How to Read a Charging Chart for 4-Ton R-32 AC Systems (And Actually Understand It)

Hey, It’s Mark 📈

Look—reading a charging chart isn’t about memorizing pressure numbers or trusting your gut. It’s about precision, especially with R-32 refrigerant, which is sensitive to overcharging and hates shortcuts.

I’ve seen installs where everything looked fine—cool air, happy customer—but the system was slowly cooking itself from the inside because the subcooling was off by five degrees.

When you’re dealing with 4-ton R-32 systems, the margin for error is razor-thin. And if you’re not reading that charging chart right, you’re flying blind.

So today, we’re going deep. I’ll walk you through how to:

• Actually read and use a charging chart

• Understand what subcooling/superheat really tell you

• Match the chart to your real-world conditions

• Avoid common mistakes I’ve seen kill otherwise good systems

Let’s get into it.

 

What Is a Charging Chart Anyway? 🤔

A charging chart is basically a map. It tells you what your system’s refrigerant pressures, temperatures, and targets should be based on a few factors:

• Outdoor ambient temperature

• Line set length

• Refrigerant type (R-32, in this case)

• System tonnage

• Metering device type (TXV or piston)

For most 4-ton R-32 systems, the charging chart is printed right on the outdoor unit data plate or in the installation manual.

It usually looks like a big grid, showing how much subcooling or superheat you should have at certain outdoor temperatures.

Need a refresher on the difference between superheat and subcooling? HVAC School breaks it down here.

 

Why You Can’t “Set It and Forget It” with R-32 ❄️

Let me be blunt: R-32 doesn’t tolerate guessing. You’re dealing with higher pressures and a tighter performance envelope than older refrigerants.

That means even a half-pound over or under charge can lead to:

• Higher energy bills

• Poor dehumidification

• TXV hunting

• Frosted coils

• Compressor burnout (yikes)

You’ve got to charge based on performance, not just pressure. That’s where the chart comes in.

 

First Step: Understand Your Metering Device

Before you even look at a chart, figure out if your system uses a:

• TXV (Thermal Expansion Valve)

• Piston or fixed orifice

Why? Because this determines whether you’ll charge by subcooling (TXV) or superheat (piston).

TXV = Subcooling is King
Piston = Superheat Rules the Day

Most 4-ton R-32 systems from Goodman, Daikin, or Mitsubishi use a TXV. But don’t assume—check the spec sheet.

Need help figuring it out? The AHRI Directory can tell you if your indoor coil uses a TXV.

 

Subcooling: The TXV Charging Method 🔍

Subcooling tells you how much liquid refrigerant is in the condenser coil before it flashes into vapor. Too much? You’re flooding the coil. Too little? You’re starving the TXV.

How to measure:

1. Hook up your digital gauges

2. Let the system stabilize (10–15 min runtime)

3. Measure liquid line temperature

4. Compare to saturated liquid temp on your manifold (based on pressure)

5. Subtract temp from saturated temp = subcooling

Example:

• Saturated temp (based on 405 psi R-32) = 122°F

• Liquid line temp = 110°F

• Subcooling = 12°F

If your system’s charging chart says to target 10–12°F subcooling at 85°F ambient, you’re golden.

Want to practice with a live chart? Goodman’s install manuals often include real charging examples for R-32.

 

Superheat: When You’re Working with a Piston 🔥

Superheat measures the temp of the refrigerant vapor leaving the evaporator. If it’s too low, you’re risking floodback. Too high? Your evaporator is starving.

To measure:

1. Take suction line temp

2. Read low-side pressure → saturated suction temp

3. Subtract the two = superheat

Example:

• Suction pressure = 120 psi (R-32 saturated temp = 40°F)

• Suction line temp = 60°F

• Superheat = 20°F

Use a superheat chart to know your target based on indoor wet bulb and outdoor dry bulb. Want a solid chart? HVAC Know It All has one here.

 

Using a Real Charging Chart: Walkthrough 🛠️

Let’s say you’ve got a Goodman 4-ton R-32 system with a TXV. Here’s a real-world example of how you’d use a charging chart:

Conditions:

• Outdoor temp: 95°F

• Factory charge listed: 210 oz for 15 ft line set

• Line set is 25 ft (add 6 oz)

• Total charge: 216 oz

• Subcooling target at 95°F: 13°F

Steps:

1. Pull vacuum to 500 microns or lower

2. Charge by weight to 216 oz

3. Start the system and let it stabilize

4. Measure liquid line temp

5. Compare to saturated liquid temp

6. Adjust charge to hit 13°F subcooling

Too low? Add refrigerant slowly.
Too high? Recover a few ounces.

Simple? Yes. But also critical.

 

Ambient Conditions Matter (A Lot) 🌡️

Charging on a 70°F day isn’t the same as a 95°F scorcher. Your subcooling target might only be 9°F at lower temps and jump to 14°F as ambient climbs.

That’s why charts often have correction tables or altitude adjustments.

If your chart doesn’t include ambient adjustment, check the COP (Coefficient of Performance) data published by your manufacturer. This can help you make informed tweaks.

 

Line Set Length: Don’t Skip This Calculation 📐

4-ton systems often have longer line sets—especially in multi-level homes. Most manufacturers include refrigerant add-on charts based on line length over 15 ft.

Goodman’s rule?
Add 0.6 oz of R-32 per foot over 15 ft of line set.

So if you’ve got 30 ft of line set:

• 15 ft included

• 15 ft extra

• 15 x 0.6 = add 9 oz refrigerant

Skip this, and your subcooling will never hit the mark.

 

Tools That Make Chart Reading Easy 🧰

Mark’s always rolling with these:

• Testo 550s – built-in R-32 profiles

• Fieldpiece SM480V – rugged with app-based chart integration

• UEi DL479 multimeter – for verifying compressor amp draw during charge

• BluVac micron gauge – for before/after vacuum pull validation

• Fieldpiece Job Link app – syncs your temp probes and auto-calculates targets

And yes, if you’re still using analog gauges and guessing numbers off a clipboard? You’re setting yourself up to fail.

 

Common Mistakes I See with Charging Charts ❌

Let me save you some grief:

• Charging by pressure only – useless without temp context

• Not adjusting for ambient temp – leads to undercharge

• Guessing line length – leads to subcooling mismatch

• Not using R-32 compatible tools – pressures are not the same as R-410A

• Not re-checking after 15 minutes of runtime – readings fluctuate

Charging is not a one-and-done thing. You measure, wait, recheck, fine-tune. That’s the game.

 

Mark’s Final Word 💬

I’ve said it before and I’ll say it again—charging a 4-ton R-32 system is not the time to freelance. You’ve got a powerful, high-pressure, highly efficient piece of HVAC engineering on your hands. If you want it to last, perform, and pass inspection?

You’ve got to get the charge right.

The good news? Once you understand the chart, it’s easy. It’s just matching your real-world readings to what the manufacturer already figured out for you.

And if you’re just getting started and want to make sure your system is built to perform, check out The Furnace Outlet’s 4-ton R-32 AC kits. They’re matched, tested, and come ready with charging specs right in the box.

Need to know how to prep your home before you install your 4-ton R-32 AC? Visit my guide: The Room-by-Room Checklist.

Keep those gauges clean, your scales zeroed, and as always…
don’t half it.
–Mark, your go-to tech 📊🔧