Why and How to Use HVAC Gauge Manifolds Apr. 10, 2015


When you are working as a heating, ventilation, air conditioning and refrigeration service technician, also known as an HVAC/R technician, your set of gauges is one of your biggest assets. Gauges are used to read the pressure of various liquids and gases in a cooling system, as well as vacuum pressure when you are pressure testing or charging the device. There are a number of different ports on the HVAC gauge set, also known as a manifold, and the main difference in the types of manifolds is the number of ports that you have available to attach accessories to, as well as the pressure that the gauge set can withstand, which is important when working with different refrigerants.

This equipment is also used to perform some HVAC service and replacement.


Gauge Manifolds

Gauge manifolds are the most basic of all refrigeration system tools. The gauge manifold is used as both a diagnosis and a service tool. A technician must have R-134a and R-12 manifolds to service modern vehicles. Many shops with refrigerant service machines also have one or more gauge manifolds to make pressure checks when the machine is being used on another vehicle. Manifold gauges are used to remove contaminated or unknown blend refrigerants from the air conditioning system, reducing the chance for cross-contaminating a service machine. In shops that perform a large volume of air conditioning work, technicians often have their own gauge manifolds as part of their toolset. All gauge manifolds have the same basic parts, although there are some variations among manufacturers. The major parts of a common gauge manifold are shown in picture below.

Manifold Body and Hand Valves

The manifold body is made of brass or aluminum. Passages are drilled in the body to connect the other manifold parts. Some manifold bodies have a sight glass to observe the flow of a refrigerant. Hand valves are used to control the flow of refrigerant through the passages of the manifold body. Note the internal passages are arranged so the gauges can read refrigeration system pressures when the valves are closed. The hand valves used on R-12 and R-134a gauge manifolds are usually arranged in the same way, or on a slant or in front of the manifold. These different arrangements make manifold identification easier. Valve wheels for the high and low sides are identified by color. The low side handwheel is made of blue plastic or has a blue decal in its center. The high side handwheel is made of red plastic or has a red decal.


The gauges used with a refrigeration gauge manifold are either analog (indicator needle), which resemble other pressure gauges, or digital. In an analog gauge, the position of the needle in relation to the numbers on the gauge face indicates the pressure or vacuum in the refrigeration system. Digital gauges provide a numerical reading indicating system pressure or vacuum. All gauge manifolds have high and low pressure gauges. A few older gauge manifolds have three gauges. The third gauge was used to measure compressor output. Older high side gauges are calibrated from 0-500 psi (3445 kPa). Newer high side gauges may be calibrated from 0-250 psi (1723 kPa). Low side gauges are calibrated from 0 to 100-250 psi (689-1723 kPa). In addition to the pressure scale, low side gauges have a provision for measuring 0-29.9” of vacuum (approximately 50 microns).

Analog vs Digital

Digital manifold gauges have began to rise in popularity in HVAC due to the readings a technician can get quickly and reliably. The HVAC world has been slow to adapt to the newer digital technology and in the past digital gauges were left to the HVAC techno-nerds, but this is changing and digital becoming much more common place.

There are no doubt people on both side of the debate on whether using digital gauges are an evolution of HVAC or just the latest gizmo. On one side many believe that digital gauges are the next step in the HVAC world and give a better tool to get a job done more quickly. On the other side many say why mess with what works.


PRO…Analog gauges have stood the test of time, and have been used for generations by heating and cooling professionals. A long time HVAC technician will easily be able to tell what pressures a unit should be reading, without ever looking at a chart.

CON…Analog gauges are prone to errors mainly due to the technicians. Reading the analog gauges give a user a rough picture of what is happening. Conversion errors by technicians compound this problem.


PRO…Digital gauges are very accurate and have many other features, due to their accuracy and no need of conversion tables, errors are less likely to happen.

CON…Digital gauges are new to HVAC technicians which require training to use them properly. Different manufactures add different features which may complicate the process.

These are just some of the pros and cons to look at...

Procedure Instructions

1. Connect the high pressure side of the cooling line to the red port on the gauge. The red port and red gauge are always the high pressure gauges. You will want to attach a code-approved red hose, which is made to withstand high pressure, using the flare fittings that are included with it, to the high pressure port on the cooling unit. The high pressure port is typically of a different size and thread pitch than the low pressure side in order to avoid the potential for accidental incorrect hookups.

2. Connect the low pressure side to the blue port on the gauge. The low pressure side will be connected with a blue low pressure hose to the blue pressure gauge and the low pressure side of the cooling unit. This will allow for the proper flow of vacuum pressure or a micron meter in order to determine the vacuum pressure in the system already.

3. Attach a waste hose or vent hose to the center of the manifold set up. If you are going to discharge a unit, or if you are going to vent freon by attaching the unit, you will need to attach the larger black low pressure hose to the center port of the manifold. By doing so, you will be able to attach a refrigerant recovery bottle to the manifold set to avoid violating federal law regarding the release of refrigerant into the atmosphere.

4. Attach micron meters or other gauges to the additional ports. The additional ports on the manifold can be used to attach micron (vacuum pressure) gauges, or even a vacuum pump, than can be attaced to some piece of furniture in order to perform the proper services to the cooling system as required for the repair that you are making.

Extra Tips

When it comes to HVAC and buying HVAC manifold gauges for most it is never as simple as buying one set and being done with it. Often multiple sets are carried around (or at least two) to be able to work on the different refrigerants in systems and oils used in refrigeration, multiple sets are usually used.

There are a few HVAC technicians that carry only one set but most carry at least two so as to not mix the different oils that are in Freon. Also sometimes you will come across an old unit that you just don’t want to put new gauges on.

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