What is Head Pressure in HVAC?

Written By Lester Mclaughlin
Updated On

Are you wondering what head pressure is in regard to your HVAC system?

You’ve come to the right place!

In this Blue National HVAC guide, you’ll learn:

  • What head pressure is
  • How HVAC technicians measure it
  • Why proper head pressure is vital for your HVAC system

And much more!

Head Pressure in HVAC

So, if you’re looking to learn about head pressure of HVAC systems, keep reading our detailed guide below to get answers to all of your questions!

What is Head Pressure?

Head pressure, also known as “high side pressure,” discharge pressure, or evaporator pressure, is the refrigerant pressure made by the compressor in an air conditioner, refrigeration system, and air-source heat pump. It is measured at the outlet of the compressor. 

The compressor is part of the “closed-loop” or an air conditioner. It is responsible for transporting the refrigerant to the evaporator coils inside the home to condenser coils outside the home. It’s the powerhouse of the AC unit, ensuring that the house cools down by keeping the refrigerant moving to transfer the heat outside the home. 

The cooling process described above is what all air conditioners use and is called the vapor-compression refrigeration cycle. The amount of refrigerant in the system is critical to this system. To ensure proper cooling is achievable, the refrigerant levels before (head pressure) and after (suction pressure) the compressor must be maintained at optimal levels. 

The value of the discharge pressure in AC units depends on several variables, including the condition of the condensing coils, cleanliness of the air filter, and the speed and size of the condenser fan. The head pressure is also highly dependent on the amount of refrigerant in the AC coils. 

The head pressure can go up and down depending on the outdoor temperature and if the air conditioner is running or not. 

However, issues with the air conditioner can cause both abnormally high or low head pressure. In these cases, the air conditioning unit should be inspected and repaired by an experienced HVAC technician. 

In summary: Head pressure and suction pressure are two very important and very common variables that HVAC technicians use to identify issues within HVAC systems. 

What is Suction Pressure? 

Suction pressure – also known as “low side pressure” – is the pressure at the intake point of the compressor in an air conditioner. Suction pressure is measured when the air conditioner runs and the compressor is pulling refrigerant into it. It is typically 100 PSIG but it can vary depending on brand and model. 

Suction pressure is commonly measured at the same time as the head pressure to ensure the system is balanced correctly and the refrigerant is not restricted. 

How Are Head Pressure and Suction Pressure Measured? 

The refrigerant within the closed loop of your air conditioning system goes through a significant change when it passes through the compressor. The pressures of the refrigerant on each side of the compressor are referred to as the head pressure and suction pressure. 

And to explain how and where they are measured, let’s recap on what the refrigerant does and where it travels in an air conditioning system. 

The Refrigerant’s Path

When the refrigerant is passed through the evaporator coils, it is low pressure and cold. Warm air is blown across the evaporator coils, and the air’s heat is transferred to the refrigerant.

The now warmer refrigerant travels through the suction line into the inlet of the compressor. It compresses the refrigerant to a higher pressure, which increases its temperature too – so much that it’s known as the superheat state. After leaving the compressor, the superheated refrigerant moves out along the discharge line to the condenser coils. 

The condenser coils in the outdoor unit (condensing unit) use a fan and airflow to remove heat from the liquid refrigerant before flowing to the thermal expansion valve (TXV or TEV). 

The thermal expansion valve provides a sudden pressure drop, which rapidly cools the refrigerant before it flows into the evaporator coils. This part of the process is subcooling.

Like the beginning of the process, the evaporator coils have warm air from inside the home blown across them, and the refrigerant vapor absorbs heat before being pumped back to the compressor. The process repeats until the home’s ambient temperature reaches the setpoint on the thermostat.

How and Where the Pressures are Measured

Now that you know the refrigerant flow path in this process and how it removes heat from the home, it will be easy to understand where and how the head pressure and suction pressure are measured. 

Before continuing, it is worth noting that refrigerant levels can only be checked legally by EPA-certified HVAC technicians. Refrigerants are harmful to the environment, and only those with proper training should handle them. 

The head and suction pressures are actually measured at the same time with an HVAC manifold gauge, sometimes called just a “manifold” for short. 

Manifold gauges come in various shapes and sizes – analog (with dials and pointers) or digital (with an electronic display). However, all of them have two pressure connections to measure head and suction pressure simultaneously. 

The manifold gauge is designed to measure the pressure of common refrigerants like R-314a and R-12, which replaced older refrigerants such as freon. They have two dials and connections: 

  • High side gauge – the high side gauge measures the head pressure and is usually marked with a red color and red hose; the hose is connected to a pressure connection on the outlet side of the compressor on the AC unit
  • Low side gauge – the low side gauge measures the suction pressure and is typically colored blue along with its hose, which hooks up to the inlet side of the compressor 

Besides the ability to measure the head and suction pressures, the HVAC manifold gauge works as a metering device too. The manifold balances the air conditioner’s refrigerant levels. 

The manifold portion includes two bleed valves, one each on the high and low sides. The valves can be opened to bleed refrigerant from the air conditioner or refill by connecting a full refrigerant tank to the service inlet of the manifold.

In other words, the HVAC manifold gauge measures both the head and suction pressure and is used to add or take away refrigerant from the AC unit to get both pressures to optimal levels. 

The refrigerant’s discharge temperature and line temperature are also commonly measured on the manifold gauges as well. 

Why Proper Head Pressure is Important

Correct head pressure (along with suction pressure) is needed to ensure the air conditioning system is running optimally. If either pressure is too high or too low, it can inhibit the flow of refrigerant.

When the refrigerant’s flow slows, the heat transfer from the home slows too. Therefore, correct head pressure is needed to ensure the house is cool and comfortable. 

If the head pressure and suction pressure are unbalanced too long, it could lead to equipment failures. For example, if the head pressure is too low because of a refrigerant leak, the compressor will continually run in an effort to cool the home. 

However, due to the lack of refrigerant, the heat exchange process can be almost entirely halted. This could lead to overuse and overheating of the compressor and other critical components, sometimes to the point of complete failure. 

Think of head pressure like your blood pressure – if it is too high, we know that it can be dangerous to your health. If it is too low, that’s a bad sign too. Just like your blood pressure, your head and suction pressure need to be at ideal levels at all times to keep the system healthy. 

How to Fix Low and High Head Pressure 

Maintaining the temperature in your home during the hot summer months is essential. However, if your air conditioner has incorrect head pressure, maintaining a cool temperature will be a struggle. 

EPA-certified HVAC technicians determine incorrect refrigerant levels. When they discover something amiss, they will begin troubleshooting the AC unit. Low refrigerant levels, an overcharge of refrigerant, leaks, condenser obstructions, a clogged air filter, and a faulty fan motor can all cause high head pressure or low suction pressure. 

Once the HVAC technician correctly diagnoses the problem, they will repair the problem and supply a refrigerant charge if necessary. 

How Blue National HVAC Can Help

Blue National HVAC’s technicians are all EPA-certified to measure the head pressure of your air conditioner or heat pump. They are trained to properly balance the refrigerant levels to ensure your AC unit has the correct suction and discharge pressure. 

Inexperienced HVAC technicians can easily contaminate your air conditioner’s refrigerant lines with air and water by not performing proper procedures. They could forget to purge the air from their manifold gauge before connecting to the system or fail to evacuate the system of air when they recharge the refrigerant in a system with a leak repair. 

Unlike these inexperienced technicians, our team of professionals always dot their i’s and cross their t’s. Their expert work ensures your air conditioner does not fail unnecessarily and lead to costly equipment failure. 

If you suspect your air conditioner has improper head pressure, give our expert team a call today!

Meet Your HVAC Expert

Lester Mclaughlin

HVAC systems are highly technical and often is the most misunderstood part of the house. From ductwork to heat pumps, I've been exposed to all sorts of issues facing homeowners. It really irks me when a homeowner is given bad advice like refilling freon vs fixing a leak in the system. I'm here to help our website readers with their heating and a/c problems.
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