Understanding Pressure Switch Fault Code 31

Contractors have likely seen the list of fault codes that are printed on the back of the furnace doors and in the installation manuals. These codes are used to troubleshoot common problems and help you find the reason your furnace isn’t working properly.

What is Fault Code 31?

The most common fault for condensing furnaces is code 31; code 31 means that a pressure switch did not open or close when it was supposed to. You may have up to four pressure switches on a given furnace, so it is not always easy to figure out which switch is causing the problem. To compound things sometimes one switch needs to open while another is closing. It is essential that you understand the sequence of operations that are listed in the installation manual so that you know how the switches are supposed to operate. Remember to look at the legend because HPS and LPS do not always mean High-Pressure Switch and Low-Pressure Switch. Next, you should look at the wiring diagram to see how the switches are wired together.

The newest versions of Bryant furnaces have a three-digit display that will more precisely identify pressure switch issues, so your troubleshooting should be easier.

Identifying Pressure Switch Problems

A pressure switch relay has been incorporated into some of the circuit boards that take the HPS pressure switch out of the control circuit for 60 seconds after the burner lights; this is to give time for the flames to settle down so that the pressure is consistent. If you find that the pressure switch consistently trips at exactly 60 seconds, you may have a bad circuit board.

Many times, pressure switch problems are caused by improperly run vent pipes, so confirm that the vent pipes are run correctly. Confirm the vent sizing using the smallest diameter PVC vent pipe allowed for the BTU input. Remember to add the equivalent length of pipe/elbow or fitting x the number of fittings to your calculation. Do not count any fittings inside the furnace or the vent terminations outside the house. Check for the proper ¼”/foot of slope back to the furnace, and make sure your intake and exhaust pipes are supported according to the Hanger Spacing Table in the installation instructions.

Consult the installation manual regarding minimum and maximum vent lengths and if you require an inlet or outlet choke. Prime the trap per the installation instructions, and make sure that the drain removes all the excess water. The drain line should also have a ¼”/foot of slope back to the drain or pump so that no water is left standing in it. It is essential that there is no back pressure on the drain or vent pipes from standing water, which could affect the pressure switches. Avoid short horizontal vent lengths. It is better for drainage if you use two 45’s and a short, angled piece of pipe. It is also easier to run a cleaning snake through 45’s rather than 90’s.

The way the furnace condensate drain is run is a huge cause of pressure switch problems. Ideally, the furnace condensate is drained separately from the A/C condensate because air pressure from the evaporator coil can backfeed into the furnace drain and affect the pressure switches. You can use a common drain if the following conditions are met. You need an air gap between the A/C condensate line and the common drain pipe. You need to have an open Tee, not an elbow, where the A/C condensate line turns to go down to the common drain. A drain trap is not necessary on the coil of an up-flow furnace; a drain trap is only needed if the drain is upstream of the blower, like it is on a rooftop unit or blower coil.

Make sure that the furnace is level or slightly pitched forward to keep water from pooling at the back of the secondary heat exchanger, not only will it prolong the life of the heat exchanger, but it can keep your pressure switch from tripping.

Make sure there is no air leakage between the vestibule and the blower compartment, which could cause negative pressure on the pressure switches. Don’t drill holes through the blower deck to run tubes or wires, and if you find that someone else has, seal them up with silicone.

If the furnace is set up for LP gas, make sure that the Low Inlet Gas Pressure Switch (LGPS) is closed and that there is enough gas pressure in the tank to operate the furnace.

If the furnace has been in for a while and works properly with the burner door off but not with the burner door on, you may have an obstruction in the intake pipe or screen before the burner. To see if the intake vent is obstructed you can go outside and place a golf ball in the intake pipe and see if it rolls all the way back to the furnace. Check for approved vent termination per the Installation Manual. Anything else could result in ice or snow buildup, choking the intake when the temperature is near zero. The termination below has frosted over because the exhaust was not goose necked above the intake.

Be sure to use the smallest diameter pipe allowed for your installation; oversized pipe diameters can lead to the exhaust velocity being too slow and the exhaust temperature being too cold, which can lead to freezing at the exhaust termination of smaller size furnaces. Ice or snow in the exhaust or intake pipes will trip the pressure switches.

A pressure switch fault in the fall or spring could be caused by an insect nest or leaves especially if the vent terminations are located on an inside corner of the house. You are not supposed to have a vent termination within 3 feet of an inside corner.

To see if the exhaust vent is obstructed, you must first disconnect the exhaust vent pipe from the furnace and put a bag or bucket over the end to catch the golf ball when it comes out. If the golf ball comes out covered in water or has debris clinging to it, you probably have something in the vent pipe that needs to come out.

Another reason that you can have pressure switch problems is if you have disconnected, cracked, or obstructed pressure tubes. Remove the pressure tubing from the pressure switch and see if you can blow air through the tubing. Very often clearing the obstruction is all that is needed to restore the furnace to operation. 80% efficient furnaces often plug their inducer pressure fitting with mineral deposits from the chimney. 80% furnaces also burn up the end of the pressure tube that connects to the inducer, so cut off the end of the pressure tube and reattach it to the fitting or replace the tubing with high temperature pressure tubing.

Be sure that when you replace a pressure switch you don’t miss wire it; transfer one wire at a time so that you don’t make a mistake. Taking a picture before you start is also a good idea, especially when dealing with multiple switches.

You can disconnect the pressure switch from its tubing and shake it above your hand with the pressure fittings pointed down, and if water comes out, it has a perforated diaphragm, replace the switch.

Low inducer voltage, a bad inducer capacitor, or a defective inducer can cause the inducer to run slowly, which will result in a pressure switch trip.

If there is excessive wind, especially with a one-pipe system, the pressure switch may trip, so be careful with your vent design. Our newest furnaces have an optional inducer boost that can help with this situation; see inducer boost in the installation instructions.

Methods to Determine the Pressure Switch Is Working Properly

Electrically testing pressure switches is done in many ways. You can use one or all of the methods to determine if your pressure switch is working properly.

Method 1. With the power off, set your meter to AC volts and put your probes across the switch terminals. Turn the power on and you should read 24 volts until the switch closes, then you should read zero.

Method 2. With the power off, remove the wires from the switch, set your meter to continuity and turn the power on.  There should be no tone at first, then after the inducer comes up to speed you should hear a tone which means that the switch has closed.

Method 3. The best and fastest way to troubleshoot pressure switches is with a two-channel digital manometer.  You look on the back of the switch and find the brake pressure of the switch, then you tee into the pressure tube and turn on the furnace.  Compare the reading on the meter to the pressure switch tag, is there enough vacuum pressure to close the switch?   If there is enough vacuum pressure and it doesn’t close the switch is bad, replace the switch. Remember that the make point of the pressure switch is approximately 0.15” WC above the break point.

There are advanced dual port manometers like the one pictured below that incorporate an adjustable variable pressure pump and test leads for testing pressure switches as well as checking gas pressure and static pressure. With this kind of a meter, you can read the back of the switch to find the break point, add 0.15” WC to know the make point and then use the built-in pump to slowly increase the vacuum and confirm that the switch is working properly. The test leads can be hooked up to the pressure switch terminals and an LED will light to show when the switch makes and breaks. This meter also shows the actual vacuum pressure on the digital readout.

Contact the B-Y customer assurance team with any questions on pressure switch faults.