4F. Venturi Combustion Flow System

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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 100oF and 140oF. 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.

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.

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