What You Should Know about Furnace Pressure Switches and Testing Them for Failure
Your residential natural gas furnace relies upon a variety of electronic controls and switches to function safely and efficiently. One such component is the pressure switch, and if there is something wrong with the switch, then the furnace won't operate. Below is more information on gas furnace pressure switches as well as how you can test your switch for proper operation:
What you should know about furnace pressure switches
Pressure switches prevent operation of the furnace if air pressure inside the vent fails to fall within a predetermined range. This function serves primarily as a safety measure to keep dangerous combustion gases from passing into living spaces; if the pressure is too high, it can be indicative of a failure to evacuate these gases at an adequate rate.
Depending on the specific manufacturer and model of your furnace, the pressure switch range of operation will vary from unit to unit. However, pressure switches typically have their range of operation printed on an attached label; the pressure scale used is "inches of water column", or Inch WC. For reference, 27.7 inch WC is equivalent to 1 pound-per-square inch (PSI).
It is important for do-it-yourselfers to note that proper vent pressure is critical for safe operation and that attempting to override a pressure switch is dangerous and could damage the unit, as well. That is why testing your system pressure is a key part of evaluating whether the pressure switch has actually failed or is indicating airflow problems inside the furnace.
Evaluate your furnace for potential pressure switch problems
Fortunately, evaluating your pressure switch isn't difficult, as the furnace itself will provide diagnostic feedback should the furnace fail to start or quit running. Gas furnaces utilize a flashing LED that blinks in particular sequences or color variations, and this is where you need to check first.
To perform a check, remove the access panel to your furnace and locate the LED. It can also be viewed through a sight glass, but the inside of the panel contains a key for reading the blinking light pattern. Once you find the light, watch it carefully for a minute or so to determine its pattern of blinking. Look for both short and long flashes, and also make note of any color changes. Next, compare the flash pattern to the key on the panel, and it will provide an explanation as to why your furnace failed to start or if it quit. If the cause indicated is pressure switch failure, then proceed to the next step to verify the switch is operating correctly.
Test the pressure switch
After checking for a possible pressure switch failure, turn off the power to the furnace to prevent possible electrical shock. Next, locate the pressure switch inside the furnace; it will be round and contain two wire connections and at least one attached air hose. Carefully pull the air hose free from the nipple on the switch and remove the connected wires.
Set a multimeter to its resistance setting, so it measures the number of Ohms of electrical resistance. Pair each electrical terminal with a lead from the multimeter, then observe the reading on the multimeter scale. Pressure switches with faulty internal electrical connections will show a reading of infinity, which may be designated as 1 on your multimeter. Good switches show no resistance at zero on the scale.
Replace a bad switch
Should the multimeter indicate the pressure switch is bad, then your next step is to replace the switch with a new unit. Be certain that you obtain an exact manufacturer's part for the replacement; never use switches manufactured for generic use or those with another pressure range than that on the original. Unapproved parts such as these can void your furnace warranty or lead to unsafe operating conditions. If you are unable to locate a suitable part, then contact your local heating repair specialist for assistance in performing the furnace repair.