What it is

Weigh-in charging means putting in an exact amount of refrigerant by weight, measured on a refrigerant scale, instead of dialing in by superheat or subcooling. It's the most accurate method and the right one for a brand-new install or any system you've fully recovered and evacuated. You use the manufacturer's factory charge, then add (or subtract) for the actual line-set length.

This is the gold standard for commissioning because it doesn't depend on the weather. Superheat and subcooling methods get shaky in mild or cold conditions; a scale doesn't care what the temperature is.

How it works

The condensing unit ships with a factory charge stamped on the data tag — enough refrigerant for the unit, the indoor coil it's matched to, and a baseline length of line set (commonly the first 15 or 25 feet, depending on the manufacturer). Your actual line set is rarely exactly that length, so you adjust.

A longer line set holds more refrigerant, so you add an allowance per foot beyond the baseline. A shorter line set than the baseline means you'd remove a little. The per-foot allowance depends on the liquid-line diameter — bigger liquid line holds more refrigerant per foot. The manufacturer's instructions give the per-foot figure for each liquid-line size; common ballpark values are on the order of 0.6 oz per foot for a 3/8" liquid line, with larger lines holding more. Always use the figure from that unit's instructions.

In the field

  1. Recover any existing charge and evacuate the system to a deep vacuum (down to ~500 microns with a passing decay test). Weigh-in only works from a known-empty starting point.
  2. Read the factory charge and the baseline line-set length from the data tag.
  3. Measure your actual line-set length and note the liquid-line diameter.
  4. Calculate the adjustment: (actual length − baseline length) × per-foot allowance for that liquid-line size. Add for longer, subtract for shorter.
  5. Total charge = factory charge ± line-set adjustment.
  6. Put the tank on the scale, zero it, and meter in the calculated weight as liquid into the high side with the system off — or as liquid metered into the low side with the system running (through a charging valve so you don't slug the compressor).
  7. Verify after stabilizing. Once it's running and stable in good conditions, confirm with subcooling (TXV) or superheat (piston) as a cross-check. The scale gets you right on the money; the operating reading confirms it.

Normal values & targets

  • Factory charge: whatever the data tag says for that specific unit/coil match. It's not transferable between models.
  • Baseline line-set length: commonly the first 15 ft or 25 ft, per manufacturer.
  • Per-foot allowance: depends on liquid-line size; a 3/8" liquid line is often around 0.6 oz/ft as a representative figure — use the manufacturer's number for your line size.
  • Evacuation target before weigh-in: ~500 microns with a decay test that holds (doesn't rise rapidly when valved off).

Common faults & what they mean

  • Weighed in correctly but readings are off after stabilizing: suspect a mismatched coil, a metering-device issue, an airflow problem, or that the line set length/size was measured wrong. The weight was right for the assumptions; one assumption is off.
  • Couldn't reach a deep vacuum: you have a leak or moisture/non-condensables. Fix that before charging — weighing in on top of a system that won't hold vacuum just traps a problem.
  • System overcharged after weigh-in: double-check whether there was residual charge you didn't fully recover, or whether you used the wrong baseline length.

Tech tips & gotchas

  • Weigh-in is for an empty, evacuated system. It assumes you started from zero. You can't accurately "weigh in" on top of an unknown existing charge — you don't know your starting point. For a top-off on a system with charge already in it, use subcooling or superheat instead.
  • Don't forget the line-set adjustment. Skipping it is a common rookie mistake on long line runs — you'll end up undercharged on a long set or overcharged on a short one. The factory charge is only correct at the baseline length.
  • Liquid-line diameter drives the per-foot number, not the suction line. The suction line is vapor and holds comparatively little refrigerant by weight; it's the liquid in the liquid line that adds up.
  • Charge as liquid for blends to avoid fractionation, but never dump liquid straight into the suction port of a running compressor — meter it through a charging device or charge into the high side while off.
  • Confirm, don't assume. Even a perfect weigh-in deserves a quick subcooling/superheat sanity check once it stabilizes. If the cross-check is way off, something in your assumptions (coil match, line length) is wrong.

Safety / code notes

  • Proper evacuation isn't optional — moisture and non-condensables left in the system cause acid formation, capacity loss, and metering problems.
  • Recover per EPA 608; never vent.
  • Use a scale and charging equipment rated for the refrigerant and its pressure.