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What is Suction Pressure?

By Paul Scott
Updated May 17, 2024
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Suction pressure is a term used to describe the pressure of a refrigerant gas at the intake point of a refrigerator or air conditioner compressor. Suction or, as it is also known, low side pressure, is one of the critical variants in the operation and diagnostic processes of air conditioning and refrigeration. A correct balance between the suction and discharge pressure ensures that the system is healthy and functioning correctly. This relationship, along with the suction and discharge temperatures and the specific refrigerant used, play an important role in determining the correct refrigerant charges for individual systems.

Suction pressure refers to the pressure of the refrigerant at the point where it enters the compressor after leaving the interior of the building. Measured when the compressor is running, this pressure will typically be in the region of 100 pounds per square inch (PSI). The exact suction pressure an air handling unit is designed to produce is a product of the intended capacity of the system, the specific components employed, and the refrigerant the system is designed to use.

A commonly held misconception is that an air conditioner blows cold air into a room or building. All refrigeration systems work on a basic heat transfer principle in which low pressure, cold refrigerant circulates through a set of coiled tubes inside the area to be cooled. This cold fluid absorbs heat from the atmosphere in the room and is then pumped outside the building via the system compressor where a fan or water spray dissipates the heat. The refrigerant is then returned to the interior of the building to repeat the process until the desired temperature is achieved.

The relationship between suction and compressor outlet pressures and temperatures are known quantities in any given air conditioning or refrigeration system. This finite relationship allows suction pressure values to be used as diagnostic aids when troubleshooting system faults. High suction pressure, for instance, may indicate a clogged condenser coil or faulty condenser fan. Low pressure values, on the other hand, may be an indication of a refrigerant undercharge or restricted indoor airflow.

This general overview of a refrigeration cycle serves to demonstrate the importance of suction pressure. For any air conditioning or refrigeration system to work correctly, the relationship between low and high side pressure needs to be maintained within the systems operating specifications. Anomalous suction pressure values can, however, be a very reliable indication of where faults lie in systems that are not operating correctly.

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Discussion Comments

By anon326484 — On Mar 22, 2013

An air conditioner takes heat from A to B. If A is inside, they heat the room; if A is outside they cool the room.

By chivebasil — On Jul 12, 2011

I used to do HVAC work and one of the biggest and most annoying problems that can happen is low suction pressure.

I remember once working on this big job, a building that was something like 15 stories tall. There HVAC system was vast and complicated in a way that I can;t really explain on this forum but suffice it to say that there was whole maze worth of ductwork and some pretty complicated cooling systems. We got called out because whole floors of the building were not getting cooled down.

It took us at least 3 days just to figure out the problem. I remember that it had something to do with the multistage pump. Once we had figured out the problem the fix was pretty simple but it took a ton of work just to get to that stage. Don't let anyone tell you that HVAC work is easy. It takes a lot of work and more brains than you would expect.

By backdraft — On Jul 11, 2011

I have heard this principle of air conditioners before, that they do not cool the air but instead just remove the heat. I never had any idea how this worked but this article did a great job of explaining it.

It is a pretty simple principle when you get down to it. It is one of those mechanical functions that seems so technical and unfathomable, but when you start to look at the details it makes a lot of sense. Now I understand why when you stand next to a big central air unit in someone back yard all the air blowing off of it is hot.

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