manufacturer requires one, or specifies that a minimum flow rate has to be
maintained while the boiler is firing. The installed bypass must then include
an automatic bypass valve (not a fixed position valve)
The use of an automatic bypass valve is
important where the system includes a large number of thermostatic radiator
valves (TRV's) When most TRV's are open, the automatic bypass remains closed,
allowing the full water flow to circulate around the heating system. As the
TRV's start to close, the automatic bypass starts to open, maintaining the
appropriate water flow through the boiler. The use of an automatic bypass is
also likely to reduce noise in the system due to excess water velocity.
An automatic bypass is always preferable to a
fixed bypass. With a fixed bypass there is a constant flow of hot water coming
out of the boiler which is fed directly in to the return at all times. This
allows the boiler to operate at higher temperature, which reduces efficiency
and restricts the amount of heat transferred to the system.
Control Functions
Compensator
Reduces boiler water temperature for space
heating according to internal/external air temperature and should increase the
efficiency of condensing boilers by reducing the average return water
temperature of the system.
Delayed Start
Reduces energy use by delaying boiler start
time when the weather is mild.
Optimum Start
Adjusts the heating start time to give the
required dwelling comfort temperature at a chosen time.
Night Setback
Allows a low temperature to be maintained at
night. Provides improved comfort and reduced dwelling warm-up time in cold
weather. A programmable room thermostat can provide this facility.
Self-adaptive function
Reduces appliance "on time" by learning from
previous temperature characteristics.
Anti-cycling control
Delays boiler firing to reduce cycling
frequency but is unlikely to provide significant energy savings. In some
circumstances their use may reduce energy consumption, but usually at the
expense of performance or comfort. Standalone units (those not supplied as
part of the boiler) are not generally recommended as they provide little or no
improvement over the minimum level of control shown in this guide.
This text has
been reproduced from the Good Practice Guide 302
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