Arkansas HVACR NewsMagazine September 2025

HVACR NewsMagazine September 2025

Tech News

Later, I learned all there was to know about sizing and replacing pistons. The installers I worked with often forgot to put in the correct size. In case you are as confused as I was, a piston is a fixed orifice metering device used in systems for many years. They are especially used in residential heat pumps and straight cool systems. Even now that TXVs and EEVs are becoming more popular, you will still see pistons being used outside as the heat mode metering device in many Carrier models.

Piston Design and Bypass Bryan Orr

I remember it like it was yesterday. It was my first day of work as a trainee at my first technician job, just a wet-behind the-ears kid fresh out of trade school.

It was a Monday morning, and the technicians and I were standing in the dusty warehouse surrounded by stacks and stacks of brand new condensing units, drinking the nasty warehouse coffee… …and I was LOVING IT. Finally, I had made it. I was one of the guys, listening to the war stories and good-natured ribbing while getting a caffeine fix for the day. One of the senior techs was telling a story of low suction pressure, and he said, “So, I figured it has to be the wrong- sized piston,” and he stopped, looked over at me, and said, “You know what a piston is… RIGHT?” It seemed like an eternity passed while the whole group stared at me. I mumbled, “A piston? Sure,” and gave a weak nod, hoping that “LIAR” wasn't emblazoned on my forehead for all to see. The tech turned and finished his story, and my mind raced. Of course, I knew what a piston was in an ENGINE or even a reciprocating compressor, but I had no clue that the little hunk of brass with a hole in it that we called a “fixed orifice” in school was called a piston.

Piston Designs

I regularly see three common piston designs. I will group them by the manufacturers I know them by, though different manufacturers may also use them.

Lennox/Rheem Type

The piston shown above is the Lennox/Rheem style. It is directional, meaning it can only be installed one way, with the cone (tapered side) pointed at the evaporator and

the other side pointed at the liquid line. This type uses seals toward the end of the cone to help