W:\ELECTRICAL\INSTRUCT\FURNACE\FURNACE GENERAL INSTRUCTIONS 2011.DOC
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01/2014
9-6
TEST DEVICE (Gas heat only)
On gas heated equipment, the test device is used as part of an inspection program to
ensure that the main and pilot shutoff valves are sealing properly. (See Appendix D -
Main Safety Shutoff Valve Leak Test Procedure and Pilot Safety Shutoff Valve Leak
Test Procedure.)
9-7
VENT VALVE
(Gas heat only, if requested by customer)
On gas heated equipment, the vent valve is used to vent the section of gas piping
between the primary and secondary shut off valves. This is installed only at the
customer=s request. The customer must pipe outlet of vent valve to a safe location
outside their building (in accordance with local codes) where any leaking gas will be
safely dispersed into the atmosphere.
9-8
POWERED FORCED EXHAUSTER
(If applicable)
As a safety device, the powered forced exhaust system is used to remove a definite
volume of air when flammable solvents, vapors, gases or products of combustion are
present in the furnace atmosphere. The exhaust rate must be properly sized to give
safe operation and is determined from the quantity of material in the atmosphere and
the operating temperature of the furnace. Reduced exhaust could result in an
explosion or fire and bodily injury or property loss.
9-9
AIR FLOW SWITCH
(If applicable)
This device senses pressure differential across an exhaust blower, recirculating
blower or combustion burner blower to indicate that these blowers are moving air.
When there is no pressure differential, the air flow switch opens and turns off the
control circuit to the main heat. In electrically heated furnaces the circuit is opened to
the heating element contactors (or SCR power controller) and separate contactors.
On gas fired equipment, the control circuit is opened to the control and safety valves.
Where failure of the air supply is critical to prevent a dangerous situation, this device
is mandatory. The air flow switch picks up typical failures such as when a motor fails
to turn the blower or when the blower drive belts are broken.
9-10
PURGE TIMER
Where it is necessary to ventilate the furnace prior to turning the heat on, a purge
timer is installed. This purge period is typically required in gas heated equipment or
when flammable solvents are present in the furnace=s load. The purge timer
ensures that the recirculating blower(s), powered forced exhauster(s) or combustion
blower(s) are operating for a preset period of time prior to turning the heat on. The
purge time is typically based on allowing sufficient time to pass four (4) furnace
volumes of fresh air through the furnace. The time period is factory preset and is
calculated based on the ventilation rate and total volume of the furnace within the
insulated walls. The ventilation rate is based on the quantity of flammables entering
the furnace and the operating temperature. In the case of gas heated equipment, the
exhaust rate is also based on the combustion venting required for the burner system.
9-11
HIGH GAS PRESSURE SWITCH (Gas heat only)
This device is used on gas heated equipment to sense an abnormally high gas
pressure which would affect the operation and safety of the equipment. If the gas
pressure exceeds a preset level, the control circuit is opened to the safety valves.
The installer must pipe, in accordance with local codes, the vent port on this switch to
a safe location outside the building. Each such vent must be piped separately.
