Metering Device & TXV Reference
The refrigerant metering devices — fixed orifice, capillary tube, thermostatic expansion valve (TXV), and electronic expansion valve (EEV) — and how a TXV is sized and its sensing bulb mounted. A TXV is matched by tons, refrigerant, and pressure drop, and the bulb placement on the suction line makes or breaks its superheat control.
Metering device types
| Type | Category | How it works |
|---|---|---|
| Fixed orifice (piston) | Fixed | A fixed-diameter bore in a metal insert at the evaporator inlet, sized per system |
| Capillary tube | Fixed | A long small-bore tube of fixed length and diameter; older A/C and small refrigeration |
| Thermostatic expansion valve (TXV) | Modulating (mechanical) | Meters refrigerant to hold a target evaporator superheat via a bulb, spring, and diaphragm |
| Electronic expansion valve (EEV) | Modulating (electronic) | A stepper-motor valve driven by a controller reading thermistors and transducers — most precise |
Sizing a TXV
- Rated in nominal tons — match the valve capacity to the system tonnage.
- Refrigerant-specific — an R-410A TXV is not interchangeable with an R-22 TXV (R-410A runs a much higher design pressure drop).
- Rated at a stated pressure drop across the valve — the actual available pressure drop changes the delivered capacity.
- Oversized → hunting: the valve overshoots the superheat setpoint and oscillates.
- Undersized → starving: the evaporator is underfed, superheat runs high, and capacity drops.
Mounting the sensing bulb
- Mount on a clean, straight, horizontal section of the suction line — never on a fitting or joint.
- Clamp with a secure metal strap for solid metal-to-metal contact — never tape or zip ties.
- Fully insulate the bulb after mounting so it reads refrigerant temperature, not ambient.
- Never mount at the 6 o’clock (bottom) position — oil pooling makes the bulb read oil temperature.
Bulb clock position by line size
| Suction line OD | Bulb clock position |
|---|---|
| ≤ 7/8" OD | 12 o’clock (top) |
| 7/8" to 1-5/8" OD | 10 or 2 o’clock |
| Over 1-5/8" OD | 4 or 8 o’clock |
Why placement is everything
A TXV controls the valve opening from the temperature its bulb senses on the suction line, so a bulb that reads wrong makes the valve act wrong. Two things corrupt the reading: poor contact and oil. Loose mounting, tape instead of a metal clamp, or skipping the insulation all let the bulb drift toward ambient temperature; mounting at the bottom of the line lets oil pool under the bulb so it reads oil rather than refrigerant. Either one throws off the superheat the valve is trying to hold, which is why placement is treated as carefully as sizing.
Because a TXV holds evaporator superheat on its own, a TXV system is charged by subcooling, not superheat — see the superheat & subcooling reference for the charging method that pairs with each metering device.
Common questions
How is a TXV sized?
By three things: the system capacity in tons, the refrigerant, and the pressure drop across the valve. A TXV is rated in nominal tons and must match the system — and it must be the right refrigerant, since an R-410A TXV is not the same as an R-22 TXV. Oversizing makes the valve hunt (overshoot and oscillate); undersizing starves the evaporator. Aim within roughly 10–20% of the actual capacity, not grossly oversized "for headroom."
Where does the TXV sensing bulb mount?
On a clean, straight, horizontal section of the suction line, clamped with a metal strap for solid contact and then fully insulated. The one hard rule is to never mount it at the 6 o’clock (bottom) position, where oil pools and makes the bulb read oil temperature instead of refrigerant temperature. The clock position moves lower as the line gets bigger — roughly 12 o’clock on small lines down to 4 or 8 o’clock on large ones.
What is the difference between a TXV and a fixed orifice?
A fixed orifice (piston) or capillary tube is a fixed restriction — it cannot adjust, so its performance drifts as conditions change. A TXV modulates mechanically to hold a target evaporator superheat across varying load, which makes it more efficient and is why it is charged by subcooling rather than superheat. An electronic expansion valve (EEV) does the same job with a controller and stepper motor for even tighter control.
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