Is your furnace trying to start but nothing more happens? Is your furnace blowing cold air? The problem could be your HVAC pressure switch. Although this might sound like a job for a pro and diagnosing the problem can get tricky for the average DIYer, it is possible to do so by following a few simple steps and using the proper tools.
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Gathering the Tools
Most of the tools required to diagnose this situation are basic. One tool that might not be so basic is a multimeter. A good multimeter is one of an HVAC pro's most important tools. Knowing how to use it is just as important as having one.
The tools typically used to diagnose an HVAC pressure switch include:
The Role of a Pressure Switch
The pressure switch is a safety device. It assures that the draft inducer or combustion blower is actually running to create proper combustion. Without this switch making a connection based on either positive or negative pressure, it will not allow the combustion process to start. In turn, the furnace will either blow cold air or give a fault code to the control. Think of it as a switch activated by air pressure that proves the combustion blower or draft inducer is working.
Pressure switches are specific to each application. If you find that your pressure switch is bad, be sure to get the right one for your furnace. There are universal pressure switches that have to be adjusted to the right pressure for safe operation.
Locating the Pressure Switch
First, you need to open the furnace with either a screwdriver or the proper size nut driver to expose the burner assembly. If you look around, you will see something that looks like a snail attached to the discharge, venting outside. This is the draft inducer. Off that snail configuration, there will typically be one or two small 1/4-inch hoses that lead to the pressure switch.
Diagnosing the Pressure Switch Using Ohms
Disconnect power at the service switch. Set your multimeter to ohms, indicated by the Greek omega symbol. Disconnect the two wires on the pressure switch and secure them so they do not short out on anything. Turn the power at the service switch back on.
Once the combustion process begins (you will know this by the start-up of the draft inducer), put the two leads from your multimeter on the two terminals you took the wires off and see if the switch makes contact with no resistance. Many multimeters will beep. If it beeps, then your switch is good. If not, the switch is bad and needs to be replaced. If the switch is good, reattach the wires you disconnected exactly where they came from.
How the Pros Do It
An HVAC pro may test the switch in different ways based on their training. Two common methods are voltage tests.
In one method, the technician resets the control by turning the service switch off and then back on. This will clear any faults given to the control by the safety circuit. The draft inducer will start up, initiating the combustion process. The technician sets a multimeter to AC volts and examines the wiring where the thermostat ties into the pressure switch, identifying the "C," or common (negative) terminal. Once a good negative or common has been established, the technician checks both sides of the switch to see if the 24 volts are going through the switch.
A pro will do this to ensure there is voltage in the control circuit, specifically to the component being tested. These circuits are typically wired in series. A series circuit is wired in such a way that anything in the series will stop the flow of power back to the ignition control. When the pro establishes anywhere from 24 to 28 VAC, then the switch is good.
Using another type of voltage test, a technician will cycle the power to reset the control; then (much like the resistance method mentioned above) the pro will check the switch by using the multimeter leads on both sides of the switch with the power on and wires attached to the terminals. When the combustion process starts, the pro will read the voltage to be 24 to 28 VAC, but when the switch makes contact, it will read 0. This proves that the switch is activated and is good.
Sean Damm, Licensed HVAC contractor
Sean Damm has been a licensed heating and cooling contractor since 2001 as well as a HVAC instructor for 15 years. He has also served as a technical advisor to contractors, business owners and homeowners specializing in many facets of the HVAC industry throughout the country.