cfm Tech Tips – Operating and Troubleshooting Furnace Pressure Switches

Beginning January 1, 1992, all furnaces were manufactured with an integrated furnace control board, often referred to as FCB (Furnace Control Board). The FCB manages all operation requirements of the furnace. Fault and Information codes are displayed by one or more LEDs on the board. The FCBs used by York/JCI have the ability to store and retain the last 5 codes in ‘non-volatile memory’ – meaning that loss of power to the control doesn’t erase the stored codes – a very helpful tool when troubleshooting furnaces. This month’s topic is Pressure Switch Operation and Troubleshooting.

Pressure Switch Fault Codes:

2 Red Flashes – Pressure Switch is closed when it should be open

3 Red Flashes – Pressure Switch is open when it should be closed

6 Red Flashes – Pressure Switch has opened 4 times during a heating cycle

All furnaces now utilize ‘induced draft combustion’, meaning that every furnace has an inducer assembly to draw combustion air through the heat exchanger. To ensure that the inducer is operating, and moving the correct amount of air, a pressure switch is selected, for each furnace size, by the design engineers to monitor inducer operation. Pressure switches typically have the Pressure Setting (Pressure Measured in Inches of Water Column) stamped on the body of the switch. The Switch Setting is the predetermined point that the switch must open if combustion air falls too low.

Pressure switches have a ‘closing pressure’ – not listed on the switch body, but equal to about 0.15” greater than the pressure stated on the switch. This means that a pressure switch rated at 0.80” will not close until approximately 0.95” of pressure is applied. Once closed, it will remain closed until pressure falls below 0.80”. This is known as Hysteresis or Differential. Pressure switches also have a ‘tolerance’ rating of approximately 10%, so the 0.80” switch could open at 0.72” or 0.88” and be considered a correctly operating switch.

Furnace Sequence of Operation related to Pressure Switch Operation:

1. Call for heat from the thermostat.
2. FCB checks the Pressure Switch to make sure it’s open.
   1. If not, the FCB immediately stops the sequence of operation and flashes a Red Code 2.
3. FCB starts the inducer.
4. FCB checks the Pressure Switch to make sure it closed.
   1. If not, the FCB immediately stops the sequence of operation and flashes a Red Code 3.
5. FCB continues the sequence of operation through the current heating cycle while monitoring the Pressure Switch, making sure it remains closed.
6. If the Pressure Switch opens during the heating cycle, the heating cycle is interrupted and a Red Code 3 is flashed until the FCB repeats the above checks and restarts burner operation.
7. If the Pressure Switch opens 4 times during a heating cycle, the FCB shuts down the burners and flashes a Red Code 6.

If an FCB is flashing a Pressure Switch fault code, it doesn’t mean that the pressure switch has failed. It simply means that the pressure switch is giving us valuable troubleshooting information. Don’t replace the pressure switch until the troubleshooting procedures indicate a failed switch. Troubleshooting a pressure switch will require two accurate test instruments that allow you to see what the switch is seeing.

• A VOM (Volt, Ohm, Milliammeter) to check voltage and/or continuity across the switch contacts.
• An accurate method of measuring inches of water column pressure in ‘tenths of an inch’. See examples below.
  • Digital Manometer
    • Available at a reasonable cost. Every Technician should own one.
  • Magnehelic Gauge
  • Incline Manometer

IE: You can’t troubleshoot a pressure switch problem without –

• A tool to see the pressure that the switch is seeing
• A tool to see the voltage/current that the switch is seeing.

Because pressure switches must monitor locations of extra high humidity – such as the condensate pan area of a condensing furnace – there is a risk of condensation collecting in the body of the pressure switch. To keep this from happening, a ‘bleed hole’ is designed into the body of the switch to allow dry air to bleed through the hole and travel through the connecting tube of the switch, keeping the switch body and tube dry. This means that if pressure is applied to the switch and then the connection is sealed off – such as pinching off the tube – because of the bleed hole, the switch will not remain closed. This is normal switch operation and doesn’t indicate a failed switch. So, if there is condensation in the body of the switch, it could indicate a plugged or missing bleed port. Because condensation in contact with the diaphragm of the pressure switch assembly nearly always results in incorrect operation, any switch with water in it should be replaced.

When troubleshooting Pressure Switches, it’s important to determine the pressure being applied to the switch during the time it is monitoring inducer operation. Because of the bleed hole in the switch body, a Technician must be able to connect his manometer into the switch tubing using a tee. If the manometer is inserted in place of the pressure switch – without the tee – it may lead a Technician astray.

Example: A pressure switch is monitoring pressure applied to a furnace condensate pan. There is corrosion in the connection of the condensate pan restricting the flow of air through it. The hole in the pan connection – because of the corrosion – is now smaller than the bleed hole of the pressure switch, resulting in a Red Code 3. Pressure inside the condensate pan is correct, but because of the corrosion in

the pan/tube connection and the bleed hole in the switch, when the switch is in place not enough pressure gets applied to it (more air is entering the bleed hole than can pass through the restricted port in the condensate pan), but when the manometer is applied directly to the condensate pan – without the bleed hole in play – pressure appears to be correct. This is not the fault of the switch – it is the fault of the corrosion in the condensate pan connection. A problem like this can only be diagnosed with an accurate manometer and would be missed if the tee (keeping the switch in the tube circuit) was not utilized.

Troubleshooting steps to resolve Fault 2.

• Is the inducer operating?
• Yes – If the inducer relay contacts are stuck closed, keeping the inducer on when it should be off, the pressure switch will be closed when it should be open.
• Troubleshoot the FCB and/or inducer relay to determine why the inducer is running when it should not be.
  • No – If the inducer is not running, the pressure switch contacts must be checked with an ohmmeter to determine if they are stuck closed.
    • • If so, it is a failed switch and must be replaced.
  • If not, troubleshoot the pressure switch wiring to determine if there is a short circuit, leading the FCB to believe the pressure switch is closed when it really isn’t.

Troubleshooting steps to resolve Fault 3.

• Is the inducer operating?
  • No – Troubleshoot the inducer and related components to determine why it’s not operating.
  • Yes – Insert a tee into the pressure switch tubing and use a manometer to determine if pressure applied to the switch is greater than switch setpoint + differential.
• If so, the switch must be replaced.
  • If not, troubleshoot the vent system and/or inducer to determine why pressure isn’t sufficient to make the switch.
    • • Remember the example above and check the tubing for cracks and tubing connection points for restrictions.
      • Obstructions commonly found in vent systems – Mud Dauber material, bird nest material, children’s toys, condensate buildup (caused by pipe sags), etc.

Important Note – As the gas/air mixture burns and the hotter the vent gasses get, the flue gas density is reduced, resulting in reduced ability of the inducer fan to build pressure. If a pressure switch is operating close to its setpoint, it could open simply as a consequence of the warm flue gasses. The vent pressure of any furnace should be well above the switch setpoint, so if the pressure switch is cycling per the above information, investigate the vent system and/or inducer assembly to ensure adequate vent pressure before combustion occurs. If this occurs on an 80% furnace, remove the inducer assembly and check for debris in the inducer housing.

Troubleshooting to resolve Fault 6.

1. Condensing Furnaces (90% to 98% efficient furnaces)
   • Check for low spots in the vent system that could be trapping condensate water. Water must flow back towards the furnace while the inducer is blowing air against water flow. This requires a minimum of ¼” slope per foot of pipe to ensure correct draining of the water in the vent.
   • If the vent pipe exits horizontally from the furnace cabinet; check for downward slope of the vent section between the inducer outlet and the first elbow. It is critical that all horizontal runs of the vent slope the necessary ¼” per foot back towards the furnace.
   • Check for debris in the condensate trap and line. A restricted condensate system can result in condensate buildup in the inducer assembly, resulting in pressure switch tripping during a long heating cycle.
     • • Particulate matter (ash material from burned bugs, dust and other airborne matter) can build up in the condensate trap of a condensing furnace resulting in slow flow of condensate.
       • Condensing Furnaces now have a ‘blocked condensate’ pressure switch wired in series with the operating pressure switch. Divide and Conquer to determine which switch is tripping.
   • Check for double-trapping.
     • • The built-in trap of the condensing furnace expects an open, sloping drain pipe to dispose of condensate water. Flexible tubing that is not lying flat on the floor can result in several traps in the line and cause condensate to back up into the furnace. All condensing furnaces should have a vent tee immediately outside the furnace to eliminate the air-locking that can occur with multiple traps.
   • Check for debris in the vent system.
     • • Obstructions commonly found in vent systems – Mud Dauber material, bird nest material, children’s toys, etc.
2. 80% Furnaces

• Nearly all Fault 6 problems can be traced to debris in the inducer housing below the inducer wheel, or a restricted vent system.
  • Temporarily remove the vent from the furnace. Operate the furnace to observe if the pressure switch opens without the vent system in place.
    • • If furnace works correctly, investigate the vent system.
      • If pressure switch cycles without vent attached, investigate the inducer assembly.
        • Items often found in the inducer housing – Bird Nest material, Mud Dauber material, sheet metal screws, etc.

Note: Debris in the inducer housing of 80% furnaces often results in a Code 7 or Code 8, not a Code 6 which can mislead a technician. This part will be covered in next month’s Tech Tips Letter.

Coming next month: Fault Codes 7, and 8 – Failure to prove flame, loss of flame after proof.

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Paul Flora

Service and Training Manager at cfm Distributors, Inc.

Paul has been with cfm Distributors, Inc. over 20 years and is currently the Service and Training Manager.His experience in the HVAC industry provides a high level of technical experience and knowledge for our customers.

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