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4E-2k. Combustion Blower
The combustion blower operates when it is
provided with 115V, 60 Hz power. Confirm 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 condition 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 combustion 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 confirm 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 attempts
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
flow system has two aspects - air flow and gas flow.
Air flow is checked by measuring pressures at service
ports on the venturi. Gas flow is checked by evaluating
venturi pressures plus the regulator offset pressure and
the gas orifice size.
In a venturi flow system the difference between
various pressures is far more important than their
“gauge” value relative to the room. The gas pressure
offset and the gas orifice pressure differential are
especially important concepts. The following sections
illustrate these and related information.
4F-1. Pressure Measurement Ports
Figure 32 shows venturi and gas control
components and service port locations. Air flow enters
the venturi from below through the filter box. It is
pulled through a converging section to the throat,
where pressure is reduced substantially. Gas flow is
pulled into the throat through an orifice inside the pipe
between the throat and the elbow. Air and gas are
does not operate or the igniter doesn’t glow, investigate
as outlined in a later sections. If both of these
components operate normally but voltage doesn’t
appear at point 8 during the trial for ignition, replace
the ignition control. If voltage appears at point 8 but
there is no ignition, proceed to analysis of other
components.
4E-2i.Burner Limit Switch
The burner limit switch is located on the front
surface of the burner plenum near the top of the heater.
Servicing this switch normally requires removal of the
top of the heater, but it can be checked by probing for
voltage at point 9 on the vent limit switch. Point 9 is
the terminal of the vent limit switch to which is
connected a brown wire coming down from above. The
vent limit switch is mounted on the vent outlet diffuser
above the combustion blower.
Voltage will be present at point 9 only during
normal operation or during a trial for ignition. If
necessary restart the heater using the Flex Temp
control to obtain one of these conditions. If voltage
does not appear at point 9, the burner limit switch is
open. This may be due to burner mis-adjustment,
incorrect gas orifice size or similar problems which
should be investigated as outlined in later sections. If
there are no such problems and there is no evidence of
excessive burner temperature, replace the burner limit
switch.
4E-2j.Vent Limit Switch
Probe for voltage at point 10, which is the gas
valve terminal having a brown wire connected to it. As
in the preceding case, this terminal will have voltage
present only during a trial for ignition or during normal
operation. If voltage is not found, the vent limit switch
is open, indicating that the exhaust temperature has
been too high.
Normal vent temperature is between 100
o
F and
140
o
F. Excessive temperature might be due to
excessive water temperature or overfiring of the
system. Investigate water temperature and possible
water flow deficiency. Confirm that the heater is
equipped for the correct fuel gas - especially that
propane gas is not being provided to a heater
configured for natural gas. To confirm correction of
the problem, re-set the switch by pushing the red
button. If vent temperature is normal and the switch
will not allow operation, replace it.
WARNING
The combustion blower, the igniter and some
terminals 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.