How to Diagnose a Bad Furnace Pressure Switch (5 Easy Things to Check in 2021)

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.

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.

pexels photo 3964704

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..

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