26.1 TXVs, Capillary Tubes, Reversing Valves & Metering Devices
Key Takeaways
- Superheat is suction-line temperature minus saturated suction temperature (SST from the P-T chart). A typical air-conditioning TXV holds about 8–12°F superheat; charge a TXV system by subcooling, not as if it were a piston.
- Internally equalized TXVs belong only on small single-circuit coils with low evaporator pressure drop (textbook under about 3 psi). Multicircuit coils with distributors need an externally equalized TXV; cap the equalizer and the valve underfeeds.
- TXV bulb: metal-to-metal on the suction line at the evaporator outlet, 10 o'clock or 2 o'clock on a horizontal run (4 or 8 o'clock on larger lines), never 6 o'clock in oil. Strap tight, insulate, and tap the external equalizer just downstream of the bulb on top of the pipe.
- Capillary tubes are critically charged — weigh in the nameplate ounces; they are not field-adjustable. Pistons are fixed orifices charged by superheat in cooling. Heat pumps need bi-flow TXVs (or a TXV plus check valve) or pistons at both coils.
- A reversing valve is not a metering device. Most U.S. brands energize it in cooling on thermostat O; some (commonly Rheem/Ruud) energize in heating on B. Land the output the outdoor unit actually wants. Class B still stops at 25 tons / 500,000 Btu in any one system.
26.1 TXVs, Capillary Tubes, Reversing Valves & Metering Devices
Trade Area D (Equipment Component Installation) is 14 percent of both Class A and Class B. After condensers, compressors, air handlers, ducts, and refrigerant piping (Chapters 23–25), the outline’s numbered cluster is the metering devices and the heat-pump reversing valve: refrigerant metering devices, heat pump reversing valves, thermostatic expansion valves (TXVs), and capillary tubes. The 2026 Air Conditioning CBT technology book is Refrigeration & Air Conditioning Technology, 9th Edition (2021). FBC Mechanical 2023 Chapter 11 still wants listed refrigerant accessories. F.S. 489.105 still fences Class B at 25 tons cooling and 500,000 Btu heating in any one system — a 30-ton rooftop’s TXV is a Class A component even though the valve itself fits in a palm.
Quick Answer: The metering device drops high-side liquid to a low-side mixture and feeds the evaporator. A TXV holds superheat (suction-line temperature minus saturated suction temperature, SST). Externally equalized TXVs belong on distributor coils. Cap tubes are charge-sensitive. Pistons are fixed orifices charged by superheat in cooling. A reversing valve is not a metering device; its solenoid may be energized in cool (O) or heat (B) depending on the manufacturer.
Why the expansion device is its own install item
The vapor-compression cycle has four processes: evaporation, compression, condensation, and expansion. Starve the evaporator and superheat climbs, capacity falls, and the compressor runs hot. Flood it and liquid returns, washing oil off bearings (Chapter 23.2). Trade D scores whether you can select, orient, bulb-mount, and equalize the device the coil was listed with — not whether you can spell SEER2.
Superheat is the degrees the suction vapor is above SST. Read suction pressure, convert to SST on the pressure-temperature (P-T) chart for that refrigerant, then subtract from the actual suction-line temperature at the bulb.
Worked superheat. R-410A suction 118 psig is about 40°F SST. A probe on the suction line leaving the coil reads 50°F. Superheat = 50 − 40 = 10°F. That is a typical air-conditioning TXV target (often 8–12°F; follow the listing). 0–2°F is flooding. 20–25°F is a starved coil — undercharge, a restriction, or a TXV that will not open. Subcooling (saturated liquid temperature minus liquid-line temperature) is how you charge a TXV system. Do not charge a TXV by superheat as if it were a piston, and do not charge a piston by subcooling as if it were a TXV.
TXVs — superheat control, bulb, internal vs external equalizer
A thermostatic expansion valve meters liquid so evaporator superheat stays near a setpoint. Three forces act on the diaphragm:
- Bulb pressure (from the remote sensing bulb on the suction line) opens the valve as suction vapor warms.
- Evaporator pressure, taken internally at the valve outlet or through an external equalizer, closes the valve.
- The superheat spring closes the valve and sets the target.
When load rises, the coil runs out of liquid, suction vapor warms, bulb pressure rises, the valve opens, and feed catches up. When load falls, the reverse happens. That is superheat control. It is not a head-pressure regulator you set to a gauge reading.
Bulb placement is the install item the exam loves:
- Mount the bulb on the suction line at the evaporator outlet, metal-to-metal, with the manufacturer’s copper straps — not a loose zip tie.
- Horizontal line: 10 o’clock or 2 o’clock. Never 6 o’clock — oil on the bottom insulates the bulb and the valve hunts or floods.
- On larger suction lines (commonly over 7/8 inch), many manufacturers want 4 o’clock or 8 o’clock so the bulb sees neither a dry vapor pocket at the top nor the oil pool at the bottom.
- Insulate the bulb after it is strapped so 130°F attic air is not a fake load.
- Keep the bulb off fittings, valves, headers, and the compressor shell.
- The external equalizer taps the suction line just downstream of the bulb, on the top of the pipe so oil does not fill the tube.
Internally equalized versus externally equalized:
| TXV type | Where it reads evaporator pressure | Use it when | Exam trap |
|---|---|---|---|
| Internally equalized | At the valve outlet (evaporator inlet) | Small single-circuit coils with low pressure drop (textbook under about 3 psi) | Putting one on a distributor coil |
| Externally equalized | At the evaporator outlet through the equalizer tube | Multicircuit coils with distributors — almost every comfort-cooling A-coil | Capping the equalizer because it hissed |
| Bi-flow TXV | Same sensing; flow either direction | Heat pumps (or a TXV plus a check valve around it) | Leaving a cool-only piston in a heat-pump indoor coil |
If you use an internally equalized valve on a coil with a distributor, the valve sees the high inlet pressure, thinks the evaporator is full, and underfeeds. Superheat stays high, the coil starves, and someone adds charge until the compressor slugs. External equalizer is the default answer on a Florida split A-coil.
Match the valve to the refrigerant, tonnage, and bulb charge type (liquid, gas, or cross-charge / maximum operating pressure, MOP). An R-22 valve on an R-410A or R-454B coil is a wrong part, not a close-enough spring. Electronic expansion valves (EEVs) use a stepper motor and a suction thermistor; they still hold superheat, and the sensor still belongs on the suction line the way a bulb does.
Pistons, orifices, and capillary tubes
A piston (fixed orifice) is a sized hole in the indoor-coil inlet. It does not hold a superheat setpoint. Superheat rises as outdoor temperature rises (more flash gas, less liquid through the hole) and falls as outdoor temperature falls. Charge a piston system by superheat in cooling at the published outdoor condition — often a target near 10–12°F on the manufacturer’s charging chart, not a TXV subcooling chart. Heat-pump indoor pistons usually sit with a check valve so heating can bypass the cooling orifice and meter at the outdoor piston.
A capillary tube is a long, small-bore tube. Length and inside diameter are the restriction. There is no adjustable stem and no bulb. Cap-tube machines are critically charged: the factory weigh-in is the charge. Add two pounds for the line set on a cap-tube appliance and you flood. Recover two pounds and you starve. Cap tubes clog on copper-oxide scale (brazing without nitrogen — Chapter 25) and on oil-logged debris; a strainer belongs at the inlet. Low outdoor temperature can overfeed a cap tube; many small machines add a suction accumulator for that reason. Do not shorten the cap tube to add capacity.
| Device | What it controls | How you charge | Heat-pump note |
|---|---|---|---|
| TXV / EEV | Superheat as a setpoint | Subcooling (and verify SH) | Bi-flow TXV or TXV plus check valve |
| Piston / orifice | Nothing — fixed hole | Superheat in cooling at the chart condition | Piston at each coil plus checks |
| Capillary tube | Nothing — fixed L and ID | Weigh-in (critical charge) | Rare on split HPs; still charge-sensitive |
| Reversing valve | Which coil is the condenser | Not a charge method | Energized O (cool) or B (heat) by brand |
Reversing valves — energized in heat or in cool
A four-way reversing valve sits in the discharge line. Four connections: compressor discharge, compressor suction, indoor coil, outdoor coil. A pilot solenoid slides the valve so hot gas goes to the indoor coil (heating — indoor coil is the condenser) or to the outdoor coil (cooling). It does not meter liquid.
Whether the solenoid is energized in heating or in cooling is a manufacturer choice. Most U.S. splits energize the valve on thermostat O in cooling. Some brands — commonly the Rheem / Ruud family — energize on B in heating. Landing O on a unit that wants B leaves the machine in the wrong mode all season. Package heat pumps use the same rule inside the cabinet. A slide stuck mid-stroke leaks discharge into suction: high suction, low head, compressor hot. Do not hammer the body to free it. Replace a failed valve with nitrogen-purge brazing and the listing orientation (often main tubes down so the slide is supported).
Heat-pump metering is bi-flow. Insulate both line-set legs (Section 20.2). Class B still stops at 25 tons / 500,000 Btu on the connected system; Class B candidates still owe the valve theory on a 40-ton package they may not contract.
Florida scenario
Sunset Coast Mechanical, certified Class B in Lee County, change-outs a 3-ton heat pump. The helper (1) hangs a cool-only internally equalized TXV on the distributor A-coil and caps the equalizer port, (2) straps the bulb at 6 o’clock with a zip tie and no insulation in a 130°F attic, (3) leaves the factory cooling piston in the indoor coil because there is already a TXV outdoors, and (4) lands thermostat O on a Rheem outdoor unit that wants B. Four independent Trade D failures: external equalizer on a distributor coil, bulb position, bi-flow metering, and O versus B. Superheat on a 95°F Fort Myers afternoon reads 24°F, and the owner reports no heat on a 45°F night. Class B is legal on 3 tons. The same helper on a 30-ton package heat pump would have been illegal under 489.105 before the first bulb strap.
Traps: (1) Internally equalized TXV on a distributor coil. (2) Bulb at 6 o’clock. (3) Charging a TXV by superheat as if it were a piston. (4) Treating a cap tube as adjustable. (5) Assuming every reversing valve is energized in cooling. (6) Calling the reversing valve a metering device.
A multicircuit evaporator with a refrigerant distributor is being piped to a thermostatic expansion valve. Which equalizer choice matches Refrigeration & Air Conditioning Technology practice?
Where should a TXV sensing bulb be mounted on a horizontal suction line leaving the evaporator?
Which statement about a heat-pump reversing valve is correct on a Trade D install?