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SERVICE INSTRUCTIONS – SAFETY DEVICES
Figure 13
Service, Safety, and Other Major Unit Heater Components
Limit Control (Overheat Switch)
The limit control, mounted on the left inner side panel (when
facing front of unit), will shut off the gas supply to the main
burner in the event of overheating. It is a single pole single
throw switch.The contacts open to shut the electric gas valve
off in the event the unit should overheat.This limit control
should operate only when something is seriously wrong with
the unit. Anytime this control operates, correct the difficulty
immediately or serious damage may result. If the limit control
cuts off the gas supply during normal operation:
1. Make sure deflector blades are open and that there are not
any obstructions in the air inlet or discharge outlet.
2. Check actual input to unit against rated input.
3. Check to be sure motor is operating.
4. On propeller units, check that fan is not loose on motor
shaft. On blower units, check belt and sheave for tightness
or damage.
5. On propeller units, check fan speed against speed on motor
nameplate. On blower units check blower speed against
Performance Data on pages 18 or 19, check for restriction
in ducts and for dirty filters.
6. Check to make sure the venting system is not damaged or
blocked. Also check to be sure unit is venting normally and
that there is not negative pressure in the building adversely
affecting draft.
7. Clean heat exchanger tubes inside and out if necessary.
8. If items 1-7 do not solve the problem, check limit control and
replace if necessary.The control is accessible by removing
the left outer side panel, held in place by screws at the rear
of the unit.
I
MPORTANT NOTE:
The limit control (overheat switch) on this unit heater will shut
off the gas should excessive discharge temperatures occur. Do
not attempt to control the fan with the limit control. Any change
in wiring to attempt to control the fan with the limit control will
result in hazardous conditions and void the warranty.
Blocked Vent Safety Switch (BVSS)
A BVSS is supplied on all gravity-vented unit heaters and is
designed to prevent operation of the main burner if the venting
system is blocked.
If the BVSS has tripped, turn off the gas and electric supply to
the unit heater. Check the entire vent system connected to the
unit heater for blockage or damage.
In the case of a restricted vent, there may not be enough
dilution air to carry away the heat radiating off the heat
exchanger top (and surrounding area), the BVSS may exceed
the temperature setting and trip.
Spillage will also cause the BVSS to trip. If spillage exceeds
five minutes, even though the vent is in compliance with the
NFGC, some type of change must be made in the vent system
to stop the spillage.These changes (improvements) could be
lengthening the vertical vent run, reducing the horizontal vent
run, insulating the vent pipe, using a larger diameter vent pipe,
or using a less restrictive vent terminal.
LIMIT CONTROL
LOCATED UNDER
LEFT SIDE PANEL
FOR
SERVICE,
REMOVE
LEFT SIDE
PANEL
COMBUSTION AIR
INLETS & BURNER
LOUVERS
DIMPLED
HEAT
EXCHANGER
FAN
MOTOR
JUNCTION
BOX
TERMINAL
BOARD
RELIEF
OPENING
VENT
VENT PIPE
CONNECTION
COMBINATION
GAS CONTROL
PILOT
TUBING
BLOCKED
VENT SAFETY
SWITCH
(FRONT, TOP VIEW)
Heater Parts from ACF Greenhouses