Domestic safety controls

Domestic safety controls
Gas Controls (Domestic)
These notes are part of the gas safety training programme from RAD Training (Midland) Ltd. They
are for information only and should not be used as a substitute for normative documents or
manufacturer’s instructions. RAD Training (Midland) Ltd. accepts no responsibility for any
consequences of their use.
Contents
2
General information
Flame Supervision Devices
3
Thermoelectric valve
4
Interrupted thermoelectric valve
Electrical thermostat
5
Vitiation sensing device
6
Air / gas ratio valve
7
Solenoid valve
8
Flame rectification
9
Liquid expansion valve
10
Multifunction valve
11
Mechanical thermostat
Safety shut-off (cooker lid)
12
Gas tap
13
Fan proving switch
14
Regulator
15
Medium pressure meter regulator
16
Safety shut-off (cooker lid)
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General information
Checking the operation of gas appliance controls is a requirement of The Gas Safety (Installation
and Use) Regulations 1998. In order to check a control, a gas engineer should:
⋅ be able to identify it
⋅ understand its purpose
⋅ know why and when it operates
⋅ (to some extent) understand how it works technologically
The key to testing many safety controls is understanding what conditions make them activate. The
gas engineer may then be able to simulate the activation conditions to see whether the control
responds correctly.
Note: some controls cannot be tested this way (outside a specialist testing facility) because doing so
would require unsafe conditions (for example appliance overheat or a build-up of vitiated air).
In general, defective controls or parts of controls should be replaced rather than repaired. Any
replacement should be fitted to the appliance manufacturer’s specifications.
Note 1: an appliance which has had any safety control bypassed or rendered inoperative is normally
classed as Immediately Dangerous. The situation would also be RIDDOR reportable.
Note 2: servicing or replacement of gas many safety controls comes under the definition of “gas
work” in the Gas Safety (Installation and Use) Regulations 1998. Only persons deemed competent
under the Regulations can legally perform gas work.
Flame Supervision Devices (FSDs)
An FSD or FFD (flame failure device) is a control designed to stop (or slow) gas flow to the burner
if the flame is extinguished. It prevents explosive build-up of gas in an appliance, chimney or room.
There are many types of FSD including thermoelectric valves, flame conductance, flame
rectification, ultraviolet FSDs and liquid expansion valves.
There are many causes of flame failure including air draughts on the burner, interruption of the gas
supply, under-pressure gas, blockage of the gas tap, or liquid overspill on cookers.
FSDs are found on virtually all modern gas appliances. If a cooker is to be installed in a multioccupancy building every burner should have an FSD. In catering establishments all new appliances
and all second-hand appliances with enclosed burners should have FSDs fitted.
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Thermoelectric valve
Flame supervision device
Alternative names: thermocouple
Wire
Thermocouple tip
Gas tap with thermoelectric valve
Electromagnetic coil and spring
The thermocouple tip, when heated by a flame, creates a small electric current which flows along
the wire to an electromagnetic coil inside the valve. The coil generates a magnetic field which will
hold open the gas valve.
If the flame is extinguished the thermocouple tip cools, the current and magnetic field disappear and
the valve springs shut.
Note: the thermocouple does not generate enough energy to open the valve from the closed
position. This must be done by the user holding the valve open until the tip has heated sufficiently.
Test
Run the appliance until it reaches normal operating temperature. Achieve flame failure by turning
off the gas supply at a suitable upstream isolation valve. Ensure the valve clicks out within the
acceptable time limit. Ensure gas flow has been stopped (e.g. with a manometer downstream of the
thermoelectric valve).
For many appliances the maximum cut out time specified by British Standards is 60 seconds, but the
appliance manufacturer may specify a different time. If click-out takes longer, or if gas continues to
flow afterwards, the valve is faulty.
If a valve takes less than ten seconds to click out then the thermocouple may be close to failure.
Inspect the tip, wire and connections carefully for signs of burn-out, corrosion or damage.
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Interrupted thermoelectric valve
Flame supervision device with secondary shut off
Alternative name: interrupter (spelt “interruptor” by some suppliers)
Electrical
connectors
This device works in the same way as a thermoelectric valve with the addition of an electrical
connection in series to an overheat thermostat or other safety device. If the safety device activates
it opens the circuit, stopping current to the thermoelectric valve and shutting down the appliance.
Test: as for thermoelectric valve. There is no means of simulating overheat conditions outside a
specialist testing facility.
Electrical thermostat
Temperature control
Connections to
solenoid valve
Phial
Temperature
control
A temperature sensing device such as a liquid expansion phial (see liquid expansion valve) or
bimetallic strip activates an electrical switch to a solenoid valve controlling gas flow to the burner.
The user determines the temperature setting. The electrical thermostat then cycles between
turning the burner on when heat is required and off when the desired temperature is reached.
This is a basic form of temperature control. It does not modulate (vary the heat output of the
burner); it either turns the burner fully on or completely off.
Test: ensure that the heating cycle correctly regulates the appliance to the desired temperature.
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Vitiation sensing device
To shut down an appliance if combustion air is vitiated (lacking in oxygen)
Alternative names: Oxy-Pilot, Oxy-Safe, precision pilot, atmospheric sensing device (ASD)
Pilots
Aeration
hole
Well-aerated flame covering
thermocouple tip
Poorly-aerated flame rising off
thermocouple tip
One of the two pilot flames is directed onto the thermocouple tip. If the air entering the aeration
hole becomes vitiated the flame will rise as it ‘searches for air’, lifting off the tip. The tip will cool
and will no longer provide enough current to hold the valve open (see thermoelectric valve). The
appliance will shut down.
Note: vitiation sensing devices have been shown to fail to detect vitiated air if the appliance is too
close to a source of fresh air such as an air vent. Check manufacturer’s instructions for any
necessary clearances.
Test
Test as for a thermoelectric valve. There is no means of simulating a build-up of vitiated air outside
a specialist testing facility. Temporarily blocking the aeration hole is not a true simulation of
vitiation.
Note: if the appliance keeps shutting down it may be that the aeration hole has become blocked.
However, it may be that the vitiation sensing device is operating correctly and is shutting down the
appliance because of vitiated air. The latter situation would be classed as Immediately Dangerous.
Correction of faults: unblock aeration hole or replace complete unit. Ensure vitiation is not actually
taking place.
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Air / gas ratio valve
To maintain a safe, high efficiency mix of air and gas to the burner at varying gas loads
Alternative names: zero regulator, zero governor, gas / air ratio valve, 1:1 valve
Outlet pressure test
point
Air signal
connection
Zero offset
adjustment (sealed)
Inlet pressure test
point
Outlet pressure test
point (additional)
Vent
Gas inlet
Gas / air ratio
adjuster (sealed)
Sigma 848 air / gas ratio SIT valve
Theory
Stoichiometric combustion happens when gas and air are mixed in proportions of 1 part gas to
approximately 10 parts air. There is exactly the right amount of air to burn all of the gas. There is
no excess air to cool the flue gases and reduce heat transfer through the heat exchanger, so
combustion efficiency is at its highest possible value. There will be no carbon monoxide.
Stoichiometric combustion requires perfect mixing of air and gas, which is not achievable in a realworld gas appliance. Having no excess air risks the production of soot and carbon monoxide if
airways become blocked or as fan efficiency decreases with age. For this reason, air to gas ratio is
set higher than 10:1 in practice to give a safety margin of excess air.
To maintain efficiency and safety it is important that air and gas remain in the correct ratio when
demand for heat (and therefore gas flow rate) changes. The air / gas ratio valve was designed for
this purpose and is found in high-efficiency condensing appliances.
Operation
The boiler’s printed circuit board (PCB) controls air flow rate by varying fan speed according to the
demand for heat. The air / gas ratio valve senses the fan pressure and automatically modulates the
gas pressure to match it. The gas is then at ‘zero’ pressure relative to the air. The airways and
gasways in the appliance are sized to give air and gas flow in the proportions needed for a safe, high
efficiency mix.
Commissioning, servicing and testing
Checking and adjustment of air / gas ratio valves should be done using a combustion analyser. Every
boiler will have its own set of testing or setting procedures. Many manufacturers seal air / gas ratio
valves to prevent or limit adjustment, preferring to set them in the factory. This is because adjusting
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an air / gas ratio valve incorrectly can produce a large amount of carbon monoxide. Adjustments
must not be attempted unless permitted by the appliance manufacturer.
Test
The engineer should ensure that the inlet working pressure to the valve is correct and that the
products of combustion agree with the appliance manufacturer’s figures for CO2 %, CO ppm and
CO / CO2.
Combustion tests should be carried out according to manufacturer’s requirements. Many
condensing boilers need to be tested at both maximum and minimum load, and provide test modes
to enable this.
Note: the same types of air / gas ratio valve are found in many different appliances but settings will
be different. It is crucial that an engineer follows the manufacturer’s instructions and data for the
correct appliance.
Solenoid valve
To enable electrically-switchable control of gas flow
coil
gasway
The equivalence of a solenoid and a bar magnet
(diagram © HyperPhysics)
Electric current, when passed through a coiled conductor, generates a magnetic field around and
inside the coil. In a solenoid valve a metal armature is drawn into the core of the coil, opening the
gasway. If the electric current is switched off the armature drops back down to close the gasway.
Examples of application include electrical thermostats and interlocks for mechanical ventilation.
Test: check that the solenoid valve controls gas flow as required by the application.
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Flame rectification
Flame supervision device
Theory
Flames consist of freely-moving electrically-charged particles called ions, which allows them to
conduct an applied electric current. This effect can be used to detect the presence of a flame.
Operation
An electric current is passed through the flame using two electrodes (or ‘ionisation probes’). If the
flame is extinguished the PCB will detect that current flow has stopped. It can then try to re-ignite
the flame via the ignition electrode. If this does not work the circuit board can shut down the
appliance so that it will have to be manually reset.
Flame rectification superseded earlier flame conductance systems which could incorrectly regard a
short circuit or soot build-up as a flame. In flame rectification systems the electrodes are of
different sizes. This favours current flow in one direction only (rectification of the current from AC
to DC). A short circuit or soot deposit on the burner will conduct current both ways equally (i.e.
the current will remain AC) so the PCB will ‘know’ something is wrong and shut down the
appliance.
Note: on some units ionisation electrodes are combined with ignition electrodes or the burner.
Test (Note: specific models may differ in details of operation)
While the appliance is running, interrupt the gas supply briefly using a suitable upstream isolation
valve. The appliance should immediately begin its ignition sequence. It should continue to operate as
normal if the gas supply is re-established within a certain time.
Turn off the gas supply again, this time leaving it off. After failing to re-ignite the gas, the appliance
should enter lock-out after a time specified by the manufacturer.
Correction of faults: check for flame lift; check for oxidisation of electrodes; replace defective parts
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Liquid expansion valve
Flame supervision device; temperature control; overheat protection
Capillary tube
Capillary tube
Phial
Gas tap
Phial
Capillary tube
Phial
Gas valve
Oven FSD
Manual
reset
Oven thermostat
Overheat thermostat
with manual reset
(electrical connections
not shown)
A phial (bulb) contains a substance which, when heated, expands along the capillary tube. At the
other end of the capillary tube a protrusion is pushed out which can control other devices such as
gas valves or electrical switches.
Oven FSD
The phial is positioned in the burner flame. While a flame is present the liquid expansion device
pushes the gas valve open to full rate. If the flame is extinguished the fluid cools and contracts back
into the phial. The valve closes to a gas rate sufficiently low as to not cause an explosive build-up of
gas in the oven.
Oven thermostat
The phial is positioned to monitor oven temperature. As the oven heats up, the fluid inside the
phial expands, eventually closing the gasway in the oven gas tap. The tap contains a ‘bypass’ (usually
a small screw with a hole through its body) which always allows a small amount of gas to flow (the
‘maintenance rate’) to keep the burner lit and to maintain a constant oven temperature.
Overheat thermostat
The phial is positioned to detect overheat in the appliance. If overheating occurs the thermostat
will break the continuity across its electrical connections, causing appliance shutdown.
Tests
Oven FSD
Check that the burner starts off on low flame and, after about 10-15 seconds, comes to full flame. If
not, the phial or capillary tube may be kinked or split.
Leave the oven running for 5 minutes. Turn off the oven and, after 60 seconds, re-light it. The flame
should have returned to a low level. If not, the gas valve is sticking in the fully open position.
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Oven thermostat
Ensure that the oven flame falls to the bypass rate (low flame) when the desired temperature is
reached. If the burner goes out the bypass screw may be blocked and should be cleaned by noninvasive methods (e.g. with warm water or compressed air).
Overheat thermostat
There is no way to test appliance overheat outside a specialist testing facility.
Multifunction valve
Combines several gas control devices in one unit
Alternative names: gas valve, Honeywell valve
Regulator
Thermoelectric valve
Solenoid valve
Inlet pressure
test point
Pilot adjusting screw
Burner pressure
test point
Test: test each component individually.
Correction of faults: replace complete unit.
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Mechanical thermostat
Temperature control
Alternative names: rod thermostat
Temperature control
knob
Valve
Gas inlet and outlet
connections
Gas enters and exits the valve through separate connections. The rod consists of a solid metal core
surrounded by a tube of higher thermal expansion rate. When heated by gases from the burner, the
tube expands more than the core, pulling the valve off its seating and closing the gasway. This has
the effect of stopping gas flow to the burner. The rod then cools and the valve reopens, allowing
the burner to fire up, and the heating cycle begins again. The control knob allows the appliance user
to adjust the temperature at which gas flow is stopped.
Note: this type of thermostat is often used in conjunction with a “relay valve” which allows control
of a greater flow rate of gas than the small connections to thermostat. When this is done the outlet
gas from the mechanical thermostat is taken to the burner via a “weep tube”, where it is burned off
as a small flame at the end of the tube.
Test: ensure that the heating cycle correctly regulates the appliance to the desired temperature.
Safety shut-off (cooker lid)
Stops hotplates operating when the cooker lid is down
Alternative names: safety cut-off
Some models of cooker use a simple lever fitted to the lid which pushes a spring-loaded gas valve
open when the lid is raised. More sophisticated versions physically turn off the hotplate taps if the
lid is lowered. Some models require a reset button to be pressed to allow gas flow to resume.
Test
Slowly and carefully lower the lid with one hotplate burner operating at low flame. The burner
should go out well before the lid gets close to the flame. Turn off the gas tap.
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Gas tap
Manual control of gas flow to a burner
Turning the knob rotates a tapered barrel inside the tap, causing differently-sized openings to allow
variable flow rates of gas to the burner.
Test and correction of faults
Check that the tap passes gas at all settings from low to high. If not, it is blocked and will have to be
dismantled and cleaned.
Check that the tap handle is stable and not stiff to turn. If the tap is stiff or if it passes gas when
turned off, dismantle it and lightly apply high temperature grease (e.g. Molykote graphite-based
grease) to the inner barrel, taking care not to block any holes. Ensure all parts are correctly
oriented when reassembling the tap and check that it operates correctly as above.
Check that the tap does not allow passage of gas when in the off position and that there is no smell
of gas. If the tap is passing gas it may need re-greasing as above or replacement.
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Fan proving switch
To shut down the appliance if the fan stops working
Alternative names: air pressure switch; fan switch
A fan causes a change in air pressure when it is working. A tube, positioned to pick up fan pressure,
is connected to a spigot on the fan proving switch. While the fan is operating, air pressure closes
the switch, allowing the appliance to operate. If the fan stops working the switch opens and shuts
down the appliance.
Note: a second tube may be connected to another part of the appliance to provide a differential
pressure.
Test
(For a fan proving switch in situ) remove the pressure sensing tubes from the switch. The boiler
should shut down. Any attempt to re-ignite the burner should fail. Re-connect the tubes and check
for correct operation of the appliance.
(For a fan proving switch not in situ) apply air pressure at the tube connections (suction at the
negative connection or blowing into the positive connection). This should result in an audible click
and electrical continuity across the switch connections.
Note: it is possible to damage the switch if too much pressure is applied when testing. The
maximum operating pressure given in the manufacturer’s instructions should not be exceeded.
Correction of faults: unblock sensing tubes or replace switch.
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Regulator
Alternative names: governor
Function
Meter regulator
(low pressure mains)
(Meter regulator):
1. To provide an outlet working pressure of 21 mbar ± 2 mbar under changing gas loads
2. To lock up when there is no gas demand so that standing pressure downstream does not
exceed 30 mbar
(Appliance regulator) – to reduce inlet working pressure to the operating pressure of the appliance
Operation
A spring-tensioned diaphragm allows gas to flow out at a certain pressure. Adjustable regulators
allow the outlet pressure to be set using a screw which adjusts spring tension. Meter regulators
should be sealed and may only be adjusted by competent persons authorised by the gas
transporter.
Test
(Meter regulators) – perform standing and working pressure tests at the gas meter. Standing
pressure (no appliances working) should be 30 mbar or lower. Working pressure (one appliance
running at full rate or three cooker hotplates on full) should be 21 mbar with a tolerance of 2 mbar.
(Appliance regulators) – ensure burner pressure is set to the operating pressure stated in the
appliance manufacturer’s requirements.
Correction of faults
(Meter regulator) – contact the gas transporter (National Grid).
(Appliance regulator) – adjust to correct pressure if possible or replace if faulty.
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Medium pressure meter regulator
To regulate medium pressure mains gas (75 mbar < mains pressure ≤ 2000 mbar) to 21 mbar ± 2
mbar at varying demands for gas.
Reset lever
Second stage diaphragm
Meter Inlet
Valve (MIV)
First stage diaphragm
Pressure relief
vent
Mesura medium pressure meter installation
Operation (Mesura-type)
This regulator reduces mains pressure in two stages, first to approximately 350 mbar and then to
21 mbar. It has over-pressure and under-pressure shut off (OPSO and UPSO) which can be
manually unlocked with the reset lever.
A pressure relief vent fitted with a flame arrestor removes excess pressure if downstream pressure
reaches approximately 35 mbar. This vent should be piped to outside air a minimum of 1000 mm
from any opening into the building. Gas venting from the relief vent is classed as an Immediately
Dangerous situation.
There should be a meter inlet valve (MIV) or ‘test valve’ fitted between the regulator and meter.
Tightness testing should be performed using the MIV, not the ECV, because the lock up mechanism
may interfere with the test. If no MIV is fitted then the ECV may be used in conjunction with the
modified tightness testing procedure given in IGEM/UP/1B Edition 3.
Note: to prevent lock up always open ECV and MIV slowly.
Test: as for a regulator (low pressure mains). Ensure no gas is discharging from the relief vent.
Correction of faults: turn off the gas supply at the ECV and inform the gas emergency service
provider (National Grid).
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