INTRO (0:00 to 0:30)
A system is sitting at 500 microns with a passed decay test. The repair is done. Now the question that actually decides whether this system performs for the next decade: how much refrigerant goes back in? Not "until the line sweats." Not "until it feels cold." A number. The engineers calculated it, the data plate prints it, and there are exactly three legitimate ways to deliver it. This video is the map: which method, when, and why.
ON-SCREEN: WEIGH-IN. SUPERHEAT. SUBCOOLING. Beneath: "the situation plus the metering device picks the method"
MAIN (0:30 to 4:30)
Beat 1: The decision tree (0:30 to 1:20)
Walk the C17 decision tree visual. First question: is this a new install, did the circuit get opened, or is the charge unknown or suspect? Any yes means weigh-in, the gold standard. No reading on a running system tells you its history; a scale does not need history. Second question, only for trim work on a sealed running system: what is the metering device? TXV or EEV: charge to subcooling, 8 to 12 degrees unless the nameplate gives its own number. Fixed orifice, the piston: charge to superheat, from a chart. And over all of it, one override: when the manufacturer's plate or chart speaks, it outranks every default.
ON-SCREEN: C17-method-decision-tree.svg, animated branch by branch
Beat 2: Weigh-in and the line set math (1:20 to 2:30)
Weigh-in is reading the factory charge off the data plate and correcting it for the real line set. The plate says the charge covers a standard length, commonly 15 feet. The liquid line is the part that matters, because it is full of dense liquid; the correction for 3/8 inch liquid line is typically 0.6 ounces per extra foot.
Whiteboard the worked example, slowly. Plate: 8 pounds 12 ounces, includes 15 feet. Actual line set: 40 feet. Extra: 25 feet. Adjustment: 25 times 0.6 is 15 ounces. Total: 8 pounds 12 ounces plus 15 ounces. And here is where careers are made: that is 27 ounces, and a pound is sixteen ounces, not ten. So the answer is 9 pounds 11 ounces. On a new install, that charge goes in as liquid, into the liquid line, with the system off and under vacuum. Whatever will not transfer after pressures equalize gets throttled in as the system runs, low side valve barely cracked, so liquid flashes to vapor before it ever reaches the compressor.
ON-SCREEN: C17-lineset-adjustment-math.svg
ON-SCREEN: C17-weigh-in-setup.svg with liquid path highlighted, then throttle path
Beat 3: Superheat method, the moving target (2:30 to 3:25)
From F6: on a piston system the orifice cannot adjust itself, so charge level directly controls how the coil is fed, and superheat is your charge gauge. But the target moves. It comes from a chart with indoor wet bulb on one axis and outdoor dry bulb on the other. Humid house, mild day: target in the twenties. Dry house, brutal afternoon: the target slides toward zero, and when the chart gives you less than about 5 degrees, the method is telling you no. Weigh in instead. And never charge a piston system by subcooling: nothing in a fixed orifice system regulates the condenser liquid level, so subcooling wanders with the weather even at perfect charge.
ON-SCREEN: C17-superheat-charging-chart.svg, finger-trace one lookup: wet bulb 64, outdoor 95, target 9
Beat 4: Subcooling method and the loop (3:25 to 4:30)
From F6: a TXV holds superheat at its setpoint, so added charge stacks as liquid in the condenser and shows up as subcooling. That is why TXV and EEV systems charge to the subcooling target. The skill is the loop: measure, compare, adjust 2 to 4 ounces, then wait until the readings hold still, then read again. Never adjust against a moving number. And superheat on a TXV is a verification, not a target: if subcooling is right but superheat is wild, the charge is fine and the valve or a restriction is your problem. Chasing superheat with the charging hose is how TXV systems end up a pound overweight with the valve calmly hiding the evidence.
ON-SCREEN: C17-subcool-charging-loop.svg, loop animated: measure, compare, adjust, wait, re-read
ON-SCREEN: Add charge: subcooling rises. Recover: subcooling falls. The truth-teller on a TXV is subcooling
OUTRO (4:30 to 4:45)
Weigh-in when the circuit was open or the charge is suspect. Superheat chart for pistons. Subcooling target for TXVs. Document every final number. In the next video Darrel runs a real weigh-in start to finish, line set math included, and proves the charge with subcooling before the panel goes back on.
ON-SCREEN: Next: Darrel weighs in a charge and verifies it