Arkansas HVACR NewsMagazine November 2020

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Trane Type

PISTON DESIGN AND BYPASS

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

Piston Facts

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 40’s up to the low 100’s. When a piston is described as being a “65 piston” that means it is 0.065 of an inch and a “104” would be 0.104 of an inch.

There are three common piston designs I see regularly and while different manufacturers may use them I will group them by the manufacturers I know them by— 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 prevent refrigerant bypass and it also uses an o-ring to seal the “chatleff” style housing.

Check Flow Operation

Carrier Type

In a heat pump system, every metering device needs some 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 allowing restricted flow in one direction and unrestricted flow in the other. This is actually where a piston gets its name; because like a piston in an engine, it is a cylinder within a cylinder that can slide back and forth.

Carrier their pistons “accurators” and maybe still do although 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