How To Replace Furnace Pressure Switch – Step-By-Step Guide

When the gas refuses to come on, users will often assume it’s an ignition problem or a failed gas pressure valve. True, these two are the most common reasons for furnace failure. However, they aren’t the only possible reasons.

The third most common reason, which surprisingly tends to go under the radar mainly because most people don’t understand it is the pressure switch.

The pressure switch directly controls the furnace’s ON/OFF operation. It can decide whether the furnace lights or remains off. Read on to find out more about gas furnace pressure switches, including what it does ad how to replace the pressure switch.

What is a Furnace Pressure Switch?

The furnace pressure switch is a pluck-shaped component found external to the furnace that controls the power from the gas source to the furnace.

It is an automatic relay system located near the motor that shuts the furnace down if it detects a negative pressure from the inducer motor. By so doing, the switch protects your home from dangerous backdraft, i.e., when exhaust fumes are reintroduced into the furnace system.

Additionally, the pressure switch guards against gas leaks and stops the furnace from running if there’s a mechanical failure.

How Does the Pressure Limit Switch Work?

As already mentioned, the pressure switch is a classic relay system. When one switch is closed, electricity passes into the furnace, allowing the unit to run. However, when the pressure switch is open, the current flow is suspended. Thus the furnace cannot run.

It’s also worth mentioning that the relay’s opening and closing happen automatically, controlled by pressure from the draft inducer fan. The fan connects directly to the switch via a visible rubber tube.

You’ll beware from your HVAC fundamental knowledge that the inducer fan is the first component that comes ON when you turn on the furnace. It begins to run even before the unit starts to produce heat. Indeed, in most furnace systems, the inducer fan runs a full minute before you experience the first wave of heat from the ducts.

When the inducer fan is turned on and running normally, it creates suction. It’s this suction that activates the furnace pressure switch. An activated switch completes the relay circuit, allowing current to flow through to the igniter, which lights the furnace.

If the switch isn’t receiving enough suction from the inducer fan, it will not turn ON. When this happens, the relay circuit will remain open, meaning the furnace won’t work.

How to Tell if your Pressure Switch is Bad?

If your furnace isn’t running and you think it’s the pressure switch, you need to perform a few tests to verify the problem. First, you need to find out if it’s a minor issue, such as a blockage, that you can fix yourself.

Here’s how to proceed;

Listen Out for the Gas Valve “Click.”

In a normally functioning furnace, you will hear a gas valve “click” within 10-30 seconds of igniting the furnace. This click signals that the gas valve is opening.

So, if you can hear the click, then the pressure switch is OK. In this case, if the furnace still won’t come on, you should troubleshoot the igniter. Otherwise, move to the next step.

Listen Out for Rattling in the Inducer Fan

As we mentioned earlier, the pressure switch’s operation is directly dependent on the health of the draft inducer fan. Therefore, a bad fan can easily cause the pressure switch to malfunction.

Signs of a troubled inducer fan include whining or grinding when starting or stopping and low top speed. In this case, replacing the fan will likely fix the pressure switch issue.

Check the Switch Hose Connection and Quality

The pressure switch can become troublesome if the hose is not tightly connected, cracked, or has a hole. Fortunately, this is another issue you can fix relatively quickly.

First, make sure the hose connection is tight enough. Then, if you find holes or cracks on it, cut away the cracked part and reconnect it. Just be careful as the inducer housing can get very hot during furnace operation.

Look for Blockages in the Flue

A blockage in the flue due to debris or a bent flue can also pressure issues. These, too, are issues you can fix with ease.

However, if none of the four solutions above help you, it’s time to test the furnace with a multimeter and replace it if it’s damaged. You can test the resistance (ohms) across furnace pressure switch in three simple steps;

  1. Shut off the power and turn off the gas.
  2. Remove the wires from the pressure switch and measure the resistance across the two terminals using the multimeter.
  3. If it’s “0” (or close to zero), then the switch is closed when it shouldn’t be. This is a sign of short-circuiting. However, if it returns “infinite” (OL), then the switch isn’t shorted and is likely just “sticking open.” In both cases, you need to replace the switch.

Alternatively, you can read the voltage between each terminal and ground with a volt-meter. A healthy pressure switch gives a reading between 24-28 volts DC in this approach.

How to Replace the Furnace Pressure Switch – Step-by-Step

If the pressure switch is bad, it’s time to replace it. The following is a straightforward way to replace the furnace pressure switch;

Step 1: Turn Off the Power

Replacing the pressure switch with electric power ON is calling for trouble. It creates a massive risk of electrocution. It’s best to switch it off at the breaker.

Step 2: Turn Off Gas

It’s just as dangerous accessing the furnace’s internal parts with gas supply ON as it creates the risk of gas poisoning and even fire. So, turn off the supply at the gas valve. The valve is OFF when the lever is perpendicular to the gas line/pipe.

Step 3: Remove the Combustion Chamber Cover

The combustion chamber cover is located on the top half of the combustion chamber. You may need to unscrew a few screws to take it off.

Step 4: Locate the Pressure Switch 

In most furnaces, the pressure switch is a disc-shaped metal component connected to a hose. It’s only one in most furnace systems, though some have two or even three. For instance, two-stage pressure switches, found in dual-stage furnaces, have two pressure switches

Step 5: Disconnect the wires 

Pressure switches typically have wires running to the control board and other parts of the furnace. Disconnect the wires at the front of the switch.

Step 6: Disconnect the Hose

As we said earlier, the standard furnace has only one pressure switch with one connecting hose. However, other furnace types, such as dual-stage models, may have two or more horses. Make sure to unplug all the hoses.

Step 7: Unmount the Old Switch 

The best approach here will vary depending on whether your new switch has a bracket. If it does, then you can remove the old one together with the bracket.

However, if it doesn’t come with a bracket, only remove the old switch, leaving the bracket in place. In both cases, all you need to do I unscrew a few screws and lift off the device(s).

Step 8: Secure the New Switch in Place

The most important thing here is to make sure the new pressure switch is secured firmly. You don’t want it hanging loosely.

Step 9: Reconnect the Wires and Tubes and Replace the Panel

Start with the last one you took off, i.e., the vacuum tubes, and work your way back until you replace the combustion chamber panel.

That’s it! You’ve successfully replaced your furnace pressure switch. If you purchased the correct switch and followed all the steps above, the furnace should work without a hitch.

What Causes a Pressure Switch to Fail?

The furnace pressure switch can fail for various reasons. The most common causes are as follows;

  • Old age, which can cause inaccurate readings
  • Extreme dust or dirt buildup causes the switch to jam
  • Holes, cracks, or other damages to the diaphragm
  • A coated or stiffened diaphragm

What Happens When a Furnace Pressure Switch Goes Bad?

If the pressure switch malfunctions, the system will go into “safe mode,” try a couple of times again, and eventually start blowing cold air into your room.

When Should I Replace My Furnace Pressure Switch?

You should only replace the furnace pressure switch if it has malfunctioned, i.e., after the resistance test.

If it’s merely dirty or jammed in a way, it’s advisable to try to fix it first before replacement. Gently remove the switch from the inducer fan housing and blow and suck on the end of it to remove trapped dirt. It should work just well afterward.

How Much Does it Cost to Replace the Furnace Pressure Switch?

The cost of replacing a bad furnace pressure switch varies from around $40 to as high as $350, depending on many factors, especially the type of switch and labor costs.

High-pressure switches found in modern variable-speed furnaces are typically more expensive, costing at least $200, including labor. Meanwhile, most low-pressure switches cost $40 to $100 to purchase. Labor charges range from $50 to $150.

The pressure switch is a critical part of the furnace, without which the furnace cannot function. Therefore, it helps to know how to replace it if your furnace breaks down in the middle of the night. Fortunately, the switch isn’t too difficult to replace.