Learn How to Read ALL HVAC Gauges Like An Expert

Josh Mitchell

Written By

Josh Mitchell

Expert Reviewed By

Holly Curell

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An HVAC technician's most important tool is a set of pressure gauges.

They provide valuable information about how much refrigerant is in the system, so knowing how to read HVAC gauges is critical to job success.

Here, I'll go over everything you need to get started.

Key Takeaways

  • Gauge manifolds are differentiated as either digital or analog and also by the type of refrigerant they can support.
  • Each type of refrigerant has a its optimal low and high readings. Make sure your gauge is capable of supporting the refrigerant you work with.
  • Abnormal readings on your gauge manifold can help you identify issues in the HVAC system

Types of Gauge Manifolds Explained

When it comes to gauge manifolds, there are a few things to look for, including:

  • Digital or analog
  • The type of refrigerant they can read
  • How many connections they support.

It's also worth noting that some modern HVAC systems have built-in gauges. Multistage and variable speed systems commonly have this feature on a small display.

XtremepowerUS

Analog vs Digital

The first thing I would consider before purchasing a new set of gauges is whether to go analog or digital.

Analog gauges feature two round gauges at the top to read the pressure, whereas a digital manifold has an LCD screen.

I have a set of HVAC analog gauges that work well for occasional use or as a backup.

However, most HVAC professionals use digital HVAC gauges because they are easier to read and provide an exact numerical value.

Besides reading high and low pressures, digital gauges can provide metrics like superheat and subcooling.[1]

Some digital gauge meters even connect to a line and send accurate readings back to a phone or tablet.

However, you would still need a regular manifold to add refrigerant.

TL;DR: Analog gauge manifolds are your typical run-of-the-mill option, however, professionals often choose to go with digital models.

Gauge Manifold for Various Refrigerants

The next thing to look at is the type of refrigerant the gauges work with.

HVAC analog gauges usually have several charts inside the pressure gauge, while digital units have a few options to flip through.

The most important thing to remember is that you cannot use an older gauge designed for R-22 with a newer R-410A system.

ACLAB Note:

Newer refrigerants operate at much higher pressures, so be on the lookout if shopping for used gauges.

I use one set of gauges for everything, but many people say this is a horrible idea because oils could mix.

However, the amount of oil getting into the manifold is negligible if I’m not constantly switching between refrigerants.

The gauges I use have markings for the three most common refrigerants. I have also switched between each system without encountering any problems.

TL;DR: This cannot be stressed enough, different gauge manifolds are designed for different refrigerants. Make sure you choose the right one.

Number of Connections

Finally, I look for a gauge with four connections.

Most refrigeration gauges have three connections: one for high pressure, another for low pressure, and a central connector for a vacuum pump or refrigerant line.

The other option is a manifold with four ports that provide another connection.

The extra connection is optional for most people, and even HVAC professionals often forego manifolds with four ports.

The extra connector can make evacuating a line set easier, as well as introducing additives and oils into the system.

TL;DR: There are 3 and 4 connector gauges. 4th ports is used for pulling vacuum.

Guide to Reading HVAC Gauges

Now that you have an HVAC manifold gauge, it is time to start using them.

It is very important to first calibrate the gauges and follow the guide for connecting the lines.

manifold gauge

Calibrating Gauge as the First Step

Before hooking up the air conditioning gauges, it is essential to check their accuracy. The high and low-pressure gauges should read 0 PSI when disconnected.

If the gauges do not read zero, remove the clear plastic cover by pulling up or turning to the left.

Use the flat adjustment screw under the clear cover to get the gauges back to zero.

A Step By Step Guide to Connecting the Gauge

  1. 1
    With the manifold gauge calibrated, I begin connecting the hoses to the air conditioner.
  2. 2
    First, I do a quick ambient temperature check. If the temperature is below 65 degrees Fahrenheit, the pressure readings will not be accurate, and I will have to adjust readings accordingly.
  3. 3
    I also double-check that the valves in my manifold gauge are closed. Then, I connect the red high-pressure hose to the unit's small liquid line. Finally, I connect the blue, low-pressure hose to the unit's larger vapor return line.
  4. 4
    If I was using a digital meter, I would clip on the temperature probes at this time. Additionally, now is a good time to turn the unit on if it's not already running. Ensure it stays running for five to ten minutes before checking the pressures. I would attach a recovery machine, refrigerant recovery container, or vacuum pump to the yellow hose if needed.
  5. 5
    The valves should remain closed unless using one of the items mentioned above. When finished, I check to ensure the valves are closed, then disconnect the low-side and high-side hoses.

ACLAB Note:

Always wear gloves designed for refrigerant, and consider a set of low-loss fittings that connect directly between the manifold port and AC service valves to prevent refrigerant leaks.

Reading R22 HVAC Gauge

R22 refrigerant operates at a relatively low pressure compared to modern alternatives. It is an HCFC, which is very destructive to the ozone.

Because of this, the US banned the sale of R22. However, many homes still use HVAC systems that run on the refrigerant.

Optimal Readings

The perfect refrigerant reading depends on several factors, like atmospheric pressure and temperature.

I like to see an average range of 58-85 PSI on the low side and 120-300 PSI on the high side. 

These indicate that the unit is running okay and should be cooling properly.

TL;DR: R22 is almost phased out and even banned. On the older system that do use it, the readings should be between 58-85 PSI (low) and 120-300 (high).

Reading R134A HVAC Gauge

R134A is an HFC, which is a slightly safer refrigerant. It poses a minimal threat to the ozone but has a high global warming potential.

Automotive air conditioners commonly use R134A refrigerant.

R134A is widely available in small cans at auto parts stores and big box retailers.

Some cans even come with a blue hose and gauge that connects directly to a car's low-pressure side.

However, I always like to check an automotive system with a complete manifold set with high-pressure gauges to ensure nothing else is wrong.

Vehicles also rely on quick-connect fittings, which use an adapter to connect to the threaded hose fittings.

Keep in mind that these quick-connect fittings can’t connect to the wrong line, as the low side is larger.

Optimal Readings

Like other refrigerants, R134A can vary greatly depending on the ambient temperature.[2]

It is vital to read HVAC gauges properly when working with R134A since automotive systems typically use less refrigerant and are, therefore, less forgiving when not changed properly.

The automotive HVAC gauge readings should be between 35-40 PSI on the low side and 145-160 on the high side, assuming an ambient temperature of 70 degrees Fahrenheit.

ACLAB Note:

The respective gauges should have a pressure temperature chart to align the ambient temperature with the proper pressure. Otherwise, it is easy to find temperature charts online.

TL;DR: R134A is an automotive refrigerant. It should read 35-40 PSI (low) and 145-160 (high). 

Reading R410A HVAC Gauge

Finally, R410A is another HFC primarily used in residential and small commercial air conditioners.

However, it only comes in tanks, and buyers must have EPA certification to purchase one.

Although HFCs are relatively good for the ozone, they are being phased out because of their high global warming potential.

Optimal Readings

R410A systems are much less forgiving, and the only way to get a perfectly charged system is with superheat and subcooling probes.

Nonetheless, the low-pressure side should read between 102 and 145 PSI. Anything below this will cause the refrigerant and lines to freeze up.

Meanwhile, depending on the ambient temperature, the high-pressure gauge can range from 200 to 480 PSI.

The cooling unit may overheat and fail prematurely when the pressure gets too high.

Newer air conditioners commonly use TXV valves to regulate refrigerant flow and improve efficiency.

These devices can mess with the pressure readings, making it harder to get an accurate reading.

TL;DR: R410A is a primary home and industrial refrigerant. It should read 102-145 PSI (low) and 200-480 PSI (high).

Refrigerant Charging for AC

Other HVAC Gauges and Their Readings

A manifold gauge set is the most common type of refrigeration gauge, but there are a few others that I regularly use.

These include a micron meter and a general manometer.

Micron Meter

Micron meters connect to a vacuum to get an exact reading when evacuating a line. Attaching vacuum pressure after opening a line is vital to remove any air or moisture.

After pulling the vacuum, I like to leave the micron gauge on the line with the yellow hose attached for about thirty minutes to ensure there are no leaks.

You can technically do this with a manifold gauge set, but the readings are less accurate.

However, I do use regular manifold gauges for automotive work because there are no solder joints, and pulling a low vacuum isn't as critical.

TL;DR: Micron gauge helps measure vacuum, which in turns helps you know if there in any moisture.

Manometer

Another type of HVAC gauge is a manometer, which doesn't connect to a high-pressure port on the air conditioning unit.

Instead, it measures the indoor air handler's suction pressure and high-pressure airflow.

The manometer may be one of the most underrated HVAC gauges and tools in general. It is vital to ensure the ductwork is sized correctly for a unit.

I’ve seen many homes that had a new system installed, but the technicians never checked the ductwork.

I equate a manometer to a blood pressure reading because it provides equally essential information, this time about how an HVAC system is running.

Older systems can tolerate pressure better than new ones that use refrigerant metering devices, so pressure testing is very important.

TL;DR: Manometer are used to measure system air pressure. They can be used to check airflow, or measure pressure in ducts. 


Diagnosing Issues Using HVAC Gauge Readings

Pressure readings can tell a lot about an HVAC system.

Elevated pressure on the high-pressure side can indicate an airflow restriction, while higher than normal pressure on the suction side means a failed compressor.

Air Not Circulating Inside Cooling Unit:

A dirty air filter or a broken indoor fan motor will cause a higher reading on the high-pressure side and a lower pressure on the suction line.

Another symptom is that the evaporator coil will freeze up. 

ACLAB Note:

Be sure to let the ice on the coils melt before turning the system on again.

Defective Expansion Valve:

Newer AC systems have a valve that controls refrigerant flow and aids in efficiency. These are TXV valves, and they can stick or fail entirely.

When this happens, the high-pressure line will go up, and the low-pressure line will go down.

I've even seen where the suction line went into a vacuum, and the low-pressure gauge read a negative PSI.

A failed TXV valve can cause the compressor to overheat and potentially burn out.

Excessive Refrigerant:

An overcharged system will present the same symptoms as one with restricted airflow.

Therefore, the high-pressure gauge will show a higher than normal reading, while the low-pressure refrigeration gauge will be lower than normal.

Refrigerant Not Circulating:

The pressure on both manifold gauges should be equal when refrigerant isn't circulating.

These will be the same numbers as when reading the gauges at atmospheric pressure with the unit turned off.

Similarly, the manifold gauges on an undercharged system will both read lower than normal.

TL;DR: The readings on the manifold gauges can highlight potential issues with the HVAC system, including no circulation of air or refrigerant, excessive refrigerant or defective valve. 


FAQs

What is High Side and Low Side in HVAC?

The high-pressure side is the small line coming out of the condensing unit that carries high-pressure refrigerant to the condenser. The bigger low-pressure hose carries refrigerant back to the condenser in a gaseous state. The red hose connects to the high side, while the blue hoses and blue gauges connect to the low side.

What Gauge Wire is Used for HVAC?

Wire gauge is relative to the size of HVAC equipment. The wire gauge must be sized according to the manufacturer’s heating and cooling equipment specs. Generally, 10-gauge wire, or thicker, is needed to allow the proper flow of electricity to the main components of the heating and cooling system. However, industrial refrigerators and cold rooms may use 6-gauge or larger. [3]

Can You Use Car AC Gauges on Home AC?

Some HVAC gauges can measure pressure on both automotive and HVAC systems. However, experts recommend not using the same manifold gauge set for different types of refrigerant.

References: 

  1. https://abrwholesalers.com/blog/post/superheat-and-subcooling-defined
  2. https://rechargeac.com/how-to/ac-system-pressure-chart/
  3. https://www.homedepot.com/c/ab/how-to-read-a-wire-gauge-chart/9ba683603be9fa5395fab9015a6d2c99
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Josh Mitchell

Founder

Josh Mitchell
My name is Josh and I am obsessed with home appliances. From portable AC units to heaters and air purifiers, I enjoy testing, learning and using these devices to improve the air quality inside my family home.

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Midea U Shaped Window Air Conditioner

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