Line Set Sizing, Length and Lift Field Card
Why this matters
The line set is the one part of a split system the maker does not build, so it is the part most likely to be wrong on a callback. Undersize the suction and the system loses capacity to pressure drop. Oversize it and oil stops coming back up a riser. Ignore the lift column and a liquid line that climbs into an attic starts flashing before it reaches the metering device, which reads on the gauges like low charge and gets "fixed" with more refrigerant. Every one of those numbers lives in a table in the outdoor unit's installation instructions, and this card is about reading that table correctly. This card owns line sizing and the line-length charge for the shelf; the charging procedures cite it.
The table, as a maker prints it
Below is an excerpt in the shape most makers use for a residential R-410A heat pump. The values are illustrative. The structure is what you are learning to read; the numbers for the job come from the unit's own installation manual, every time.
LINE-SET TABLE (ILLUSTRATIVE) R-410A HEAT PUMP
Nominal Liquid Suction Suction alternate
tons OD in std OD in OD in, limit
2.0 3/8 3/4 5/8, 25 ft max length
2.5 3/8 3/4 7/8, coil level/above
3.0 3/8 7/8 3/4, 50 ft max length
4.0 3/8 7/8 1-1/8, coil level/above
5.0 3/8 1-1/8 7/8, 50 ft max length
Factory charge covers : 15 ft of line set
Add beyond 15 ft : 0.6 oz per ft (3/8 liquid)
Max total length : 150 ft
Long-line kit required : over 80 ft total
Max lift, outdoor ABOVE indoor : 50 ft
Max lift, outdoor BELOW indoor : 25 ft
Lengths are equivalent ft (fittings included)
Five things are on that page, and each protects something different.
What each column is protecting
Liquid diameter. The liquid line has one job: deliver solid liquid to the metering device. Too small and friction drops the pressure below saturation before the TXV or EEV, so flash gas forms in the line. Too large and it holds more refrigerant than the charge was built around, which is why the added charge per foot is tied to a liquid diameter. Residential tables almost always land on 3/8 inch from 2 through 5 tons; the occasional 1/4 inch or 5/16 inch line on small equipment and ductless units has its own per-foot figure.
Suction diameter. This is the real sizing decision. Bigger suction means lower pressure drop and more delivered capacity; smaller suction means higher vapor velocity, which is what carries oil back to the compressor. The standard size is the maker's balance point. The alternate column is where the maker tells you how far you may trade one against the other, and it always carries a condition.
Factory charge length. The outdoor unit ships holding enough refrigerant for itself, a matched indoor coil and a stated length of line, commonly 15 feet on residential equipment. Every foot past that needs refrigerant added; a line shorter than the stated length does not mean you remove any unless the manual says to.
Added charge per foot. This is a liquid-line number, because the liquid line holds almost all the refrigerant in the line set by weight. The suction line is full of vapor and contributes little. A per-foot figure applied to the wrong liquid diameter is a common way a long-line calculation goes wrong.
Length and lift limits. These are the edges of the maker's tested envelope. Past them, the maker either requires accessories or does not support the installation at all.
Equivalent length, not tape length
Most tables are written in equivalent feet: tape length plus an allowance for each fitting, because a long-radius elbow costs pressure like a short stretch of straight pipe. Makers either publish an equivalent-length figure per fitting or tell you to count only the tape and leave margin. Read which one your table means before you compare a run to the 50 foot limit on an alternate suction size, because a 44 foot tape run with six elbows can sit over 50 equivalent feet.
The added-charge calculation is different: it uses the actual length of liquid line, since refrigerant mass depends on the volume of pipe, not the friction in it. Two lengths, two uses. Keep both on the job sheet.
Lift: which way is up decides the problem
Vertical separation is not symmetric, which is why the table gives two lift limits. The flows below are described in cooling. On a heat pump in heating they reverse: the liquid runs from the indoor coil to the outdoor metering device, so with the outdoor unit above, it is the liquid that climbs, which is one reason heat pump tables set their own lift limits.
Outdoor unit ABOVE the indoor coil (condenser on a roof, air handler in a basement). In cooling the suction vapor has to climb from the coil up to the compressor, carrying oil with it. That is a velocity problem: the suction line must stay small enough to keep oil moving up the riser at the lowest operating capacity. This is where an oversized suction does real damage, and on long rises some makers call for an oil trap at the base of the riser.
Outdoor unit BELOW the indoor coil (condenser on grade, air handler in an attic, or a multistory building with the coil on an upper floor). Now the liquid line climbs. A column of liquid R-410A weighs about 0.43 psi per foot at a 100 F liquid temperature, and every psi lost to that column moves the liquid closer to saturation. The subcooling built in the condenser is what pays for the climb. Too much lift and the liquid flashes before the metering device. This is why the outdoor-below limit is usually the tighter of the two.
OUTDOOR BELOW INDOOR (the tighter limit)
attic air handler
+-------------+
| TXV coil |
+--+-------+--+
| |
liquid| |suction
climbs| |falls (oil drains
loses | |toward compressor)
psi | |
| |
+--+-------+--+
| outdoor | grade
+-------------+
When the long-line rules apply
Past the long-line threshold in the table (80 feet in the excerpt above), most makers require some combination of accessories, listed in the manual by name and function:
- A crankcase heater, because a long line holds more refrigerant that can migrate to a cold compressor during the off cycle and slug it on start.
- A liquid line solenoid or similar off-cycle control, on some equipment, to stop that migration at the source. On a heat pump, where the liquid line carries flow in both directions, the maker specifies the bi-flow arrangement.
- A hard start kit on some single-phase compressors, where the off-cycle pressures do not equalize quickly enough.
- Reduced suction alternates or a forbidden alternate, because pressure drop now costs more capacity.
The capacity loss over a long line is real even when every limit is met. Most long-line appendices publish a capacity correction factor by length and suction size; on a long run, that factor belongs on the load calculation, not discovered at the first heat wave.
When a bigger or smaller suction is the right call
Reuse of an existing line is the usual reason to leave the standard column. The house has a 3/4 inch suction and the new 3-ton unit wants 7/8. The table's alternate entry answers the question: in the excerpt, 3/4 inch on a 3-ton is acceptable to 50 equivalent feet with a capacity penalty the appendix prices out. If the existing run is 60 feet, the alternate is outside its limit and the line gets replaced, whatever a supply house counter says about it "working fine on lots of jobs." Whether that line is clean and tight enough to reuse at all is a separate question. See related: How to Check an Existing Line Set Before Reusing It.
Upsizing the suction is legitimate when the run is long and horizontal, the indoor coil sits level with or above the outdoor unit, and the maker lists the larger size. It is wrong when the coil is below the outdoor unit and the vapor has to climb, or when the system is two-stage or variable capacity. Low stage moves a fraction of full-load mass flow through the same pipe, so a suction sized for comfortable velocity at full load can lose oil return at minimum speed. Inverter equipment tables are often stricter about alternates for exactly this reason.
The rule of thumb never beats the table. "3/8 by 7/8 up to 4 tons" is a decent memory aid and nothing more. Ductless and inverter equipment in particular may size differently, use flare connections at fixed diameters, and charge per foot at a different rate. See related: Mini-Split Flare Torque and Flare Size Field Card.
Worked example: a 3-ton heat pump with the air handler in the attic
A new 3-ton R-410A heat pump, outdoor unit on a pad at grade, air handler in the attic. Using the illustrative table above:
Measured run. Tape length 62 feet, with five long-radius elbows. The maker's appendix lists an equivalent length for each elbow; say it is 2 feet each here (illustrative). Equivalent length is 62 + (5 x 2) = 72 equivalent feet.
Lift. The outdoor unit is 22 feet below the air handler's liquid connection, measured vertically, not along the pipe. That is outdoor BELOW indoor, limit 25 feet. 22 is under 25, so it is inside the envelope with 3 feet to spare.
Length. 72 equivalent feet is under the 150 foot maximum and under the 80 foot long-line threshold, so no long-line kit. Had the run picked up five more elbows (82 equivalent feet), it would have crossed 80 and the crankcase heater and off-cycle control would be required.
Line sizes. 3/8 inch liquid, 7/8 inch suction, the standard column. The alternate 3/4 inch suction would have been limited to 50 equivalent feet; at 72 it is not an option.
Added charge. Use actual liquid line length, not equivalent: 62 feet minus the 15 feet the factory charge covers leaves 47 feet. 47 x 0.6 oz = 28.2 oz, call it 28 oz, which is 1 lb 12 oz weighed in after evacuation.
What the lift costs the liquid. 22 feet x 0.43 psi per foot = about 9.5 psi of static loss. Near a 100 F condensing temperature, R-410A saturation moves about 4.5 psi per degree, so 9.5 / 4.5 = about 2 F of subcooling spent on the climb alone, before friction. A condenser delivering 10 F of subcooling arrives at the TXV with roughly 8 F, less whatever friction takes. That is fine. A unit already running low subcooling from a marginal charge would arrive at the TXV with almost none, and the symptom would be erratic superheat and a hissing metering device.
Final charge. The weighed-in 28 oz is the starting point; the charge is finished by the maker's subcooling target at the outdoor unit. See related: How to Calculate and Add Charge for a Long Line Set, and How to Charge a TXV System by Subcooling.
What would change this answer
- Move the outdoor unit to the roof with the air handler still in the attic below it. The lift now runs the other way, the limit becomes 50 feet, and the concern shifts from liquid flash to oil return up the suction, so an oversized suction becomes the thing to refuse.
- Swap to a two-stage outdoor unit. The maker's two-stage table may drop the 3/4 inch alternate entirely, and the subcooling target and per-foot rate may differ.
- Swap to an R-454B unit. The per-foot charge rate differs, and the added charge raises the total system charge that the A2L room-area check is run against. See related: A2L Equipment Install Requirements Field Card.
References
- Outdoor unit installation instructions: line-set table, long-line appendix, equivalent-length allowances, capacity correction factors
- See related: How to Calculate and Add Charge for a Long Line Set; How to Install and Insulate a New Line Set
- See related: HVAC Superheat and Subcooling Method Reference; Evacuation and Vacuum Decay Verification
- See related: Condensing Unit Nameplate Read for Install and Service