AC Piston & Flowrator Orifice Size Chart

A piston — Goodman calls it a Flowrator, Carrier calls it an AccuRater, Lennox calls it an RFC orifice — is a brass plug with a precisely drilled hole that meters refrigerant into the evaporator. The number stamped on it is the bore in thousandths of an inch. A .068 has a 0.068″ hole.

Almost everybody arrives here looking for one thing: what size piston does a 3 ton system take? The honest answer is that the question has no single answer, and this page exists to show you why and then get you to the right number anyway.

Which style is it? — and which one we stock

Before any chart matters, work out which family of metering device you are holding. The bore is only half the part. The body it seats in, how it is captured and how it seals differ between families, and they are not interchangeable even when the hole is the same size.

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Telling them apart from the old part in your hand

Why “3 ton = .074” is wrong

The piston is specified by the AHRI-matched combination — the specific outdoor unit paired with the specific indoor coil — not by tonnage. Manufacturers are blunt about this in their own literature.

The counterexample: one brand, one tonnage, five answers

Carrier’s own piston guide, 3-ton (036) outdoor units. We do not stock Carrier pistons — this is here because Carrier publishes the clearest proof that tonnage does not determine the answer, and the same principle governs the Flowrator brands we do stock:

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Goodman, 3-ton R-410A. Six answers, and one of them is not a piston at all:

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The same outdoor unit, different coil, different piston

This is the part that catches people. Nothing changes outside — only the coil:

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Where to find your number, by brand

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Generic tonnage charts — the last resort

If the rating plate is gone, the coil model is unreadable and you cannot find the OEM chart, these are the published fallback tables. We are showing you three, from three manufacturers, because they disagree with each other. That disagreement is the honest reason not to treat any one of them as the specification.

ADP’s own instructions put it best: size the piston to the outdoor unit manufacturer’s recommendation, and only if the manufacturer does not give one, use the generic chart.

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Allstyle Coil — the one Texas brand that publishes a straight chart

Allstyle is a Houston manufacturer and a lot of Dallas–Fort Worth air handlers carry their coils. They ship a bi-directional Flowrator as standard and, unusually, publish sizing by outdoor capacity rather than by model pair:

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Allstyle’s own kits are RP31109 (1.5–2T R-22), RP31105 (2.5–3T R-22), RP31106 (3.5–5T R-22), RP31117 (1.5–3T R-410A) and RP31119 (3.5–5T R-410A). Their procedure calls for a backup wrench, verifying the piston slides freely in the body, Teflon seal in place, and 20 ft-lb.

First Co — the Dallas brand nobody documents

First Co is made here in Dallas and turns up constantly in local apartments and light commercial. Their pistons are the CP81-xx family and the suffix is the bore — CP81-51 is .051, CP81-59 is .059, CP81-63 is .063, CP81-76 is .076. Whether a unit even has a piston is in the model nomenclature: P for piston-type metering device, V for a TXV.

First Co publishes no piston sizing chart anywhere public — their literature says to contact the factory. We have not found one either, and we are not going to invent one. If you are working on a First Co unit, the old piston and the model number are the two things worth bringing to the counter.

The Flowrator pistons we stock11 sizes

These are Goodman B1789 part numbers and the last three digits are the bore — but because the Chatleff-style piston is one part across the family, these are also your Lennox, Rheem, Ruud, Allstyle and First Co pistons wherever the bore matches. Buy by bore, not by the badge on the air handler.

The applications listed below are the ones from Goodman’s own published charts, because Goodman is the only brand in the group that publishes a real model-by-model chart. Note how many bores serve more than one system — which is precisely why the piston in your hand does not tell you what tonnage it came out of.

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We also stock the flowrator inlet tube and the Teflon seal and nut that go with these, plus universal and Chatleff-style TXV kits in 2 and 3 ton if you are converting rather than resizing.

R-410A vs R-22 sizing

For the same nominal tonnage, the R-410A piston is the same size or smaller than the R-22 piston — never larger — and the gap widens as tonnage goes up. Goodman’s own charts, same brand, same tonnage:

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At 1.5 tons the difference is two or three thousandths. At 5 tons it is five. Notice also that the heat pump column does not track the air conditioner column — at 2.5 and 3.5 tons the R-22 heat pump actually takes a larger piston than the R-22 air conditioner.

Symptoms of a wrong piston

Neither superheat nor subcooling alone tells you anything — it is the pair that identifies a metering problem. Read them together.

Wrong piston is a diagnosis of exclusion. High superheat can mean underfeed from any cause, and low superheat can just as easily be an airflow restriction or an overcharge. Verify airflow first — filter, blower speed, return and supply static — then confirm the charge is at target superheat, and only then read subcooling and start suspecting the orifice.

Piston vs TXV, and converting between them

A piston is a fixed hole sized for one design condition. A thermostatic expansion valve modulates to hold superheat as load and outdoor temperature move. The trade is cost and simplicity against performance across a range.

There is one underrated advantage on the piston side: a fixed orifice equalises system pressure shortly after the compressor stops, so the unit restarts unloaded. That is why fixed-orifice systems generally do not need a hard start kit — and why converting to a non-bleed TXV on a reciprocating compressor means you now do.

When the manufacturer requires a TXV

• To make the efficiency rating. Goodman marks whole product families “TXV only” on its chart, with a required kit. ICP states its R4H3 heat pumps do not reach minimum 13 SEER with piston fan coils.
• Two-stage condensers. ICP is explicit that piston fan coils are not appropriate for two-stage equipment.
• Long line sets. Carrier directs you to consult the distributor beyond about 50 feet of interconnecting tubing.
• Wide load and ambient swings, where a single design point does not serve.

Conversion also means the bulb goes on the suction line at 10 or 2 o’clock, insulated, with an external equalizer tapped downstream of it. And once you have converted, stop charging by superheat — charge to the manufacturer’s subcooling spec. The nameplate charge weight was stated for the original metering device and line length, so it is no longer the authority.

We could not find any manufacturer document that warns against converting — every major OEM sells a kit for it. The warnings that exist point the other way, at getting the metering device size wrong.

How to change one

Charging: superheat, not subcooling

Fixed orifice → charge by SUPERHEAT  ·  TXV → charge by SUBCOOLING

The reason is worth understanding rather than memorising. A TXV holds superheat — that is its whole job — so superheat reads about the same whether the system is correctly charged or somewhat over. The extra refrigerant only shows up as subcooling at the condenser outlet. A fixed orifice regulates nothing, so superheat becomes the free variable and moves directly with charge: high means undercharged, low means overcharged.

There is no single correct superheat number. Target superheat is a two-variable lookup — outdoor dry bulb and indoor return-air wet bulb — and it ranges from roughly 4°F to 40°F across normal conditions. Here is Carrier’s published table:

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Three cautions that separate a good charge from a guess:

• Verify airflow first. Trane’s superheat chart is valid only between 375 and 425 CFM per ton. Low airflow mimics an oversized piston — low superheat — and the fix is the blower, not the orifice.
• Run it at least 15 minutes before reading anything, with indoor conditions within a couple of degrees of normal.
• Heat pumps in heating mode get weighed in. Carrier is explicit that heating check charts are for reference only. On a heat pump, take the vapor line temperature between the accumulator and the compressor.

Published target superheat tables disagree with each other by several degrees at the same conditions. Use the chart in your own equipment’s literature, or a calibrated digital manifold that calculates it — not a table copied off the internet, including this one.

The mistakes that cost the most

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Questions we get at the counter

Sources: Allstyle Coil evaporator coil installation instructions (2024) and Style Quest guide; First Co HCX/HBX spec sheets and A2L/TXV installation guide; Nordyne B6BM air handler and split-system heat pump instructions; Danfoss Chatleff piston listings; Goodman Piston Kit Charts PKC-00W (2005), PKCG-4001J (2015) and PKCG-4001ZB (2020); Goodman CAPF coil installation instructions and TXV kit instructions IO-630; Carrier Piston Guide for Residential Split-Systems and Carrier superheat charging procedure; ICP Product Information Bulletin PIB10-18, “R-410A Indoor Piston Sizes”; Rheem RCBA coil installation instructions; Lennox 13ACX product bulletin and C33/CX34 coil bulletins; ADP coil installation instructions (P/N 96654008); Trane/Oxbox J4MXC coil installer guide; HVAC School and ACHR News on metering devices, superheat and charging method. Verified July 2026.

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