How To Test A Gas Furnace Pressure Switch With Multimeter


How To Test A Gas Furnace Pressure Switch With Multimeter

The pressure switch is a small and useful component but can lead to a lot of failures if not understood well. This post will provide you all the required information on “how to test a gas furnace pressure switch with multimeter“. But before that, let’s know what is it and how it works because it’s the pressure switch that might create the biggest of the problems. Focus on the important things first before you change the gas furnace pressure switch.

What Is A Furnace Pressure Switch?

 A furnace pressure switch is a safety component which sends the signal that the inducer motor is working. The inducer motor pulls out toxic gases coming out from the flames. The pressure switch is used as a safety to allow passage of gas. Basically, it acts as a safety mechanism.

Before you learn about “how to test a gas furnace pressure switch with multimeter“; let’s first know how does a switch work!

How Does The Switch Work?

First of all, the switch bears a diaphragm. Whenever a vacuum is applied, the diaphragm comes out. The diaphragm doesn’t come out in the absence of vacuum. A switch is connected to the diaphragm which tells whether the diaphragm is opened or closed! In the presence of vacuum, the diaphragm comes out and closes the switch while in its absence, the switch remains open. In such a case, the furnace remains off. 

What does the switch do?

A switch is simply a connection to allow the power to pass. If it is a light switch, pushing the switch on manually will turn on the light but a pressure switch turns on because of the pressure of air. We can rather say that suction created her turns on the pressure switch. The inducer fan is responsible for the suction created. Turning on the fan will pull the vacuum present in the switch with the help of the rubber tube. The rubber tube connects the fan and the pressure switch which is responsible for pulling the switch in to close a circuit. Hence, the power is passed to the igniter.

Main Purpose Of The Switch

The main purpose of the switch is to assure that the whole unit is working alright. If there is something wrong with the unit then it will not work properly. Ultimately, the toxic gases won’t get removed and there is surely a risk. The best way to know this is that the switch does not allow the unit to ignite until it gets The desired amount of suction.

Causes Of Failures

The various causes for failures due to a furnace pressure switch are:

  • Suction Tube being low
  • Loosely connected wires
  • Bad switch
  • Blockage in chimney

What Can Be Done In Case Of A Failure?

Before you know “how to test a gas furnace pressure switch with multimeter“, get informed of the methods that can be applied in case of a failure. Keeping a check on the causes of failures can help you out. Following are the ways to resolve in case of a failure:

  • If the pressure switch doesn’t work, you won’t be able to hear the opening of the valve. On the contrary, the sound of the opening valve indicates that the pressure switch is in working condition and everything is perfect.
  • Another method is to turn off the furnace. Next, check if there is a sound coming from the inducer fan. Try to remove it yourself first. If the motor is slow or you notice something else the there is a need to get the motor changed and the switch is working well.
  • Check out for the tightness of the switch hose. Tightening a loose switch hose can fix up the problem whereas a pit in the hose might need to be sealed. One can also cut out the damaged part and reconnect the hose. Before you do any of the replacements, make sure the housing is cool. Ultimately, the switch will work properly after the hose is fixed up.

If you have ruled out that none of these problems are present then there’s a possibility that the switch is bad. To confirm if the switch is bad, you can apply this method to know “how to test gas furnace pressure switch with a multimeter“.

Testing with the multimeter

A multimeter is used to test the pressure switch and check the continuity of the switch whether it is closed or it is opened. The switch is being tested to check if its closing when the diaphragm is pulled in. The steps are as follows:

  •  The switch wires have to be removed first.
  • Touch the two switch wires with the multimeter leads. Make sure that the meter reads the continuity.
  • The furnace will be turned on next. The draft inducer motor also turns on.
  • Now commence blowing air out of the vent so that a vacuum is created.
  • The vacuum makes the diaphragm to pull in and hence the switch gets closed.
  • Use the multimeter to check if the switch closes.
  • The multimeter will check of any change.
  • If the switch closes, then it shows continuity and is working properly.
  • If the switch remains open then there is not change in continuity. The switch is bad.
  • You have to replace the switch now to resolve the issue.

Other possibilities

Before you change an inducer motor or decide to change the switch, sone other things too must be kept in mind. The other possibilities:

  • A hose that’s kinked
  • Clogging of the flue pipe
  • Any other things that intervenes the inducer motor from pushing the air out of the vent

All these possibilties can also stop the gas furnace pressure switch from working well. Make sure you have a look at these things too before you finally decide to change the pressure switch. If none of these are present and you have tried all the possible ways to eliminate the causes and check for the faults, it’s time you change the pressure switch and don’t take a risk with it.


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