Central Heating Guide
C/H Controls Introduction
C/H Thermal Controls
C/H Valve Controls
C/H Function Controls
C/H System Replacement
C/H Controls Installation
Energy efficient home
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Heating appliance valve controls
This section explains the range of valve controls that are commonly available, what they
do and why they are important. Valve controls are only for wet heating systems,
and are fitted away from the heating appliance.
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 gains.
THERMOSTATIC HOT WATER TEMPERATURE LIMIT VALVE
Self-acting valves without motors that are used to
limit hot water temperature 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. See ‘Definitions of controls’ on page 22 for
an explanation of different types.
BOILER INTERLOCK via MOTORISED VALVE
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 or cylinder
thermostat (or a boiler energy manager). In many
cases the interlock will also apply to the pump
operation but any requirements 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 or cylinder thermostat
switches the power supply 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 circuit must then
include an automatic bypass valve (not a fixed
position valve).
The use of an automatic bypass is important where
the system includes a large number of thermostatic
radiator valves (TRVs). When most TRVs are open,
the automatic bypass remains closed, allowing the
full water flow to circulate around the heating
system. As the TRVs 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 into the return at all times. This allows the
boiler to operate at a higher temperature, which
reduces efficiency and restricts the amount of heat
transferred to the system.
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