Troubleshooting a Furnace Pressure Switch (5 Easy Things to Check)
Hey guys, today we’re going to talk about Troubleshooting a Furnace Pressure Switch.
I wanted to expand on our recent gas furnace troubleshooting series by going into each
part of the sequence of operation of a furnace. In this video I’ll fill you in
on what the pressure switch does and why it’s important. And towards the end of the video,
I’m going to give you 5 easy things to check when you’re troubleshooting a furnace pressure switch.
That's coming up here on Fox Family Heating & Air.
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The furnace sequence of events
First, as a technician, you have to know the sequence of
events that occurs for a gas furnace to start up properly. It's really straightforward,
and you should have this memorized before you can even consider being qualified to troubleshoot.
Power to the furnace control board
Thermostat signals the call for heat
Inducer motor kicks on
Pressure switch proves the inducer operates correctly
Ignitor activates
Gas valve energizes
Flame pours across burners
Flame sensor proves all burners are lit
The blower 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 that creates 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 up, the pressure switch activates
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 that 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
really 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 heating season, so this happens to alert the occupant that 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
Lets 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 on to the pressure switch.
Place one lead on 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 begin 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, say,
-1" wc. It could be less, it could be more, but it can't be less than the number on the pressure
switch. Meaning, 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 I've seen pressure switches either fail
or not close to allow the rest of the system fire up are:
A clogged port on the collection chamber to the pressure switch.
An obstruction in the flue
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.
On the collection chamber, check to see if the port itself is clear of any calcium deposits,
dirt, or other debris that would prevent air from flowing through the port. If there is,
take a small wire like some thermostat wire and clean that port out. 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. I have found that 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 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 that the nest (or the bird)
has fallen all the way 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, and that's great! But your switch is on borrowed time,
and 9 times out 10, the switch will fail again. If I tell you it will fail tomorrow, it will last
for years. If I tell you it will last for years, it’ll fail tomorrow. Personally, I would recommend
trying to convince the customer to replace the pressure switch now so there’s no surprises.
One way to see if the pressure switch is stuck open or closed is to breathe lightly into the hose
leading to the switch. You'll hear the diaphragm open and close. It doesn't mean the switch will
work properly but it gives you more information to troubleshoot a furnace pressure switch.
Because pressure these things are almost impossible to rebuild,
a new switch needs to be ordered.
A pressure switch hose is damaged
There have been diagnostic service calls that I've been on where I 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 in your service van 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 that
gas into moisture that flows back down the flue pipe and into the inducer motor assembly. Remember
that 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. One time,
I replaced a pressure switch with a universal and mounted it horizontally, parallel with the ground,
and the switch failed a few days later. Of course, it didn't happen when I tested it the
day I installed it, but another tech had to come out and fix the problem a few days later.
This could have been avoided had I read the section in the installation instructions that said
not to mount it in the horizontal position. I never did that again!
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—another
good reason to have extra hose on your van. 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.
Recap
So, just to recap. 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 that 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,
letting you know something is wrong with your furnace.
Be patient and check the things we went over today before condemning a pressure switch.
It could be one of a few things.
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Thanks so much for watching, and we'll see you on the next video.