Informational Guide

HVAC Low Pressure Switch: Testing & Replacement Guide

Our easy to follow steps will have your HVAC unit tested quickly and correctly.

by Ian Haynes

Is your furnace or AC attempting to activate, but nothing happens? Or is it blowing cool air? If that’s the case, the problem might be your low-pressure switch HVAC.

Though you may think it’s too tricky to DIY troubleshoot it, you would be surprised to know you can easily test your HVAC low-pressure switch just by following some easy steps.

Let’s dive into more details!

An HVAC low-pressure switch is handy and provides several functions in a house. They can be easily found on various devices such as air compressors, water heater units, and centralized ventilation systems, especially fans.

Often, homeowners have to check low-pressure switches to see if the devices have malfunctioned or don’t work correctly.

Testing switches for functions will assist you in isolating or troubleshooting the malfunctioning source. Also, you can use low-pressure switch troubleshooting for the high-pressure switch.

So, how to test HVAC low-pressure switches exactly?

HVAC Tester with Nodes

Step 1

First thing first, take out the device’s power by either unplugging or turning off the electric breaker supplying the power.

Step 2

Afterward, remove the low-pressure switch cover. You can easily do it by taking out its middle screw using an appropriate screwdriver.

Step 3

Now, separate wires that run from low-pressure switches to terminal blocks and disengage them instantly. Are you wondering how you will discern these wires?

You’ll know these two wires through their colors: either blue or brown. While doing so, make sure you don’t disconnect the white and black wires as these two wires are for energy/power.

Step 4

Utilize the air compressor for applying air to an HVAC low-pressure switch.

Step 5

After that, use a lead of an ohmmeter and apply it to one of two exposed wires. Now, use the other lead and apply it to the second exposed wire while atmospheric pressure is applied to an HVAC low-pressure switch.

You’ll get a ‘0’ reading on an ohmmeter if the low-pressure switch closes during atmospheric pressure is applied. This means that the HVAC low-pressure switch works appropriately and perfectly.

Step 6

Turn the setpoint screw counter-clockwise as you’re applying atmospheric pressure to an HVAC low-pressure switch.

If the pressure switch still doesn’t close, it means your switch is broken/damaged and may need immediate replacement.

When To Test The Low-Pressure Switch: Signs Of Bad Pressure Switch

Here are a few tell-tale signs when you should test your HVAC low-pressure switch:

Intermittent Air Conditioning
Portable AC Unit

This air conditioner symptom can appear in various ways. For example, you may notice the AC unit starts and stops for a while.

Or, it may work every once in a while, leaving you and your house hot for the most part. This can be highly frustrating in the scorching heat of summers.

Air Conditioning Stops Working

What’s even worse than an air conditioning system working intermittently? Not turning on at all! If your air conditioning system doesn’t operate, chances are the refrigerant low-pressure switch sensor could be defective.

However, as an AC unit is a mixture of so many components tied together, there might be a chance the issue could be different.

Warm Air Is Blowing

Let me throw you a question: why have you purchased an air conditioner? To provide cold air in the summer and make your house more comfortable, correct?

When you switch on the AC system, you expect it to throw cool air. But what if instead of throwing cold air, it pours out warm air? If that’s the case, it means the air conditioner’s pressure switch has gone wrong.

However, low refrigerant can also trigger this issue, so check it routinely.

Strange Air Conditioning System Noises

If the AC compressor clicks on and off continuously, it will generate some unusual sounds. It may sound like clicking occurs whenever the AC compressor starts up, but the air won’t function.

So, always listen to distinct clicking sounds, which tell you the pressure switch turns the compressor on and off.

Related: AC Repair – Guide to Simple Fixes

How To Replace A Low-Pressure Switch

You have got a faulty low-pressure switch and want to replace it. Here’s your ultimate step-by-step guide to replacing your HVAV low-pressure switch. Beware, though, that to purchase or handle the various refrigerants in the US, you must be certified. If you’re not certified, you absolutely want to reach out to a professional to do this for you.

Clear The Refrigerant

To start with, take out the existing refrigerant from the system. But, in doing so, don’t forget to adhere to environmentally-conscious steps to handle such types of gasses.

Discharging the refrigerant in the air isn’t allowed as it adds to the overall greenhouse gasses.

You’ll have to attach the hoses of a recovery machine with the AC valves so that it discards the refrigerant safely.

Replace The Switch
Unscrewing an HVAC AC unit

If you don’t know where the pressure switch is located, you can check its exact position by looking into the manufacturer’s manual. Once you find it, just follow the steps below:

  • Unscrew the pressure switch from the position using your screwdriver
  • Check if the new component matches the existing one
  • Now, replace the existing rubber ring with the newly purchased one
  • Fit your new HVAC pressure switch using the wrench
Remove The Moisture

Make sure you remove the humidity entirely from the AC unit before filling it again with refrigerant. Here’s how to do it:

  • Set the AC gauge and connect it with pressure ports aptly detailed in one of the first steps of this procedure
  • Connect the HVAC vacuum pump from a gauge and activate it along with service valves
Restore The Refrigerant

Once the humidity has been entirely removed from the unit, you’ll have to shut service valves and disengage the HVAC vacuum pump.

  • Fit in the can available with a gauge set.
  • Now attach a refrigerant can to it
  • After that, you’ll need to open up the can along with service valves on a gauge set, providing the unit with a new refrigerant.
  • Keep repeating the process till the unit gets full. Grab your manufacturer’s manual and see how much is enough, and that’s all.

What Is The HVAC Low-Pressure Switch?

The air conditioner pressure switch is the safety device mounted either on the lower or upper side of the AC unit.

It assures you that the combustion blower or draft inducer runs properly to create combustion.

When pressure increases or decreases excessively (on the system’s side where a sensor is scaled), the HVAC pressure switch shuts off a compressor to prevent any damage.

How To Find The HVAC Low-Pressure Switch

Looking to find the HVAC low-pressure switch on your unit? It’s easier than you think. Here’s what you need to do:

  1. First, open your furnace using an appropriate-size nut driver or screwdriver to expose burner assembly
  2. If you inspect carefully, you’ll see a thing that seems like the snail connected to discharge. It’s a draft inducer
  3. Off that configuration, you’ll find one (possibly two) ¼ inch hoses leading to the HVAC low-pressure switch

People Also Ask (FAQ)

How Do I Test An AC Pressure Switch With A Multimeter?

Here’s how to test an AC pressure with a multimeter:

  • Place a multimeter in an ohms position
  • Locate the terminals
  • Touch each terminal with your leads. Remember, the closed pressure switch reads ‘0’ ohms; however, if it provides an ‘infinity’ reading, it means you have a faulty pressure switch.

How Many Ohms Should A Pressure Switch Be?

The atmospheric pressure should shut when the air applies. The ohmmeter will give a ‘0’ reading when the pressure switch closes, denoting the switch works properly.

If it doesn’t close, the ohmmeter will give an ‘OL’ or ‘infinity’ reading.

Related: How to Read HVAC Gauges

How Does A 40 60 Pressure Switch Work?

When the switch senses a drop to 40-psi, it sends electrical signals to the pump to activate. When the faucet is turned off, the pump continues pumping till the pressure returns to 60-psi. This is when the switch sends the signal to the pump to turn off, and that’s how a 40/60 pressure switch works.

What Causes A Pressure Switch To Be Stuck Closed?

There can be several things causing a pressure switch to be stuck closed, such as damaged or faulty inducer and wheel blockage in vents, among other things.

How Do You Bypass A Pressure Switch?

Here’s how to bypass a pressure switch:

  1. Shut off the unit along with the breaker providing power
  2. Eliminate the nuts that are holding the service cover using your wrench
  3. Unclip the wires from metal contacts
  4. Now, join the connectors together using electrical tape, and that’s all

Conclusion

HVAC pressure switches are vital parts of a system’s proper functioning. The low-pressure switch keeps the unit safe and ensures it works properly.

However, a faulty pressure switch will cause several problems. If you face a broken switch, just follow this ultimate guide to test and troubleshoot the low-pressure switch.

Last Updated on February 3, 2022

Ian Haynes

Ian Haynes is an HVAC writer, and researcher. He has worked on hundreds of articles relating to home cooling, heating and air quality with a vast knowledge of the technical aspects of these types of appliances. Outside of his work, Ian likes exploring Brooklyn with his Labrador. Learn more about the AC Lab team here.

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