How to Diagnose a Bad Furnace Pressure Switch
There are 5 easy things to check when you are troubleshooting a furnace pressure switch. But before starting troubleshooting or assessing any parts of your furnace, you have to know the sequence of events that occurs for a gas furnace to start up properly.
It’s straightforward and you should have this memorized before you can even consider being qualified to troubleshoot.
There is a high and low voltage which can shock a person. There are also lots of moving components which can damage body parts. The furnace also produces hot surfaces within the furnace compartment and around the housing. This can cause severe burns.
An actual flame produced by the ignition and start-up of a gas furnace can cause severe burns and damages to the property.
Furnace’s Operation Sequence
- Power to the furnace control board.
- The thermostat signals the call for heat.
- The inducer motor kicks on.
- The pressure switch proves the inducer operates correctly.
- Ignitor activates.
- The gas valve energizes.
- Flame pours across burners.
- The flame sensor proves all burners are lit.
- The blower turns on and forces air through the ducts.
First, the inducer motor starts when a furnace begins a new cycle. The inducer motor is the first thing you should see kick on. 120 volts is applied through the wires coming from the control board.
This starts the inducer motor for up to 60 seconds before anything else even happens. It’s a safety feature creating a negative pressure or draft which purges the heat exchanger of any poisonous gasses, namely the byproducts of combustion.
It makes the air inside the hollow tubes of the heat exchanger cleaner when the flame kicks on. When we have cleaner air inside the heat exchanger at the time of combustion, the efficiency of the furnace increases.
Next, a safety device called a pressure switch activates when the diaphragm inside of it recognizes the suction or purging action of the inducer motor. The pressure switch is a normally open switch which closes upon the manufacturer’s specifications for required negative pressure.
If the inducer turns on and is working normally, the pressure switch should activate. There’s no time lag on this either. The inducer motor creates this draft pretty quickly within 5 to 7 seconds in most cases and the rest of the furnace starts up from there.
If the pressure switch doesn’t activate, the furnace will then shut down, wait a bit and try again. If the pressure switch doesn’t close after 3 to 5 tries, the control board will stop sending voltage to the inducer motor, essentially locking it out from attempting it anymore.
You can tell the system is on some sort of safety lockout when the furnace’s fan (or blower) pushes room temperature air through the ducts and into your rooms. No one likes cool air blowing into their house when it’s a heating season,
This happens to alert the occupant the system isn’t working right and they should call an HVAC company to come out and troubleshoot the system.
How to Troubleshoot a Furnace Pressure Switch
Let’s assume the inducer motor is running properly. But the pressure switch doesn’t seem to be closing. With your meter, you can trace the 24 volts coming from the control board, through the safeties, and onto the pressure switch.
Place one lead on the ground, or a solid piece of metal attached to the furnace. Place the other on the incoming terminal of the switch. If you have 24 volts on the incoming terminal, but not at the terminal leaving the switch, you can assume the pressure switch has not closed.
Another way of doing this is testing with your leads across the two terminals. When the pressure switch is open, your meter will read 24 volts. When the switch closes, you’ll read 0 volts.
Remember, the pressure switch doesn’t close until the inducer motor comes on and provides the necessary suction for the pressure switch to close. The required suction is listed on the pressure switch.
Pressure Switch Not Closing
When we’re troubleshooting a furnace pressure switch, we can do a few things. We can take our manometer and make sure the inducer motor is creating the vacuum by hooking up the meter’s hose directly to the collection chamber that the pressure switch tubing is connected.
Take that pressure switch hose off and put your manometer’s hose on the same port.
Once you put the hose on and start the system up, the inducer comes on, and the manometer should start reading the induced draft as it begins to rise. This number on the meter needs to be greater than the number on the pressure switch.
So, if you’re testing a pressure switch that closes at -0.7 inches water column, then the suction being read by the meter should be around, let’s say -1” water column. It could be less, it could be more but it can’t be less than the number on the pressure switch.
It means if you’re reading -0.4” wc, something is causing the pressure to be low.
5 Easy Things to Check
Some of the more common reasons of pressure switches either fail or not close to allow the rest of the system to fire up are:
- A clogged port on the collection chamber to the pressure switch.
- An obstruction in the flue pipe.
- A diaphragm that’s ruptured or stuck.
- The pressure switch hose is damaged.
- The pressure switch hose has water in it.
A clogged port on the collection chamber to the pressure switch.
check to see if the port itself is clear of any calcium deposits, dirt or other debris which would prevent air from flowing through the port. If there is, take a small wire like some thermostat wire and clean the port.
Whatever the substance is, it should be brittle enough to be scraped off allowing the port to become clear.
An obstruction in the flue pipe.
Remember the inducer motor causes a draft to allow the gasses to be drawn out of the heat exchanger and into the flue pipe where it terminates outside the building, usually the roof.
Bees, wasps and birds like to build their nests in and around the flue pipe. It’s not likely to happen during the winter but for sure it can happen over the summer.
So, if the season is early and the furnace hasn’t been run yet, it’s good to check at the roof vent. It’s also not uncommon to see the nest (or the bird) has fallen down the pipe to the base of it, where the pipe meets the furnace.
A diaphragm that’s ruptured or stuck.
Commonly, the pressure switch fails because the diaphragm inside the casing has become stuck or it has ruptured. Ruptured diaphragms can sometimes make a flapping noise. Stuck diaphragms just won’t budge on the required draft.
Sometimes, a little tap with your finger on the flat part of the casing will free the stuck part which is great but your switch is on borrowed time and 9 times out of 10, the switch will fail again. We recommend replacing the pressure switch now so there are no surprises.
One way to see if the pressure switch is stuck open or closed is to breathe light into the hose leading to the switch. You’ll hear the diaphragm open and close, but it doesn’t mean the switch will work properly.
It gives you more information to troubleshoot a furnace pressure switch. Because these things are almost impossible to rebuild, a new switch needs to be ordered.
A pressure switch hose is damaged.
If you could tell the port was clean, the flue pipe was clear, the inducer motor was pulling a proper draft and the diaphragm was functional, but the pressure switch would still not send 24 volts across to the other terminal.
Is the hose itself in good condition? Rats like to chew these hoses up and leave holes in them. Other hoses can become brittle and crack. Keep some extra tubing for cases like this.
A pressure switch hose has water in it.
Another thing that could be going on with the tubing to the pressure switch is water could be stuck inside it. Condensing furnaces send the flue gasses up the pipe but latent heat will turn the gas into moisture.
It flows back down the flue pipe and into the inducer motor assembly. Remember, naturally flowing water flows downward. If there is a low spot in the hose leading to the pressure switch, you’ll find it won’t close.
Try draining the hose by unplugging it from the port. Just be careful. There can be a lot of water in the hose so maybe have a bucket handy.
Installing a Furnace Pressure Switch
During the pressure switch installation, you want to make sure it’s mounted in the correct position. The pressure switch you took out was in a vertical position for a reason. Diaphragms, don’t activate as easily when they have to fight gravity.
OEM switches will usually just screw right back into their old spot, but universals sometimes need to be creatively mounted. This might mean you need to use a longer hose to get to the switch which is another good reason to have an extra hose.
And make sure there are no dips in the hose so water doesn’t accumulate causing the switch to fail again in a couple of days.
When a furnace begins a new cycle, the inducer motor is the first thing you should see kick on. A safety device called a pressure switch activates when the diaphragm inside of it recognizes the suction or purging action of the inducer motor.
The pressure switch is a normally open switch which closes upon the manufacturer’s specification’s required negative pressure. If the pressure switch closes normally, the rest of the furnace sequence of operation will continue.
If the pressure switch will not close, the system will go into safety mode, try a couple more times and eventually, just start blowing cold air in your room. It’s like the furnace letting you know something is wrong with your furnace.
Be patient and check the things we went over before condemning a pressure switch. It could be one of the said few things.
If this is your first time troubleshooting your furnace and you are at the point you are not sure what to do, please don’t take a risk and call the experts. Call us at Red Deer Heating and AC.