How to Charge 4-Ton R-32 AC Systems Without Screwing It Up

Alright, Here We Go Again.

I can’t tell you how many times I’ve been called in to fix a 4-ton R-32 system that was “acting weird” — short cycling, blowing warm air, or tripping on high head pressure.

You know what 9 out of 10 of those cases had in common?

Bad charging.
Either it was overcharged by a pound, undercharged by a few ounces, or worse — "eyeballed" by a tech who thought they could beat the charts.

Listen, when you’re working with a 4-ton AC system and using R-32, you’re dealing with tight tolerances, higher operating pressure, and a refrigerant that doesn’t forgive sloppiness.

Let’s get it right. I’ll show you:

  • What tools to use

  • What mistakes to avoid

  • How to charge a 4-ton R-32 like a pro

 

Why 4-ton R-32 Systems are Tricky to Charge

First, let’s talk size. A 4-ton unit means you’re moving 48,000 BTUs per hour — that’s a lot of cooling, and a lot of refrigerant.

With R-32’s higher pressure, a slight mistake in your charge quantity can mean:

  • high head pressure lockouts

  • floodback and oil washout

  • short cycling

  • compressor damage

  • poor dehumidification

On top of that, 4-ton units often have:

  • longer line sets

  • bigger coils

  • multiple expansion devices (TXVs or EEVs)

  • large filter-driers and suction accumulators

These all impact charge volume. So if you’re “just topping off” without weighing it in or checking superheat/subcooling? You’re flying blind.

 

Get Your Tools Right

Before you even think about cracking open a refrigerant tank, you better have these on deck:

Digital manifold gauge set – Analog won’t cut it. You need accurate high/low pressure readings. I use a Fieldpiece SM380V or similar.

Refrigerant scale – No kitchen scales. Use a proper, A2L-rated refrigerant scale with gram accuracy.

Vacuum pump with core removal – Pull down to 500 microns or less, and verify with a vacuum gauge. Core removal tools are non-negotiable on big systems.

Nitrogen regulator and tank – You’ll need this for brazing and pressure testing. Nitrogen purging isn’t a “nice-to-have” on 4-ton R-32 jobs — it’s required.

A2L recovery tank – You cannot use a leftover R-410A recovery tank for R-32. You’ll need one with proper labeling and pressure ratings. Not sure what qualifies? AHRI lays it out clearly in their updated A2L refrigerant recovery and safety guidance.

 

Charging Procedure for 4-ton R-32 Units

Let’s go step-by-step.

 

Step 1: Know the factory charge

Every outdoor unit comes with a tag that says factory refrigerant charge. This is typically for 15 feet of line set.

Example:
“Pre-charged with 119 oz of R-32 for 15 ft. of 3/8” and 7/8” line.”

If you’re running a 25 ft line set, you’ll need to add additional charge — usually around 0.6 oz per foot of added tubing. But always check the manufacturer’s instructions.

A good reference for this? Trane has a detailed residential split system charging guide that works great as a baseline, even if you’re not using their equipment.

 

Step 2: Pull a proper vacuum

This isn’t optional on a 4-ton system. With this much refrigerant in play, moisture and non-condensables will destroy efficiency.

  • Use core removal tools

  • Pull vacuum to 500 microns or less

  • Let it sit for 10 minutes to ensure no rebound

No moisture = no compressor burnout down the road.

 

Step 3: Charge by weight (not pressure)

With R-32, always start by charging by weight — especially with a brand-new system or long line set.

  • Put the tank on the scale

  • Invert if needed (check tank label)

  • Open slowly to avoid shocking the system

  • Add refrigerant until the full calculated charge weight is in

Then — and only then — go to pressure testing.

 

Step 4: Fine-tune with superheat/subcooling

Now the real work begins. Run the system for at least 15 minutes, and check:

  • Subcooling (condenser-side) — typically 10–15°F

  • Superheat (evaporator-side) — depends on TXV or fixed orifice

Use the charging chart provided by the manufacturer. Don’t freelance. That 4-ton unit’s performance curve is tight, especially with R-32’s thermodynamic properties.

Want more depth? The team at HVAC School put together a fantastic charging guide for R-32 that breaks it down in plain English.

 

Top Mistakes to Avoid (Especially on 4-ton Installs)

Let me save you some callbacks. Here’s what I’ve seen kill performance:

Charging by sight glass – Not reliable with R-32. You’ll under- or overcharge it every time.

Guessing on added line set charge – Measure. Use the table. Add with precision.

Liquid into the suction line – Don’t do it. Always meter vapor into the low side slowly or use a charging cylinder.

Skipping altitude adjustments – At high elevations, pressures behave differently. Forane’s charging altitude chart is a handy tool if you’re up in the mountains.

Charging when airflow’s restricted – Dirty filters, iced coils, or blower issues? Don’t even attempt to charge until airflow’s dialed in. Need a full checklist? Energy Vanguard covers it well.

 

Final Thoughts from Tony

4-ton systems don’t give you much margin for error — and R-32’s higher pressure and tighter operating window make charging one a precision job.

If you’re not using a scale? You’re guessing.
If you’re not reading charts? You’re risking.
And if you’re not pulling a full vacuum? You’re gambling with a $1,500 compressor.

Don’t be that guy. Take your time, check your numbers, and charge like a technician — not a handyman.

Need a system that’s worth dialing in?
👉 Shop 4-Ton R-32 Air Conditioners at The Furnace Outlet

They’ve got the specs, the support, and the gear to back you up.

Need more troubleshooting tips? Visit my guide: Common Troubleshooting Issues with 4-Ton R-32 Systems (and How to Solve Them).

Until next time,
Keep it tight, keep it clean, and don’t forget your micron gauge.

– Tony the Trusted Tech

Tony’s toolbox talk

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