Arkansas HVACR NewsMagazine September 2025

HVACR NewsMagazine September 2025

Tech News

Check Flow Operation

prevent refrigerant bypass, and it also uses an o- ring to seal the “chatleff” -style housing.

In a heat pump system, every metering device needs a method of bypassing the metering device when the refrigerant flows in the opposite direction. This is done in TXVs by means of an internal or external check valve, but with a piston, the piston itself is allowed to slide in the housing. That allows restricted flow in one direction and unrestricted flow in the other.

Carrier Type

Carrier their pistons “accurators“ and maybe still do, though I haven't heard that term for years. These pistons can be installed in either direction but still use the same “chatleff” -style housing as Lennox. used to call

Trane Type

The Trane style has a much smaller size and is directional. The Trane housings do not use O-rings.

Piston Size

The physical exterior dimensions of the piston must be the same as all the others for that brand/series. Otherwise, it will not fit properly. It is only the internal bore diameter that changes. Pistons are sized in decimals of an inch, like a gas orifice, usually from the 40s to the low 100s. When a piston is described as a “65 piston,” it is 0.065 of an inch, and a “104” would be 0.104 of an inch.

That is actually where a piston gets its name; like a piston in an engine, it is a cylinder within a cylinder that can slide back and forth. Any carbon, wax, or other solid material that gets into the piston housing can cause one of three undesired conditions. If something gets into or covers the orifice bored into the piston, it can cause a restriction, resulting in low evaporator pressure, low suction, high superheat, and normal to high subcool. When a piston is restricted, and the system is a heat pump with a liquid line filter/drier properly installed, we will often alternate the system between cool Piston Restriction in the Desired Mode