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.
If this is your first time watching our channel, please click subscribe down here on the
bottom right. And if you click that little bell next to it, you'll be notified
of all our videos as they come out. 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.
If this is your first time watching our channel, please click subscribe down here on the
bottom right. And if you click that little bell next to it, you'll be notified
of all our videos as they come out. Thanks so much for watching, and
we'll see you on the next video..
