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problem, re-set the switch by pushing the red button.
If vent temperature is normal and the switch will not
allow op er a tion, replace it.
WARNING
The combustion blower, the igniter and some ter-
minals of the ignition control utilize 115V power and
require appropriate servicing precautions. Note that
wires and terminals of these components may be
"hot" even when the component is not operating.
4E-2k. Combustion Blower
The combustion blower operates when it is
provided with 115V, 60 Hz power. Confi rm this
by connecting the voltmeter between the "neutral"
wirenut and point C, which is the terminal marked F2
on the ignition control. Unless the system is in lockout,
115V should be present at point C whenever a call for
heat reaches the venturi pressure switch (i.e. 24V at
point 6). To be sure that a lockout con di tion is not in
effect, turn the heater off and then back on at the Flex
Temp control.
If 24V is present at point 6 but 115V does not
appear at point C (and there is no lockout) the ignition
control is defective. If 115V power is available at point
C and the com bus tion blower does not run, the blower
is defective and should be replaced. Note, however,
that the blower has an internal overtemperature limit
and before replacing it confi rm that the environment of
the heater is not excessively hot. If so, ventilation of
the heater room may be necessary.
4E-2l. Igniter
The igniter should glow during ignition at tempts
as outlined in the earlier section on ignition control. If
it does not, connect the voltmeter between terminals
marked D and E on the ignition control. If 115V is
present during the igniter-on portion of the ignition
sequence but there is no glow, the igniter is probably
defective. If voltage is not found, the ignition control
is defective and should be replaced.
An igniter which will not glow should be
checked by disconnecting the wires from the ignition
control and measuring the igniter resistance. A cold
igniter should have resistance in the range of 40 to 75
ohms. Replace the igniter if resistance is outside of
this range.
4F. Venturi Combustion Flow System
Verifying proper operation of the combustion
fl ow system has two aspects - air fl ow and gas fl ow.
Air fl ow is checked by measuring pres sures at service
ports on the venturi. Gas fl ow is checked by evaluating
venturi pres sures plus the regulator offset pressure and
the gas orifi ce size.
In a venturi fl ow system the difference between
various pressures is far more im por tant than their
GAS VALVE
SERVICE
PORT
Figure 33. Venturi fl ow system components.
"gauge" value relative to the room. The gas pressure
offset and the gas orifi ce pressure differential are
especially important concepts. The following sections
illustrate these and related information.
4F-1. Pressure Measurement Ports
Figure 33 shows venturi and gas control
components and service port locations. Air fl ow enters
the venturi from below through the fi lter box. It is
pulled through a con verg ing section to the throat,
where pressure is reduced substantially. Gas fl ow is
pulled into the throat through an orifi ce inside the
pipe between the throat and the elbow. Air and gas are
combined in the throat and mix thoroughly as they
proceed through the venturi tailpipe to the burner.
Service ports are provided at three places. On
the right side of the venturi an inlet ser vice port is
provided just above the air fi lter box. Also on the right
side, a throat service port is provided - at the narrowest
section of the venturi casting. A gas pressure port
is provided on the outlet face (right side) of the gas
valve.
To evaluate system operation requires ac cu rate
measurement at these ports. A digital manometer,
known to be accurately calibrated, is ideal. Other
instruments may be used, but the "positive/negative"
nature of the readings must be very clear. Also, gas
pressure offset measurements are at very low levels
(0.2" WC) and the instrumentation must be ca pa ble of
determining it accurately.
4F-2. Venturi System Checkout
WARNING
When servicing is complete be sure to replace pres-
sure service port plugs and assure that pressure
switch and equalizer tube connections are unaltered
and fi rmly in place.
VENTURI
THROAT
SERVICE
PORT
VENTURI
INLET
SERVICE
PORT
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