It’s a cold winter evening. You’re just from work and, because of the freezing conditions, the first thing you do when you get to the house is to reach for the furnace switch.
However, the furnace won’t come ON. Maybe it blows cold air, i.e., the fan seems to run, but it’s not heating the house. Or, perhaps it doesn’t run altogether. Additionally, you’ve noticed that it flashes three times – as if to warn you.
What could be wrong, and how can you fix it? Also, can you fix it yourself, or do you need to call the pros? Let’s find out.
If your furnace flashes three times, it’s a sign that it has an issue within the pressure switch circuit. This applies to nearly all furnaces unless the manufacturer says otherwise. It’s a sign that the pressure switch is open when it should be closed.
What’s the Pressure Switch and What Does it Do?
The pressure switch is a specialized circuit built into the furnace to protect you and your family from the backdraft of exhaust gases.
You see, during combustion, the furnace burns fuel (natural gas, propane, etc.) and oxygen, producing several byproducts, some of which are poisonous gases. Carbon dioxide, carbon monoxide, nitrogen dioxide, and sulfur dioxide are just a few examples. Pretty much all these gases are poisonous in large quantities. They can cause shortness of breath and even asphyxiation.
Furnaces deploy a draft inducer motor/fan to remove these gases from the furnace and outside the house.
However, there must be a mechanism to verify that the draft inducer is working perfectly, right? Otherwise, you’d be banking on guesswork. That’s where the pressure switch comes in handy.
The pressure switch is designed to only close (to complete the furnace circuitry) if the draft inducer fan removes exhaust gases at a pre-determined level. Below a certain level, or if the fan isn’t working at all, there won’t be enough negative pressure within the draft inducer box to close the switch. Thus, it remains open.
What Causes an Open Furnace Pressure Switch?
The underlying causes of an open pressure switch vary widely, from obstructions in the flue to a defective switch. The following are the main culprits;
A clogged port: The pressure switch has a port to help hold the connecting hose sturdily. This port also helps ensure a leak-proof connection to prevent loss of gas and gas poisoning. Unfortunately, the port is exposed to all kinds of debris and dust, which can clog the area, partially or fully blocking it over time.
An obstructed flue pipe: As we mentioned earlier, the draft inducer must clear exhaust gases from the furnace before the pressure switch can close. Furnaces use flue pipes to direct the poisonous gases out. However, imagine if the flues become blocked. This isn’t uncommon. For instance, birds very often nest in furnace flues during the summer. So, your flues may be partially blocked at the onset of the heating season.
A stuck or ruptured diaphragm: The pressure switch relies on a unique diaphragm to suck the lever that ultimately closes the relevant circuitry. When the furnace is ON, and the draft inducer motor is working fine, the diaphragm is sucked inward due to the negative pressure within the switch. Now, imagine if you’re dealing with a stuck or raptured diaphragm! It wouldn’t respond to pressure changes.
The pressure switch hose is damaged: The pressure switch connects to the draft inducer motor housing via a hose that transfers the pressure values from the motor housing to the switch. If this hose is compromised, it may fail to relay current pressure readings.
For instance, if the hose is cut or doesn’t fit snugly, it may suck in air from the surrounding space to fill the vacuum inside. This would lead to high pressure, thus keep the switch open.
How to Troubleshoot an Open Pressure Switch
Unfortunately, it’s impossible to pinpoint the exact problem until you perform further tests. Therefore, if your furnace flashes three times, it may be helpful to switch it off for troubleshooting.
- Switch off power and gas: Never work on your furnace with the power on and gas supply running. It’s too risky. So, first, locate the electrical switch and flip it off. Then, find the gas valve and turn it off too. The valve is OFF when the lever is perpendicular (rather than parallel) to the gas pipe.
- Inspect the chimney flue: Look up the chimney to make sure there’s no blockage. If it’s daytime, you should see sunlight at the other end. Otherwise, there’s a blockage. Fix the issue, then move to the next step.
- Remove the front panel: The pressure switch is located behind the front panel. To access it, unscrew the panel and put it aside
- Locate the pressure switch: A pressure switch is a disc-shaped object with a black, flexible tubing that connects to the draft inducer blower. Standard ACs have just one tube. However, condenser models have two. You’ll also notice that the switch has two wires connecting to it.
- Inspect the tubing: Disconnect it at the blower end first. Then trace it to the pressure switch and disconnect it on this end too. Holding the hose in your hand, blow through it to make sure it’s not blocked. Also, inspect it for cuts and general damage. If it’s okay, great. Otherwise, replace it.
- Inspect the port: The port where the hose connects to the pressure switch is notorious for collecting dirt and debris. With the hose still off, inspect it too. You can use a small wire to clear the gas hole and surrounding area to ensure a firm connection with the hose. If everything is great, put back then hose.
- Inspect the wiring: Do you have any loose connections? Are some of the wires naked, thus creating the risk of short-circuiting? If they are okay, then great. Leave them in place. Otherwise, it would be best if you replaced the wires. You need an electrician for this process.
- Try to light the furnace again: If it was a hose, wiring, or switch port issue, the furnace should light without further issues. However, if your furnace still produces the three flashing LED lights, it’s most likely that your pressure switch has malfunctioned.
How to Bypass the Pressure Switch
If you’ve concluded that your pressure switch is likely damaged, you can confirm it by jumping the switch. Bypassing the switch can also offer a temporary fix as you wait for a more permanent solution.
To bypass the switch, shut down the furnace and open the front panel to access the switch. Then, pull out the two electric wires connecting to the switch and touch the wires directly. If the three flashing lights are gone, you’ve just confirmed that the pressure switch is defective.
How to Replace the Pressure Switch
The best solution to a malfunctioned pressure switch, however, is to replace it. First, purchase a replacement, ideally an OEM product, from a licensed dealer. Then, remove the old one and install the new unit.
Furnace pressure switches from around $6 to $150, depending on the furnace model and type of switch. Meanwhile, professional installation costs from $60 to $150.
That’s all. Now you know where to look and what to do when your furnace LED flashes three times. But, again, we strongly recommend professional servicing and installation in case you need to replace the switch.