HVAC flow rate piston size for a heat pump or AC range from .052” to .093”. These sizes are accurate for Trane, Carrier, Lennox, York, Goodman, Rheem and most other major brands.
The right HVAC piston size for a heat pump or AC coil is determined by the tonnage of the coil, from 1.5 to 5 tons. Whether the system is R22 or R410A also makes a difference. The piston orifice size for R22 refrigerant systems is larger than for R410A systems.
These HVAC coil parts are used in both A-coil and Z-coil types. They are also called flow rate pistons and flowcheck or flow check pistons. Most come in kits with a Teflon O-ring and gasket.
R410A Piston Size Chart
| HVAC Size | Piston Size |
| 1.5 Ton | .049 |
| 2 Ton | .055 |
| 2.5 Ton | .059 |
| 3 Ton | .068 |
| 3.5 Ton | .074 |
| 4 Ton | .080 |
| 5 Ton | .089 |
* The piston sizes in the table are from the following brands: Trane, Carrier, Goodman, Lennox, Bryant and related brands.
The piston size for a 2 ton evaporator coil is .049”.
The correct piston orifice size for a 3 ton system is .068”.
A .080 piston is used in a 4 ton R410A system.
Tip: HVAC technicians refer to pistons by the last two numbers. So, for example, a .068 orifice is called a 68.
R22 Piston Size Chart
| HVAC Size – Outdoor Unit | Piston Size |
| 1.5 Ton | .052 – .059 |
| 2 Ton | .055 – .065 |
| 2.5 Ton | .059 – .071 |
| 3 Ton | .071 – .082 |
| 3.5 Ton | .078 – .090 |
| 4 Ton | .082 – .090 |
| 5 Ton | .088 – .093 |
* The piston sizes in the table are from the following brands: Trane, Carrier, Goodman, Lennox, Bryant and related brands.
A 3 ton R22 system takes a .071 to .082 orifice. The best size orifice for a 3 ton system is a .071.
What is the best piston size? When the tonnage of the outdoor unit and the indoor coil are the same, there is generally one best piston size. It is always best for the units to match.
But when the indoor coil is a slightly different size, then there are allowable ranges for each tonnage.
For example, when both the outdoor unit and indoor unit are 3 tons, the most precise R22 piston size is .071. But when the indoor coil is a 3.5 ton size, the piston can range from .073 to .078.
The efficiency of the heat pump or central air conditioning system will be changed very little by slight differences in coil piston sizes.
R22 Piston Size for Matching Outdoor Coil and Indoor Coil Sizes
| Size | Best Piston Size – Orifice Diameter in Inches |
| 1.5 Ton | .052 |
| 2 Ton | .059 |
| 2.5 Ton | .065 |
| 3 Ton | .071 |
| 3.5 Ton | .078 |
| 4 Ton | .082 |
| 5 Ton | .090 |
The right piston size for a 4 ton R410A heat pump coil is .082”. A 5 ton system requires a .090 piston.
These pistons are called 82 and 90 for easy reference.
Pro Tip: R410A piston sizes are generally about 3% to 11% smaller than R22 piston orifice sizes. The larger the piston, the closer an R410A piston and R22 piston will be in size.
For example, the correct R22 1.5 ton coil piston size is .055. And the proper piston size for an R410A 1.5 ton coil is .049, a difference of 11%.
However, the 5 ton R22 piston orifice size is 0.092. The 5 ton R410A coil piston size is 0.0.89, a difference of less than 4%.
Knowing R22 vs R410A piston size comparison will be useful if you can only find the right size orifice for one refrigerant type but you need the piston for the other type.
How to Measure Piston Size
To measure piston size, measure the diameter of the piston orifice opening.
Measure piston size – the diameter of the orifice – using a micrometer. A digital micrometer can be purchased for $14 to $40 depending on how much you want to spend.
An alternative option if you can’t measure piston size is to know the correct piston orifice size for the coil you are working on.
The Charts above are a R22 Piston Size Chart and an R410A Piston Size Chart that gives you the right piston size for each HVAC system size and refrigerant type.
What Does the HVAC Piston Do?
The piston allows a steady, consistent flow of refrigerant into the evaporator coil. Pistons are used in most older R22 systems and some R410A systems.
The piston is part of a fixed metering device within an air conditioning system or heat pump.
Fixed metering means that it delivers a consistent amount of refrigerant flow. The evaporator coil is the indoor coil located in your air handler or furnace. Fixed metering is a simpler system than a variable refrigerant flow system.
The amount of refrigerant that passes into the coil is determined by the piston orifice size. The larger the system size, from 1.5 to 5.0 tons, the larger the orifice.
Alternative to Changing the HVAC Piston
Many HVAC technicians choose an alternative when they need piston sizing help: Instead of replacing or installing a new refrigerant piston, they remove the piston and replace it with a TXV valve. A TXV is a thermostatic expansion valve. It eliminates the need for the piston. And it has other advantages.
A TXV protects the condenser when the condensing unit and the rest of the system are not produced by the same manufacturer. This is called a mix-match system by technicians. Adding a TXV will protect the system, increasing longevity and reducing the risk of incurring the cost of evaporator coil replacement.
The TXV will optimize the efficiency of the system because it delivers a variable amount of refrigerant based on the temperature and the pressure of the refrigerant.
Are Mini Split Heat Pump and Central Heat Pump Piston Sizes the Same?
The sizes are not always the same for a central heat pump and a mini split heat pump. Always be sure you are buying a piston for the right type of system and the right capacity or tonnage of the system.
