1 / 48

Table of Contents

Table of Contents. Safety Circuits. Induced Draft Motors. Vent Pressure Safety Switches (3-11) High Limit Controls (12-14) Flame Roll-out Limit Switches (15-17) Induced Draft Blower Housing Limit Switches (18-19) Measuring Flame Current (20-22).

naiya
Download Presentation

Table of Contents

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Table of Contents Safety Circuits Induced Draft Motors Vent Pressure Safety Switches (3-11) High Limit Controls (12-14) Flame Roll-out Limit Switches (15-17) Induced Draft Blower Housing Limit Switches (18-19) Measuring Flame Current (20-22) Checking Shaded Pole Induced Draft Motor Windings (35-36) Checking PSC Induced Draft Motor Windings (37-38) Checking Earlier Variable Speed Induced Draft (39-40) Motor Variable Speed 3 Phase Induced Draft Motors (41-42) Gas Valves ECM Blower Motors ECM Blower Motor Quick Test (43-45) LitePort Furnace Control Information (46-48) Measuring Inlet Supply Gas Pressure (23) Manifold Pressure Adjustment (24-30) Hot Surface Ignitors Checking Silicon Carbide Hot Surface Ignitors (31-32) Checking Silicon Nitride Hot Surface Ignitors (33-34)

  2. Fig. 1 Fig. 2 Vent Pressure Safety Switches The vent pressure safety switch on 80% efficient furnaces is connected to the induced draft blower housing. The vent pressure safety switch monitors the negative pressure developed by the induced draft blower assembly. If the induced draft blower pressure falls below the vent pressure safety switch minimum closing pressure, the switch will open. On 80% efficient gas furnaces, this switch (fig. 1) is used to prove the presence of adequate combustion air. It does not prove any operation of the vent system, chimney, because 80% furnaces do not use the induced draft blower to vent products of combustion. (THRU CHIMNEY) These furnaces are Category 1 appliances. Category 1 means these furnaces vent the products of combustion by natural draft. The vent pressure safety switches on 90 % efficient gas furnaces (fig. 2) are connected to the secondary heat exchanger area. The vent pressure safety switch on these models monitor the secondary heat exchanger area to ensure there is adequate negative pressure present. The induced draft blower assembly draws in combustion air and also vents the products of combustion.

  3. Fig. 3 Fig. 5 Fig. 4 Vent Pressure Safety Switches These furnaces are Category 4 appliances. Category 4 appliances use the induced draft blower to create a positive pressure on the vent system. Vent pressure safety switches on Category 4 appliances protect the furnace from operating with a lack of combustion air and from blockages or restrictions in the flue system. The vent pressure safety switch circuit is powered by 24 volts. The switch is a normally open device that closes on an increase in negative pressure. Modern furnace integrated furnace controls (IFC) monitor the position of this switch during a call for heat via a connection to either a 9 pin or 12 pin plug located on the IFC board. The 9 pin plugs (fig. 3) are for single stage furnaces (fig. 4) and the 12 pin plugs are for 2 stage furnaces (fig. 5). Continued on Next Slide…

  4. Fig. 6 Vent Pressure Safety Switches Continued… The plugs have two pins for each vent pressure safety switch. One pin sends 24 volts potential to the switch and the other pin receives the 24 volt potential back to the IFC. If the furnace is a two stage model, there will be one pin for voltage out to both switches and then two pins to receive the 24 volt potential back at the IFC board (fig. 6). When a call for heat is received by the IFC, the IFC sends 24 volts out to the vent pressure safety switch. If the switch is open, the IFC will not receive 24 volts back. However, if the switch is shorted, or stuck in the closed position, the IFC will receive 24 volts back. The IFC will then lock out and flash an appropriate fault code on its LED. If the IFC determines the vent pressure switch is open, it will start the induced draft motor for a pre-purge cycle if all other safety devices are in their proper position. Notice: Prior production variable speed vent motor models had the pressure switches report to the microprocessor on the vent motor assembly. The status of the pressure switches was then communicated by the microprocessor to the IFC board.

  5. Fig. 7 Fig. 8 Fig. 9 Vent Pressure Safety Switches Notice there are two vent pressure safety switches shown (fig. 7), they are installed on a two stage gas furnace. The front switch is for second stage induced draft motor speed and the back one is for first stage induced draft motor speed. The first stage switch will have a closing pressure that is lower than the second stage switch (fig. 8). The switch has a lower close pressure to match the slower speed of the induced draft blower when on low fire. During low speed operation the high speed switch should be open and the low speed switch closed.  During a call for second stage heat the induced draft motor goes to high speed and generates an increased amount of negative pressure. Both the low speed switch and high speed switch should be closed during both low and high speed induced draft motor operation. The vent pressure switch must be in the open position prior to induced draft motor operation (fig. 9). If the furnace control senses a closed vent pressure switch when the induced draft motor is off, the furnace control will lockout. The control will not allow heat operation until it senses the switch has opened. If a technician tries to bypass the switch with a jumper, the furnace will lockout on the next call for heat cycle.

  6. Fig. 10 Vent Pressure Safety Switches This safety feature prevents furnace operation when an insufficient level of combustion air is being drawn into the burners. The switch also prevents operation of the burners on 90% efficient models when the flue is restricted. If the furnace was allowed to operate with inadequate combustion air, the furnace would malfunction and produce carbon monoxide. If the burners were allowed to operate with insufficient vent, products of combustion would spill into the home. How to Check for a Shorted Vent Pressure Safety Switch 1. Remove power to the furnace. 2. Disconnect the wires from the vent pressure switch terminals. 3. Using an ohmmeter, check for continuity across both vent pressure switch terminals (fig. 10) . 4. A shorted vent pressure switch will measure resistance. Replace the switch if it is shorted. 5. An open vent pressure switch will measure infinite resistance. The switch is not shorted. 6. When finished, replace the wires and remember to restore power to the furnace.

  7. Fig. 11 Vent Pressure Safety Switches How to Check for Proper Pressure Switch Close Operation To determine if a vent pressure safety switch is closing at the proper pressure you will need to measure the negative pressure being sensed by the switch. A good instrument for measuring induced draft negative pressure is a Magnehelic gauge. For 80% efficient gas furnace induced draft pressure, the gauge scale should be capable of reading accurately down to as low as .05 inches of water column and as high as 3 inch of water column. For 90% efficient gas furnace induced draft pressure, the scale should be capable of reading accurately down to as low as .05 inches of water column and as high as 2 to 3 inches of water column. To sense the pressure at the back of the pressure switch, a tee is placed in the tubing between the switch and inducer housing (fig. 11). One opening in the tee is connected to the induced draft blower housing, one opening to the LOW PORT of the Magnehelic gauge, and the other tee port is connected to the vent pressure switch. The HIGH PORT of the Magnehelic is left open to atmosphere. If the furnace is a sealed combustion model, the high port of the Magnehelic is connected to the tube leading to the burner enclosure. Continued on Next Slide…

  8. Fig. 12 Fig. 13 Vent Pressure Safety Switches Continued… When the induced draft motor runs, the pressure being sensed by the vent pressure safety switch is displayed on the Magnehelic gauge fig. 12). If the pressure is greater than the closing set point of the switch, yet the switch does not close, replace the switch. If the pressure is not high enough to close the switch, there is a problem with the induced draft blower assembly or the vent/combustion air system. Switching Function Check 1. Connect a Magnehelic gauge between the induced draft motor housing and the pressure switch (fig. 13). 2. With the induced draft motor running, note the negative pressure reading on the Magnehelic gauge. The pressure must be greater than the close pressure of the pressure switch being tested. (continued) Continued on Next Slide…

  9. Fig. 14 Fig. 15 Vent Pressure Safety Switches Continued… 3. Use a voltmeter to check for 24 volts at the pressure switch VOLTAGE IN terminal. Check by placing one voltmeter lead to ground and the other to the VOLTAGE IN terminal (fig. 14). (MOTOR RUNNING) You should measure 24 volts. If the furnace is a two stage model, check both the low and high fire pressure switches to ensure you have 24 volts at the VOLTAGE IN terminals. (MOTOR RUNNING) Next, check for 24 volts at the VOLTAGE OUT terminal. Place one voltmeter lead to ground and the other to the VOLTAGE OUT terminal (fig. 15). You should measure 24 volts. If you do not measure 24 volts, the pressure switch is open. Replace the switch. If you measure 24 volts, the switch is in the closed position. (MOTOR RUNNING)

  10. Vent Pressure Safety Switches Causes of Low Induced Draft Blower Housing Pressure 80% Efficient Models · Seized induced draft blower wheel. · Failed induced draft motor, or capacitor. · A lack of combustion air supply. · Kinked or plugged tube from induced draft blower to pressure switch. Causes of Low Vent Pressure on 90% Efficient Models. · Plugged vent or induced draft assembly. · Failed vent motor or capacitor. · Plugged heat exchanger. · Improper sized vent piping. · Blocked condensate drain. LED Flash Code Response (40” Furnace Models) If a White Rodgers IFC board has detected a vent pressure safety switch fault, the diagnostic LED will flash 3 times and lock the furnace out until the pressure switch fault has been corrected.

  11. Fig. 1 Fig. 3 Fig. 2 Fig. 4 High Limit Controls High limit controls protect against abnormally high air temperature in the heat exchanger airstream area. These limit switches (fig. 1) are 24 volt operated automatic reset devices that open when the air temperature they sense exceeds the switches open setting. The open setting of the limit switch is usually not found on the switch itself, but rather on the furnace nameplate. The high limit is wired in series with other furnace limit switches (fig. 2). Modern furnace integrated furnace controls (IFC) monitor the position of limit switches during a call for heat via a connection to either a 9 pin or 12 pin plug located on the IFC board. The 9 pin plugs (fig. 3) are for single stage furnaces and the 12 pin plugs (fig. 4) are for 2 stage furnaces. The plugs have two pins that connect to the furnace limit devices.

  12. Fig. 5 High Limit Controls One pin sends 24 volts potential to the limit switches and the other pin receives the 24 volt potential back to the IFC. When a call for heat is received by the IFC, the IFC sends 24 volts out to the high limit circuit. If the high limit switch is open, the IFC will not receive 24 volts back. If the high limit or any of the other limits open, the furnace will shut down burner operation and run the furnace blower and induced draft blower in an attempt to cool down the furnace. Check Procedure with Voltmeter 1. Call for heat 2. Place your voltmeter leads across the high limit switch terminals (fig 5). 3. You should measure 0 volts if the limit is closed. If the limit is open you will measure 24 volts. 4. If you measure 0 volts across the high limit, yet the furnace is indicating an open limit fault, check for 24 volts to ground at each side of the high limit switch. If you do not measure voltage, another limit switch is open.

  13. Fig. 6 High Limit Controls Check Procedure with Ohmmeter 1. If the furnace is hot, allow it to cool down, and then remove power to the furnace. 2. Remove the two wires from the high limit switch. 3. Using an ohmmeter, check for continuity across the switch (fig. 6). 4. You should measure 0 ohms if the switch is closed. If the high limit is open, replace it. Causes of High Limit Trips · Dirty filters · Dirty A/C Coil · Undersized ducting · Blower motor failure · Dirty blower assembly · Over-firing of the furnace burners

  14. Fig. 1 Fig. 2 Flame Roll-out Limit Switches Flame roll-out limit controls (fig. 1) protect against abnormally high air temperature in the furnace burner area. These limit switches are 24 volt operated manual reset or one time failure devices that open when the air temperature they sense exceeds the switches open setting. The switches can be either snap disc or fuse link devices. The open temperature of the switch is normally not listed on the part. The flame roll-out switch circuit is powered by 24 volts. The flame roll-out switch is wired in series with other furnace limit switches (fig. 2). (TCO) Modern furnace integrated furnace controls (IFC) monitor the position of limit switches during a call for heat via a connection to either a 9 pin or 12 pin plug located on the IFC board. Continued on Next Slide…

  15. Fig. 3 Fig. 4 Fig. 5 Flame Roll-out Limit Switches Continued… The 9 pin plugs (fig. 3) are for single stage furnaces and the 12 pin plugs (fig. 4) are for 2 stage furnaces. The plugs have two pins that connect to the furnace limit devices. One pin sends 24 volts potential to the limit switches and the other pin receives the 24 volt potential back to the IFC.  If the limit opens, the furnace will shut down burner operation and run the furnace blower and induced draft blower in an attempt to cool down the furnace. Check Procedure with Ohmmeter 1. If the furnace is hot, allow it to cool down, and then remove power to the furnace. 2. Remove the two wires from the flame roll-out switch. 3. Using an ohmmeter, check for continuity across the switch (fig. 5). 4. You should measure 0 ohms if the switch is closed. If the switch is open, you will measure infinite ohms. If you measure infinite ohms reset the switch if it is a reset type, if it cannot be reset, replace the switch.

  16. Fig. 6 Fig. 7 Fig. 8 Flame Roll-out Limit Switches Check Procedure with Voltmeter 1. Call for heat. 2. Place your voltmeter leads across the limit switch terminals (fig. 6). 3. You should measure 0 volts if the limit is closed. If the limit is open you will measure 24 volts. 4. If you measure 0 volts across the high limit, yet the furnace is indicating an open limit fault, check for 24 volts to ground at each side of the switch (fig. 7 ) (fig. 8). If you do not measure voltage, another limit switch is open. Causes of Flame Roll-out Trips · Low gas pressure · A lack of combustion air · Plugged burner orifice · Plugged heat exchanger

  17. Fig. 1 Fig. 2 Fig. 3 Induced Draft Blower Housing Limit Switches The induced draft blower housing limit control (fig. 1) protects against abnormally high flue temperature in the induced draft blower housing. This limit is only present on 90% efficient models.  These switches are 24 volt or 115 volt operated auto reset devices. The switches are mounted to the plastic induced draft blower housing. The open temperature of the switch is normally not listed on the part. Furnaces wired to break 115 volt power to the induced draft blower, report a pressure switch error when this switch opens because the vent motor never comes on during a call for induced draft blower operation. (See fig. 2) Furnace models may use a 24 volt switch that is wired directly to the furnace IFC board 9 or 12 pin plug. Furnaces wired in this manner have the capability to report an open induced draft blower limit switch on the diagnostic LED. (See fig. 3) Continued on Next Slide…

  18. Fig. 4 Fig. 5 Fig. 6 Induced Draft Blower Housing Limit Switches Continued… The IFC board plugs have two pins that connect to the furnace limit devices. One pin sends 24 volts potential to the limit switches and the other pin receives the 24 volt potential back to the IFC. When a call for heat is received by the IFC, the IFC sends 24 volts out to the induced draft blower limit circuit. If the limit switch is open, the IFC will not receive 24 volts back. If this limit is open, the IFC will flash a fault code. The induced draft blower and the indoor blower motor will run until the switch re-closes. Check Procedure with Ohmmeter 1. If the furnace is hot, allow it to cool down, and then remove power to the furnace. 2. Separate the plug assembly located near the limit switch (fig. 5). 3. Using an ohmmeter, check for continuity across the two plug pins (fig. 6). Causes of Flame Roll-out Trips · Overfiring of the furnace. · Excessive supply air temperature due to a lack of indoor air volume (Plugged secondary.)

  19. Fig. 1 Fig. 2 Fig. 3 Fig. 4 Measuring Flame Current The flame sensing circuit (fig. 1) proves the presence of flame. Without this circuit the furnace would not know when flame is present. The circuit consists of a flame sensing rod (fig. 2) located at the opposite end of the burner assembly from the furnace gas manifold connection. The flame sensing rod is positioned in front of the last burner. The flame rod has an electrical terminal connection with one wire (fig. 3). The wire connects the flame sensing rod to the furnace 9 or 12 pin plug located on the IFC board (fig. 4). On some models of furnaces, the flame sensing wire is connected to a terminal on the IFC board. The voltage to the flame sensing rod is typically around 80 volts AC when measured with a digital voltmeter.

  20. Fig. 5 Fig. 6 Measuring Flame Current When a call for heat takes place and the burners ignite, flame envelopes the flame rod. The flame is a conductor and can conduct a very small amount of current. The voltage potential at the flame rod now has a conductor path through the flame. Since the flame rod is much smaller than the burner ground area, more current flows in one direction than in the other. This is called flame rectification. The current flowing in one direction is now greater than in the other direction so it is DC current. The amount of current that actually flows is in the microamp range. Typically from 2 to 4 DC microamps. The minimum flame current necessary to maintain ignition on White Rodgers ignition controls is 1 DC microamp. How to Measure Flame Current with a Digital Multimeter Capable of Reading DC Microamp Range 1. Disconnect power to the furnace. 2. Set the multimeter to the DC microamp scale. 3. Remove the wire from the spade terminal on the flame sensing rod. 4. Connect one end of a jumper wire to the flame sensing rod spade terminal (fig. 5). 5. Connect the other end of the jumper wire to one lead of the multimeter (fig. 6). Continued on Next Slide…

  21. Fig. 7 Fig. 8 Measuring Flame Current Continued… 6. Connect the other lead of the multimeter to the wire removed from the flame sensing rod (fig. 7). 7. The meter is now wired in series between the flame sensing rod and the IFC board (fig. 8). 8. Restore power to the furnace and call for heat. 9. When flame is established the actual flame current will be displayed on the multimeter. 10. When you are finished with your measurements, remove the call for heat. Allow the furnace to run through the complete sequence of operation and then remove power to the furnace. Reconnect the wire to the flame sensing rod and restore power. Causes of Low or No Flame Current · Dirty flame sensing rod · Cracked ceramic insulator on flame sensing rod · Cracked or broken flame sense rod wire · Poor furnace ground · Reverse polarity of 115 volt power

  22. Fig. 1 Fig. 2 Fig. 3 Measuring Inlet Supply Gas Pressure Inlet supply gas pressure requirements for single stage natural gas models is 5 to 7 inches water column. LP models require an inlet supply pressure of 11 to 13 inches water column. The gas valve has a pressure port tap from which to measure the inlet gas pressure. Inlet supply gas pressure is measured with a manometer or Magnehelic gauge. Procedure for Reading Inlet Gas Pressure 1. Turn off power to the furnace and shut off the gas supply to the furnace. 2. Remove the pressure port plug on the inlet side of the gas valve body (fig. 1). 3. Insert a hose barb fitting into the port and tighten till snug (fig. 2). 4. Connect a manometer to the hose barb (fig. 3). 5. Restore power to the furnace and call for heat. If the furnace is two stage, make sure both stages of heat are calling. HAVE ALL GAS APPLIANCES ON WHEN CHECKING INLET GAS PRESSURE. 6. Once the gas valve has energized, the inlet pressure will be displayed in inches of water column on the manometer. If the inlet pressure falls above or below the inlet requirement, contact the gas company for correction. 7. When finished replace screw cover. End call for heat. Remove power. Disconnect hose barb and replace the pressure port plug (fig. 4). 8. Restore power. Fig. 4

  23. Measuring Manifold Gas Pressure Manifold Pressure Adjustment Manifold pressure requirements for single stage natural gas models is 3 to 3.5 inches water column. LP models require a manifold pressure of 10.5 to 11 inches water column. The gas valve has a pressure port tap from which to measure the manifold pressure (fig. 1) and has an adjustment screw to raise or lower the manifold pressure (fig. 2). Manifold pressure is measured with a manometer or Magnehelic gauge. Before making an adjustment to the manifold pressure, make sure the inlet gas pressure is within acceptable limits. Procedure for Reading Inlet Gas Pressure 1. Turn off power to the furnace. 2. Remove the pressure port plug on the outlet side of the gas valve body (fig. 3). Fig. 1 Fig. 2 Fig. 3 Continued on Next Slide…

  24. Manifold Pressure Adjustment Continued… 3. Insert a hose barb fitting into the port and tighten till snug (fig. 4). 4. Connect a manometer to the hose barb (fig. 5). 5. Restore power to the furnace and call for heat. 6. Once the gas valve has energized, the manifold pressure will be displayed in inches of water column on the manometer. 7. If the manifold pressure must be adjusted, remove the slotted cover to gain access to the manifold pressure adjustment screw (fig. 6). Turn the screw clockwise to increase pressure and counterclockwise to decrease pressure (fig. 7). 8. When finished replace screw cover. End call for heat. Remove power. Disconnect hose barb and replace the pressure port plug. 9. Restore power. Fig. 4 Fig. 5 Fig. 6 Fig. 7

  25. Fig. 7 Fig. 8 Fig. 9 Manifold Pressure Adjustment Gas Pressure Manifold Adjustment Two Stage Furnaces Before making an adjustment to the manifold pressure, make sure the inlet gas pressure is within acceptable limits. Manifold pressure requirements for two stage natural gas models is 1.4 to 1.7 inches water column first stage and 3 to 3.5 inches water column second stage. LP models require a manifold pressure of 6 to 6.2 inches water column first stage and 10.5 to 11 inches water column second stage.  The gas valve has a pressure port tap to measure the manifold pressure. There are two adjustment screws to raise and lower manifold pressure for first and second stage fire rates. The first stage adjustment screw is on top of the gas valve (fig. 8) . The second stage adjustment screw is next to the pressure port plug on the outlet side of the gas valve (fig. 9). Manifold pressure is measured with a manometer or Magnehelic gauge. Continued on Next Slide…

  26. Fig. 11 Fig. 10 Fig. 12 Manifold Pressure Adjustment Continued… 1. Turn off power to the furnace. 2. Remove the pressure port plug on the outlet side of the gas valve body (fig. 10). 3. Insert a hose barb fitting into the port and tighten till snug (fig. 11). 4. Connect a manometer to the hose barb (fig. 12). 5. Restore power to the furnace and call for first and second stage heat. 6. Once the gas valve has energized, the second stage high fire manifold pressure will be displayed in inches of water column on the manometer. Continued on Next Slide…

  27. Fig. 13 Fig. 14 Manifold Pressure Adjustment Continued… 7. If the manifold pressure must be adjusted, remove the slotted cover on the outlet side of the gas valve to gain access to the manifold pressure adjustment screw (fig. 13). Turn the screw clockwise to increase pressure and counterclockwise to decrease pressure. 8. Once the second stage manifold pressure has been set, replace the high fire slotted cover. 9. Remove the low fire slotted cover (fig. 14) and reduce the call for heat to first stage only. Note the drop in gas pressure on the Magnehelic gauge. If the pressure needs adjustment, turn the adjustment screw to get the proper pressure. 10. When finished replace screw cover. End call for heat. Remove power. Disconnect hose barb and replace the pressure port plug. 11. Restore power. NOTE about LP gas pressure adjustment: When adjusting the gas pressure on LP equipped furnaces, first turn the high fire adjustment screw clockwise until it bottoms out before setting the low fire propane fire rate. If you do not do this, you will not be able to set the low fire rate to its proper level.

  28. Fig. 15 Fig. 16 Manifold Pressure Adjustment Gas Pressure Manifold Adjustment Sealed Combustion Furnace Sealed combustion burners (fig. 15) are not at atmospheric pressure. The burners are actually at a negative pressure compared to atmosphere. Therefore, you must reference the burner box pressure when measuring manifold pressure. To measure manifold pressure on a system that has sealed combustion you must have a manometer or magnehelic gauge with a high and low pressure connection. To measure gas pressure with sealed combustion (fig. 16), first, connect a tee into the hose connecting the gas valve to the burner box. Next, connect one end of the tee to the tube leading to the burner enclosure. Next, connect the last tee opening to the low port of the magnehelic gauge or manometer. Connect high port of the magnehelic gauge or manometer to the pressure port tap on the gas valve. The pressure you read will be the manifold pressure referenced to the burner box pressure.

  29. Fig. 17 Fig. 18 Fig. 19 Fig. 20 Fig. 21 Manifold Pressure Adjustment Pressure Measurement: New 36G Gas Valves The new White Rodgers 36G gas valves (fig. 17) require a special kit to measure the inlet and outlet gas pressure. The kit number is KIT07611. The kit consists of a 5/16 tube, a connector, and a 3/32 hex wrench (fig. 18). The kit is used to connect a gas pressure manometer to pressure bosses located on the gas valve body. The kit’s 5/16 inch tubing fits over the inlet and outlet pressure bosses of the gas valve. The kit’s connector connects the manometer to the 5/16 inch tubing. The hex wrench opens the port inside of the pressure bosses. Rotating the screw inside of the boss counter clockwise one turn will open the pressure port. To close the port, rotate the screw clockwise one turn (fig. 19). Always follow the safety instructions included with the pressure tap adaptor kit. New production units will have new easy to service White Rodgers 36 series gas valves. The valves feature easy to access pressure adjustment screws and easy ON/OFF pressure access bosses on the gas valve body (fig. 20 and 21). The small size of the valve makes service a breeze.  LP conversion kits are available. The two stage valve shares the same spring as single stage models.

  30. Fig. 1 Fig. 2 Fig. 3 Checking Silicon Carbide Hot Surface Ignitors Silicon carbide hot surface ignitors (fig. 1) are used as the ignition source on some furnace models. The igniters are 120 volt operated devices that are energized by the IFC board during a trial for ignition period. The igniters are connected to the IFC board at the igniter HOT and igniter neutral terminals (fig. 2). When a call for heat occurs, the furnace goes through a pre-purge cycle. After the pre-purge cycle is completed, the trial for ignition cycle is started. The igniter goes through a warm-up period and the gas valve is energized. If the igniter fails to glow, the trial for ignition will fail. Checking the Igniter 1. Turn off power to the furnace. 2. Let the furnace go through the pre-purge cycle and enter the trial for ignition period.

  31. Fig. 4 Fig. 5 Checking Silicon Carbide Hot Surface Ignitors Measure the voltage at the plug (fig. 4). There should be 120 volts. If 120 volts is present and the igniter does not glow, replace the igniter. If there is no voltage, check for 120 volts at the IGNITER HOT and IGNITER NEUTRAL terminals on the IFC (fig. 5). If voltage is present, repair the wiring between the IFC and the plug. If there is no voltage at the IFC board, replace the IFC board.

  32. Fig. 2 Checking Silicon Nitride Hot Surface Ignitors Silicon nitride igniters (fig.1) are used as the ignition source on some furnace models. The igniters operate at a voltages below line level. Typically in the 70 to 80 volt range. These igniters cannot operate at 120 volts. If 120 volts is applied to the igniter over an extended period of time, the igniter will fail. Furnaces that have silicon nitride igniters have IFC boards that monitor furnace line voltage levels. The IFC will regulate the voltage to the igniter based upon the line voltage level (fig. 2). By reducing voltage to the igniter, the life-span of the igniter is extended. When a call for heat occurs, the furnace goes through a pre-purge cycle. After the pre-purge cycle is completed, the trial for ignition cycle is started. The igniter goes through a warm-up period and the gas valve is energized. If the igniter fails to glow, the trial for ignition will fail. Fig. 1

  33. Fig. 3 Fig. 4 Checking Silicon Nitride Hot Surface Ignitors Check Procedure 1. If the igniter fails to glow, disconnect power to the furnace. 2. Unplug the igniter wiring harness and access the pins in the plug assembly leading to the igniter (fig. 3). 3. Using an ohmmeter, measure the resistance through the igniter (fig. 4). At room temperature you should measure between 11 and 17 ohms. If the resistance is infinite or zero, replace the igniter.  4. If the igniter ohms out OK, replace the IFC board. Ignitor Malfunction Check voltage from ground to neutral. It should never measure more than 10 volts. Cause: Loose neutral wire connection.

  34. Fig. 1 Fig. 2 Checking Induced Draft Blower Motor Checking Shaded Pole Induced Draft Motor Winding 80% efficient gas furnace models use shaded pole motors to turn the induced draft blower assembly (fig. 1). These motors are 115 volt operated motors that have an internal overload switch.  When a call for heat occurs, the motor is energized by the IFC board. The IFC has a HOT INDUCER terminal that has 115 volts present when motor operation is called for. The IFC also has a terminal connection for NEUTRAL. Check Procedure If the motor does not run when it should, the motor winding may be open, the motor may be seized, the internal overload may be open, or the IFC board may not be switching 115 volts to the motor. 1. Disconnect power to the furnace. 2. Unplug the motor from the wiring harness (fig. 2). Continued on Next Slide…

  35. Fig. 3 Fig. 4 Checking Induced Draft Blower Motor Continued… 3. Using an ohmmeter, check for continuity between the two pins on the plug (fig. 3). You should measure a small resistance. If you measure infinite resistance, the Internal overload may be open, or the motor winding may be open. If the motor is cool, replace the motor. If the motor is hot, allow time for it to cool and recheck to see if the internal overload resets. Check for a seized motor. 4. If the motor windings are OK, connect the motor plug back into the wiring harness. 5. Restore power to the furnace and call for heat. 6. Check for 115 volts at the IFC board terminals that power the induced draft blower (fig. 4). There should be 115 volts present. If there is no voltage, replace the IFC. 7. If voltage is present, check the wiring harness between the IFC and the induced draft blower motor.

  36. Fig. 1 Checking PSC Induced Draft Motor Windings 90% efficient gas furnace models use PSC motors to turn the induced draft blower assembly. These motors are 115 volt operated motors that have an internal overload switch. The motors have start and run windings along with a run capacitor. If the furnace is a single stage model, the motor will have one run winding speed tap if the furnace is a two stage model, there will be two run winding speed taps. When a call for heat occurs, the motor is energized by the IFC board (fig. 1). The IFC has a HOT INDUCER terminal that has 115 volts present when motor operation is called for. The IFC also has a terminal connection for NEUTRAL. If the furnace is a two stage model, there will be a low and a high speed terminal on the IFC board. Check Procedure If the motor does not run when it should, the motor winding may be open, the motor may be seized, the internal overload may be open, or the IFC board may not be switching 115 volts to the motor, or the run capacitor may be bad. 1. Disconnect power to the furnace. 2. Unplug the motor from the wiring harness. Continued on Next Slide…

  37. Fig. 2 Fig. 3 Checking PSC Induced Draft Motor Windings Continued… 3. Using an ohmmeter, check for continuity between the white wire pin on the plug and both black high speed pin and the red low speed pin (fig. 2). You should measure a small resistance between white and black and white and red. If you measure infinite resistance, the Internal overload may be open, or the motor winding may be open. If the motor is cool, replace the motor. If the motor is hot, allow time for it to cool and recheck to see if the internal overload resets. Check for a seized motor. If the motor is not seized, make sure the run capacitor is not bad. 4. If the motor windings are OK, connect the motor plug back into the wiring harness. 5. Restore power to the furnace and call for heat. 6. Check for 115 volts at the IFC board terminals that power the induced draft blower (fig. 3). Make sure you check the proper speed tap. There should be 115 volts present. If there is no voltage, replace the IFC. If voltage is present, check the wiring harness between the IFC and the induced draft blower motor. Remember to check the run capacitor!

  38. Fig. 1 Fig. 2 Fig. 3 Checking Earlier Variable Speed Induced Draft Motor Some variable speed gas furnaces use a DC drive induced draft motor that has an on board microprocessor control (fig. 1). The vent pressure switches report directly to the vent motor assembly. Pressure switch status is reported to the IFC board on a communication bus between the IFC board and the microprocessor board on the vent motor. There are status LED indicator lights on the IFC that communicate diagnostic information received from the vent motor microprocessor. The vent motor has a 12 pin plug that has a line voltage connection (fig. 2). The 115 volt power must be present for the vent motor to run. If the power is not present at the vent motor, the induced draft blower housing limit switch may be open. The 12 pin plug also has a 13 volt DC pin that must have power present for the vent motor microprocessor to function. The 13 volt DC power is received from the IFC board’s 5 pin plug. The IFC tells the vent motor whether to operate at first stage speed or second stage speed. The stage signal is sent out on the 5 pin IFC plug (fig. 3). The voltage levels are 1 volt DC on or 12 volts DC off. For example, the first stage speed pin would have 1 Volt DC if the IFC is calling for first stage induced draft motor operation.

  39. Fig. 4 Fig. 5 Fig. 6 Fig. 7 Checking Earlier Variable Speed Induced Draft Motor Checking Vent Motor Quick Test 1. If the induced draft blower does not run, make sure there is 115 volts to the Vent motor 12 pin plug. Check the voltage between NEUTRAL and pin 12 of the 12 pin plug on the vent motor (fig. 4). If voltage is not present, check to make sure the induced draft blower limit is closed. If it is open, replace it. 2. If voltage is present, the IFC board green LED labeled RPM should be flashing slow. If it is not, check for 13 volts DC on the IFC board. Measure at the 5 pin plug between pins 2 and 3 (fig. 5). If voltage is not present, replace the IFC board. 3. If 13 volts DC is present at the 5 pin plug, jumper pin 4 on the 5 pin plug to ground (fig. 6). The motor should start. If the motor starts, replace the IFC board. 4. If the motor does not start, replace the vent motor. NOTE: To see if the motor will change speeds, start the motor on low speed by jumpering pin 4 to ground. Once the motor is running remove the jumper from pin 4 and rapidly jumper pin 5 to ground (fig. 7). If the motor does not change speed, replace the motor.

  40. Fig. 1 Fig. 2 Variable Speed 3 Phase Induced Draft Motors Some variable speed furnace models feature variable speed 3 phase induced draft motors (fig. 1). The motor speed is adjusted to optimize venting performance. The motor speed is controlled by the furnace IFC board. A 4 pin plug connects the motor to the IFC board (fig. 2). The IFC board sends out three phase voltage to change the speed of the motor. If the motor fails to run, the problem is either with the motor or the IFC board.

  41. Fig. 3 Fig. 4 Variable Speed 3 Phase Induced Draft Motors Check Procedure 1. If the motor does not run, make sure there is power to the furnace. Ensure that a shorted vent pressure switch is not present. 2. Disconnect power to the furnace and unplug the vent motor from the wiring harness (fig. 3). 3. Using an ohmmeter check the resistance between all three motor leads (fig. 4). L1 to L2, L1 to L3, L2 to L3. You should measure equal resistance through all winding combinations. If any combination reads infinite, the winding is open. Replace the motor. If all combinations read infinite resistance, the internal overload is open. If the motor is cool, replace the motor. If the internal overload resets, check for a seized motor.  4. If the motor windings are OK, make sure the motor is not seized.  5. If the motor is not seized, replace the IFC board.

  42. Fig. 1 Fig. 2 ECM Blower Motor Quick Test ECM blower motors are used on some furnace models. These motors are variable speed. They will adjust their RPM in an attempt to deliver the CFM that is programmed by the installing technician. The motors operate on 115 volts if installed on furnaces and 230 volts if installed on air handlers. The motor has two plugs (fig. 1). One plug is a five pin plug that connects the line voltage to motor. The line voltage must be present for the motor to operate. The second plug is a 16 pin plug that connects to the furnace IFC board (fig. 2). This plug carries control signals between the IFC and the ECM Motor.

  43. Fig. 3 Fig. 4 ECM Blower Motor Quick Test Check Procedure If the motor does not run, make sure you have power to the IFC board and then check the voltage between pins 4 and 5 of the 5 pin plug on the motor. You should read 115 volts. If voltage is not present, the motor will not run. Check for a break in the wiring between the IFC board and the motor. If the motor is equipped with a choke coil, the choke coil may be open. If line voltage is present to the motor and the motor does not run, jumper R to G on the thermostat terminal strip (fig. 3). If the motor runs the problem is in the low voltage thermostat wire. If the motor does not run, remove the 16 pin plug from the IFC board. Locate pins 12 and 15. Connect 24 volts from R to pin 12 and pin 15. Connect pins 1 and 3 to the common side of the 24 volt transformer (fig. 4). The motor should run. Continued on Next Slide…

  44. Fig. 5 Fig. 6 ECM Blower Motor Quick Test Continued… If the motor does not run, unplug the 16 pin wiring harness from the motor (fig. 5). Put 24 volts to pins 12 and 15 and 24 volt common to pins 1 and 3 at the motor (fig. 6). If the motor starts, the fault is in the harness. If the motor does not run, replace the motor module.

  45. LitePort Furnace Control Information Control Response RED LED Fault Display Past IFC boards lost all fault information once power was removed to the furnace. New production LitePort equipped boards will retain fault information after power has been removed. If power is interrupted to a furnace that has detected faults, the faults will be retained in memory. When power is restored to the furnace, the last four faults in the IFC board memory will be displayed on the Red LED. The faults are displayed in order last in first out. The LED will not display these faults again, until power is removed and then restored. If a fault is present, the LED will display that fault only. The new LitePort series IFC boards store event history in memory. The event history can be recovered by using the LitePort optical coupler and related software. • Some of the events stored in IFC board memory include: • Number of recycles. • Low flame current events. • Line voltage irregularities. • Pressure switch faults. • Limit circuit faults. • Calls for heat over a period of time. • Number of calls for cooling over a period of time. • Safety trips.

  46. LitePort Furnace Control Information • Why the RED diagnostic LED flashes every so often. • The furnace control board communicates diagnostic information at least every 20 seconds. The information is contained in the light emitted by the RED LED. This information can be read with a LitePort optical coupler system. • If information is being read by the optical coupler, the information could be delayed by as much as 60 seconds from the actual time the event occurred. If the IFC gathers information during the transmit period, the information you read may be delayed for a very short period of time. • How to clear out the fault history • Remove power to the furnace. • Turn heat and cool demand off at the thermostat. • Turn fan mode to ON mode. Not auto. (At thermostat) (OR JUMPER R to G at Furnace Board.) • Turn the power back on. • Jumper the following low voltage connections at the IFC thermostat terminal strip within 8 seconds of power up: • W1 on and then OFF three consecutive times. ( R to W1 at the low voltage terminal on the furnace IFC.) • Control will blink the red and green LED for 2 seconds to confirm fault history cleared.

  47. LitePort Furnace Control Information • To clear event history. • Turn power off. • Fan to ON position at the thermostat. (OR JUMPER R to G at the furnace control board.) • At the thermostat turn heat and cool demands off. • Turn the power back on. • Jumper the following low voltage connections at the IFC thermostat terminal strip within 8 seconds of power up: • R to W1 On and then off. • R to Y On then off. • R to W1 On then off. • Red and Green LED’s will blink for 2 seconds to confirm erase of history.

More Related