How Do I Troubleshoot and Test My Furnace Pressure Switch?

We’ve talked about the importance of draft inducer fans numerous times and how they are essential for safe and clean combustion inside a gas furnace. A furnace pressure switch is a safety component designed to keep things in check alongside the control board. When it fails, your system is rendered inoperable which is why troubleshooting pressure switches are key when a problem occurs.

How a Pressure Switch works

Some people think turning on the heat is as simple as adjusting the thermostat. While that’s true to a degree, a lot is going on behind the scenes. When the temperature in your home reaches a certain point, the thermostat sends a signal to the furnace which tells it to turn on the heat.

When this happens, the inducer motor will start 30 to 60 seconds before the system ignites. This creates a draft that ensures there are no leftover gases in the heat exchanger. The pressure switch is attached to the motor and detects changes which in turn tells it to open or close the diaphragm.

Under normal conditions, the pressure switch activates and then allows your furnace to start safely. If there is an issue and the switch doesn’t work, the furnace will still attempt to start again after a short period of time, however.

a high and low-pressure switch

There are also two types of switches. Single-stage furnaces only need one pressure switch, but two-stage units will have a high and low-pressure switch to handle the first and second heating stages. 2-in-1 or dual furnace pressure switches are also an option on high-capacity systems.

Signs of a Faulty Furnace Pressure Switch

While designed for safety, your HVAC system will still continue to operate with a bad pressure switch. You just won’t get any gas, however, which means no heat and only room temperature air coming from the vents.

If the valve fails after several attempts, the control board receives an error code and goes into lock-out mode. This will result in airflow, but no heat. That is usually a sign of an issue with the igniter or pressure switch, and something homeowners notice quickly when only room temperature air comes through the vents.

Other signs something could be amiss with the pressure switch in a furnace include unusual noises and intermittent issues with heat when your furnace attempts to start. If you’re wondering what causes a pressure switch to fail, that boils down to a handful of factors.

Common Problems with Furnace Pressure Switches

As with any part, a furnace pressure switch has a limited lifespan no matter how well it’s built. That means eventually it will fail, which could be far before or after components like blower fans and capacitors fail.

One of the main reasons for a problem with a furnace pressure switch is blockages. That includes a blocked flue pipe, which may not happen in the winter when your system is in use. During the warmer months, birds or other creatures may make a temporary residence in that space, however.

You can check the flue each year before firing up the heater and clear any obstructions that may affect the switch. Another thing that can clog over time is the port leading to the collection chamber. If it becomes clogged, it will lead to issues with the switch.

The diaphragm itself can also be an issue if it becomes stuck or damaged.  You can fix a stuck diaphragm by tapping on the casing, although a replacement may be in order sooner than later. If you hear a “flapping” noise like a flat tire, there’s a chance the diaphragm is busted and will need to be replaced immediately.

Two other issues can occur with the pressure switch hose. One is a hose that’s become damaged. It could simply be brittle due to age or have a hole but is something that can easily be replaced. The hose can also fill with water from moisture caused by heat which goes down the flue and into the pipe. You remove the hose from the port to drain it or check for any potential issues.

Testing Pressures Switches in Furnaces

When you want to test the pressure switch further, you can do so with a multimeter. Before attempting to test the switch, you will need to shut off the power to the system. As you will need to detach a few wires, there can be no power going to the unit during this test. As you can see from the video below, you may also need an extra set of hands depending on the style and size of your furnace.

If you are uncomfortable dealing with electricity or just don’t have the DIY skills to tackle a repair yourself, this is the time to call for a technician. An HVAC professional can test the switch with a manometer and give your furnace a quick check to catch any issues a homeowner may miss. You can expect to pay around $180 to $350 to have a pressure switch replaced in a furnace.

Pressure Switch Replacement

If you know the pressure switch is bad and want to replace it yourself, nothing could be simpler. As long as you have easy access to the furnace and a few simple tools, this repair should take less than 10 minutes.

That’s quicker than it could take you to locate the actual part, which may be challenging on older systems. You’ll need to get the part number off the old switch along with the model of your furnace to find the right fit. In most cases, an OEM replacement for a furnace pressure switch will cost between $12 to $30.

  1. Shut off the gas and power to your furnace.
  2. Remove the access panel. Locate the pressure switch and disconnect the hose and wires.
  3. If the new switch comes with a bracket, you need to remove the old bracket along with the switch from the furnace. Otherwise, make sure the new switch will fit securely into the old bracket.
  4. Install the new pressure switch onto the furnace and bracket if necessary.
  5. Reconnect the hose and wires to the switch, make sure everything is secure, and then put the access panel back onto the furnace.
  6. Turn the power and gas back on, set a heat cycle, and then test your system to make sure everything is in proper working order.

Final Thoughts

While it’s possible to tinker with a pressure switch and get it working again, chances are if it’s giving you trouble, failure is not far behind. It’s not something you will have to deal with often, but worth the minor investment as your furnace won’t function properly without it.

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