There are 5 things to check for a furnace pressure switch.
I want to make it clear that anyone watching this is not an experienced technician.The video is only for educational purposes.Fox Family Heating & Air does not recommend anyone other than a professional to open the furnace and try to diagnose the problem.
A person can be shocked by high and low voltages.There are a lot of moving parts that can damage hands and fingers.Severe burns can be caused by the hot surfaces within the furnace compartments and around the housing.A flame produced by the start-up of a gas furnace can cause serious burns to a person or property.
You need to know the sequence of events for a gas furnace to start properly.You should memorize this before you even consider being qualified to fix it.
The inducer motor is the first thing you should see when a furnace begins a new cycle.The control board has wires coming from it.The inducer motor can be started for up to 60 seconds.It is a safety feature that creates a negative pressure or draft which purges the heat exchanger of any poisonous gasses.When the flame kicks on, it makes the air inside the hollow tubes of the heat exchanger cleaner.The efficiency of the furnace increases when we have cleaner air in the heat exchanger.
A safety device called a pressure switch is activated when the inducer motor is in motion.Normally open, the pressure switch closes upon the manufacturer's specifications for required negative pressure.
The inducer should turn on if it is working normally.There is no time lag on this.This draft is created quickly by the inducer motor.In most cases, the rest of the furnace starts up within 5 to 7 seconds.
The furnace will shut down if the pressure switch doesn't work.The control board will stop sending voltage to the inducer motor if the pressure switch doesn't close after 5 tries.
When the furnace's fan pushes room temperature air through the ducts and into your rooms, the system is on a safety lock.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 fix it.
The inducer motor should be running correctly.The pressure switch is not closing.The control board, safeties, and pressure switch can be traced using your meter.Attach a piece of metal to the furnace.The other should be on the switch's incoming terminal.You can assume the pressure switch has not closed if you have 24 volts on the incoming terminal but not at the terminal leaving the switch.
The inducer motor has to come on before the pressure switch can close.The pressure switch has a requirement listed.
We can fix a furnace pressure switch.The manometer can be used to make sure the inducer motor is creating the vacuum by hooking up the meter's hose to the pressure switch tubing.Put the pressure switch hose on the same port as the manometer hose.
The manometer should start reading the draft when the inducer comes on after you put the hose on the system.The number on the meter is more important than the pressure switch number.Suppose you are testing a pressure switch that closes at 0.7 inches.The reading on the meter should be around -1′′ wc.It can't be less than the number on the pressure switch.Something is causing a low pressure reading.
There are some things that could be going on here.I have seen furnace pressure switches fail or not close enough to allow the rest of the system to fire up.
If the port is clear of calcium deposits, dirt, or other debris, you can check the collection chamber.Take a small wire and clean the port out.The port should be clear if the substance is brittle enough to be removed.
The inducer motor causes a draft to allow gasses to be drawn out of the heat exchanger and into the flue pipe outside the building.Birds and bees like to build their nest in and around the flue pipe.It can happen over the summer, but not during the winter.If the furnace hasn't been run yet and the season is early, it's a good idea to check the roof vent.The nest falls all the way down to the base of the pipe, where it meets the furnace.
Sometimes the pressure switch fails because of a stuck or rupturing diaphragm.Ruptured diaphragms can make noise.The required draft won't budge on the stuck diaphragms.Sometimes you can free the stuck part with a little tap on the flat part.Nine times out of 10, the switch will fail because it is on borrowed time.It will last for a long time if I tell you it will fail tomorrow.It will fail tomorrow if I tell you it will last for years.There are no surprises if the customer is persuaded to replace the pressure switch now.
One way to see if the pressure switch is open or closed is to breathe lightly into the hose leading to the switch.The diaphragm is open and close.It doesn't mean the switch will work perfectly, but it will give you more information to fix it.
There have been diagnostic service calls where the port was clean, the flue pipe was clear, and the inducer motor was pulling a proper draft.The pressure switch wouldn't send 24 volts to the other terminal.Is the hose in good shape?Rats chew the hoses and leave holes.Other hoses can crack.If you have cases like this, keep some extra tubing in your van.
Water may be stuck inside the tubing to the pressure switch.The flue gasses are sent up the pipe, but the heat from the furnace will cause the flue pipe to become moist and into the inducer motor assembly.Remember that the water flows downward.The pressure switch won't close if there is a low spot in the hose.The hose should be unplugged from the port.There can be a lot of water in the hose, so be careful and have a bucket handy.
It is important that the pressure switch is mounted in the correct position during the installation.There was a reason the pressure switch was in a vertical position.When they have to fight gravity, diaphragms don't work as well.I replaced a pressure switch with a universal and mounted it on the ground, but it failed a few days later.It didn't happen when I tested it, but another tech had to fix it a few days later.
I could have avoided this if I had read the installation instructions that said not to mount it in the horizontal position.I never did that again.
Universals sometimes need to be creatively mounted, but they usually just screw back into their old spot.An extra hose on your van is a good reason to use a longer hose to get to the switch.The switch will fail again in a couple of days if there are no dips in the hose.