76-500H
THE TECHNICAL TIMES
TWO PATHS TO IMPROVED PRODUCT SAFETY
Redundant Gas Valves & Energy Cut-Off Devices
Over the years, methods of ensuring the safety of gasfired appliances have been developed and refined. Added
safety is a prime concern of all manufacturers of gas-fired
equipment, and efforts are continually being made to
improve the safety of the products wherever possible. One
area of product safety is that related to the ability to turn
off the gas supply to the burner of an appliance in the
event that the equipment should operate abnormally and
cause the equipment to overheat.
The need to turn off the gas supply to the burner in the
event of overheating was recognized almost from the first
day that gas-fired appliances were introduced. The most
common method of satisfying this safety concern is to
supply the equipment with a temperature sensitive control
which is electrically interlocked with the automatic gas
valve of the appliance. THis temperature sensitive control
is often referred to as a “high temperature limit” or
“overheat control”.
The overheat control is located in a strategic position
within the appliance and is designed to monitor the
temperatures experienced in the immediate vicinity of the
control. The location of the control, and the normal
operating temperatures at that location are determined
through tests conducted when the appliance is being
designed. At the same time, the temperature at which the
limit control must function to turn off the gas supply to the
burner is also determined. If for any reason the normal
operating temperatures within the sensing zone of the
limit control exceed those for which the appliance was
designed, those abnormal temperatures will be sensed by
the limit control and the gas supply to the burner will be
turned off by interrupting power to the main gas valve of
the appliance.
An overheat or high limit control normally consists of
a temperature sensitive probe, or bi-metallic element
which is mechanically linked to a normally closed
electrical switch. This switch is wired in series with the
electric supply to the main gas valve of the appliance. An
Figure 1
High Limit Control
When heated, the internal stresses of the bimetal cause the disc
to reverse its curvature with a snap-action at a fixed, preset
temperature and operate the electrical contacts. A decrease in
the ambient temperature below the reset temperature of the disc
relieves the internal stresses in the disc. The disc returns to its
normal curvature and the contacts assume their normal
operating position.
abnormal operating temperature will cause the
temperature sensitive mechanical component of the limit
control to be activated, and upon activation the limit
control’s normally closed electrical switch will open,
cutting off the electric supply to the main gas valve and
causing the gas valve to close. (See Figure 1)
The use of a high limit control to monitor appliance
temperature and turn off the gas supply in the event of
overheating is a time proven method of providing safety
shut-off for the equipment, however national safety
agencies recognized that additional safety features could
be added to heating appliances to act as a back-up system
to the high limit control. It was recognized that, all be it
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remote, an electric gas valve had a potential failure mode
which could prevent the limit control alone from shutting
off the gas supply to the appliances. As the gas valve
consisted of an electrically controlled solenoid valve, the
possibility existed that even though the electric power to
the valve was interrupted, the valve itself could have a
mechanical failure which could result in the valve
mechanically sticking in the open position, causing the
appliance to overheat. If the cause of appliance
overheating was due to the valve mechanically hanging
up, breaking the electric supply to the valve would not
cause an interruption of the gas supply to the appliance.
Because of this potential hazard, a back-up system to the
limit control was desirable.
The recognition of the desirability of a back-up
system to the limit control to prevent accidental appliance
overheating lead to the development of the two most
commonly used back-up systems, the “redundant gas
valve” and the “energy cut-off” device. Both of these
methods of providing safety back-up are accepted by the
American National Standaards Institute (ANSI), and both
are approved for use on industrial and commerical heating
products. Which one is used is at the option of the
equipment manufacturer, however one of the methods
must be employed.
The redundant gas valve method utilizes a specially
designed gas valve which incorporates within its design
two internal valve seats which are arranged in series with
the gas flow thru the gas control valve. Because the two
valve seats are arranged in series, gas cannot flow thru the
control valve unless both valve seats are in the open
position, hence the term “redundant” gas valve. The
theory behind the design of the redundant gas valve is, “it
is highly unlikely that a mechanical failure of the gas
valve will result in both valve seats within the body of the
control to be mechanically stuck open, thus each seat
provides back-up or redundancy to the other.” Using this
method, the electrical component of the redundant gas
valve is still wired in series with the electric switch of the
high limit control, and in the event of an overheat
condition, the limit control will interrupt the electric
supply to the main gas control and both (or at least one) of
the valve seats within the redundant valve will close to
shut off the gas supply to the appliance burner. (Figure 2
shows the main components of a redundant gas valve.)
The second accepted method of procviding back-up
safety shut-off is the energy cut-off or “ECO” method.
The ECO method takes advantage of the standard design
of standing pilot gas valves and their thermopile pilot gas
Figure 2
Redundant Gas Valve
safety shut-off systems. Standing pilot gas control valve
are designed with two different types of valve seats within
the gas control. One seat is an electrically operated main
solenoid valve, andthe other is an electromagnetically
operated or thermopile pilot gas valve. These valves
utilize a thermocouple which monitors the pilot gas flame.
If the flame is present the thermocouple will generate a
small voltage which will activate an electromagnet which
holds the pilot gas valve in the open position. The pilot
gas valve must be in the open position before gas can flow
thru the main solenoid valve in the gas control. As long as
the pilot flame remains lit, the necessary voltage will be
generated by the thermocouple to hold the pilot gas valve
open.
If, by some other means, the electric flow thru the
thermocouple can be distrupted, the pilot valve cxould
also serve as a back-up safety shut-off device to the high
limit control, as well as a safety pilot shut-off. This is
where the energy cut-off (ECO) device comes in to play.
An ECO device is a thermally fused device which is
temperature dependant. As such, an abnormal appliance
temperature will cause the thermal fuse within the ECO to
break, resulting in a disruption in the flow of electricity
thru the device. If the ECO also happens to be wired in
series with the thermocouple of the gas control valve, loss
of electrical flow thru the ECO will also disrupt the flow
the electricity from the thermocouple to the pilot valve’s
electromagnet and the pilot gas valve will close. WIth the
pilot valve closed, no gas can flow thru the main gas valve
and the gas supply to the appliance burner is shut off (See
Figure 3). Like the high limit control, the ECO’s
temperature sensitive fusible link is located at a site within
the appliance where it can monitor abnormal temperatures
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and turn off the gas supply in the event the temperature at
its location exceeds those expected under normal
operating conditions.
method. All intermittent pilot ignition system utilize the
redundant valve technique. Other manufacturers may opt
for ECO’s on all standing pilot systems and redundent
valves only on IPI systems. In either case, both methods
are accepted by ANSI and are approved by the American
Gas Association and Canadian Gas Association. And, if
one method is specified, the other methos is automatically
an accepted equal.
Figure 3
Energy Cut-off Device
The fusible link of the ECO is wired in series with the
thermocouple. If the fuse in the ECO opens, power from the
thermocouple to the pilot valve is automatically interrupted.
Because ECO’s rely on gas valves which use a
standing pilot, they can not be used on equipment
supplied with automatic intemittent pilot ignition (IPI)
systems or direct spark ignition (DSI) systems. Therefore,
both redundant valve systems and ECO systems may be
used by a single appliance manufacturer.
Which system a manufacturer chooses to use will
depend on the method which best suits the manufacturing
needs of the equipment maker. Modine has chosen to use
both the redundant valve and ECO method on our
standing pilot gas control systems, depending on the size
of the unit involved. On the smaller units, 300,000 btu/hr
input and below, we have chosen the redundant gas valve
method, and the larger standing pilot systems use the ECO
Commercial HVAC&R Division
Modine Manufacturing Company
1500 DeKoven Avenue
Racine, Wisconsin 53403-2552
Phone: 1.800.828.HEAT
Fax: 262.636.1665
www.modine.com
© Modine Manufacturing Company 2002
2/02 - Litho in USA
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