FurnaceFurnace Pressure Switch Troubleshooting (9 Problems and Its Fix)

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Furnace Pressure Switch Troubleshooting:
9 Things to try before calling a Professional

In order to optimize the lifespan of your home’s furnace and prevent toxic gas build-up, repairing the furnace pressure switch is an essential key when things go awry. In this article, we look at what the pressure switch does, the parts it works alongside and what to do when it is open and not operating correctly. The pressure switch failure to close can leave your furnace inoperable, and it’s easier to try the following steps before calling an HVAC professional if nothing else solves the furnace pressure switch error.

But First,
What is a Pressure Switch and What does it do?

A furnace pressure switch is a safety device which is designed to prevent the furnace from running if the venting air pressure is incorrect. It is put in place to detect the negative pressure created by the draft inducer motor when the furnace turns on and to shut down the furnace ignition if the air pressure isn’t adequate in removing the exhaust fumes. A round-shaped switch that is easily noticeable located near the draft inducer motor after you have removed the furnace’s front cover.

When a furnace starts a new cycle, the draft inducer motor starts when 120 volts is implemented to the control board. This creates a negative pressure which allows the heat exchanger to remove any poisonous gasses (such as carbon monoxide) or anything else that is present after combustion from the previous heating cycle. During combustion, the combustion blower creates negative air pressure between the inlet side of the combustion blower and the inside of the burner box of the furnace. Once the inducer motor begins its purging action, the diaphragm in the pressure switch is activated. The pressure switch is on until the required negative pressure threshold is met in which case it will turn off.

If the draft inducer motor is on and working correctly, the pressure switch should also turn on. After this process takes place, the furnace is kicking on and starting its cycle. If the pressure switch doesn’t activate, the furnace will shut down and try again. If the pressure switch doesn’t close after 3-5 reboots, the inducer motor will stop receiving power from the control board. If the furnace is running correctly, the pressure switch senses the accurate amount of negative pressure and remains open (off). However, if the vent pipe gets clogged or certain parts in the unit malfunctions, then the pressure switch senses that the negative pressure amount is off and it turns on which closes the circuit and shuts the furnace down. With the furnace switch stuck open or not functioning properly, there is potential for exhaust fumes and toxic gas to filter through the living spaces of your home if the furnace shuts down because of this. If the pressure switch is open with inducer on, we’ll have to test the pressure switch using a manometer.

Want to know more about Heat Pump vs. Furnace and what is right for your home?

Furnace Pressure Switch Problems

Maintaining your home’s furnace unit is an important part in not only saving you money in the long run but also ensuring you are doing what is most energy and cost efficient for your home. Some small ways to ensure that you’re doing this include reducing your home’s heating load, replacing the furnace air filter every 3 months, and making sure that nothing is blocking the air vents in your home. For homes with gas furnace’s, verify that the flue pipe leading to the outside is clear from any obstructions. Be sure to schedule an annual maintenance routine call with your local hvac company. For Sacramento residences, Alpha Mechanical is here for the call. If you’re in the business for a new furnace unit, our experienced technicians complete furnace installation in Sacramento and so much more.

A number of issues can occur that result in your furnace pressure switch not turning on:

  • Draft inducer motor failure
  • Ruptured/Stuck Diaphragm
  • Restricted intake air vent
  • Constricted combustion air vent
  • Leaks around assemblies
  • Condensate drainage blocked
  • Electrical failure of the pressure switch
  • A single-stage conventional furnace has one pressure switch with one hose running down to the draft inducer fan.
  • A single-stage condensing furnace has one pressure switch with one hose for sensing the pressure at the draft inducer/burner enclosure and the other for recognizing the right amount of venting pressure at the condensate collector box.

Furnace Pressure Switch Troubleshooting

Before getting started on how to fix a pressure switch on a furnace, some tools that you may need for troubleshooting include screwdrivers, flashlight, multimeter and a pressure switch hose (if it is noticeably damaged

  1. Turn off the power to your furnace and turn off gas to your furnace
    Switch off the disconnect switch located near the furnace and if you don’t see one, turn off the furnace’s breaker in the home’s breaker box (service panel).
  2. Inspect the Pressure Switch Hose
    Take the furnace doors off and inspect the hose(s) connected to the pressure switch to make sure it is connected at both ends and is in good condition. If the hose is not in standard condition, and all other moving parts are working, the pressure switch will not send 24 volts across to the other terminal when checked with a multimeter. If the hose is cracked, frayed, or old, replace it.
  3. Check for any Hindrance in the Hose
    Remove the pressure switch hose(s) after observing and keeping in mind where it is connected (if there is more than one, remove them one at a time). Observe whether or not the hose is obstructed on the inside; do not blow into the hose.
  4. Check the Hose Port
    Using your flashlight, look into the hose port to check that it is clear of any blockage. Remove debris with a small screwdriver or something similar but be careful not to hit the switch body as it could damage the pressure switch.
  5. Inspect the Furnace Cover
    Inspect the vent slots on the furnace cover and clean them if necessary.
  6. Look for Vent Blockage
    If you have a condensing furnace, examine the vent pipe for blockage. The vent pipe is a PVC pipe that usually extends out of the side of the wall or through the roof of the house.
  7. Check the Pressure Switch Body
    If the inducer will not come on, remove the wires off the pressure switch. If the inducer then comes on, the furnace pressure switch stuck closed. Tap lightly on the pressure switch body because this may open it back up and the unit may start. (This does not count as a repair or a fix, the pressure switch will still need to be replaced, this is a short term solution. If you are in need of a furnace pressure switch replacement in Sacramento, you can trust Alpha Mechanical for the job).
  8. Test the Switch for Resistance
    Another thing you can do if the pressure switch open fault, disconnect the wires to the pressure switch by pulling the wire connectors off of the switch terminals. Set the multimeter to ohms (testing for resistance). Touch each probe to one of the switch terminals. If the reading is anything higher than 0 or close to 0, the switch has failed and needs to be replaced by a qualified HVAC technician.
  9. Test the Furnace Operation
    Rejoin the wires to the pressure switch and put the furnace access panel back, turning on the furnace again to test if the furnace is functioning correctly. If the furnace still will not operate, the problem may be elsewhere and it is best to hire a certified HVAC professional to diagnose the issue.

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We specialize in maintenance, repair and replacement covering any HVAC problems that may come up in your home. Our experienced technicians will provide an upright overall cost of the job while letting you know what’s most cost efficient for your home’s system overtime. It is our top priority to leave you satisfied and comfortable in your home. We supply our customers with coupons and offers throughout the year and financing for your home’s next HVAC project so that you can be prepared even for the unknowns.

Final Thoughts

To recap on all of the information we covered in this article, if the pressure switch is not closing, your furnace will go into safety mode. Your pressure switch is a safety device that detects the inducer motor purging action and is activated through the parts diaphragm. After attempting to start again a few tries on its own, the unit will start to blow cool air through your vents, letting you, the homeowner know that something is off. Nothing is worse in the winter months of northern California than feeling cold air temperature blowing through your home when you were expecting to warm up. There are some easy minimal steps to try to restore your furnace pressure switch but when those troubleshooting steps do not succeed, it is time to call a professional.

Frequently Asked Questions

Frequently Asked Questions

What are the most common furnace pressure switch problems?

Electrical failure, draft inducer motor failure, blocked vents/drainage, leaks in the assemblies, etc.

What tools/materials do I need for furnace pressure switch troubleshooting?

Screwdriver, flashlight, multimeter, manometer, parts to be replaced if necessary.

What is the average cost for furnace pressure switch replacement?

The cost on average is $225-250 but the price may vary a little depending on your furnace. Alpha Mechanical is a company of integrity, notifying you of what you’re being charged and providing fair pricing and we also provide HVAC financing in Sacramento.

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