Charging an HVAC system means setting its refrigerant charge.
The methods the OEM instructions and ACCA's verification protocol name: weighing the charge in, charging to a superheat target, or charging to a subcooling target — and for the gauge methods, the target that counts is the equipment manufacturer's number for that model.
Around the work sits federal law: EPA Section 608 restricts who may buy refrigerant and, for substitutes such as HFCs, requires recovery and prohibits venting.
Weigh-in vs superheat vs subcooling: the charging methods, compared
OEM instructions and ACCA's installation-verification protocol name the ways to set a charge, and the metering device decides between the two gauge methods.
ANSI/ACCA 9 QIvp requires the subcooling test on systems with thermal expansion valves and the superheat test on systems with fixed metering devices — fixed orifice, capillary tube or piston — leaving the other test optional in each case.
Goodman's own instructions teach the same split from the manufacturer's side: its older split-system condensing-unit instruction charges capillary-tube and fixed-orifice systems by superheat from the manufacturer's superheat table, and expansion-valve systems by subcooling.
Charging to subcooling starts from the reading itself, which Goodman's GVXC20 instructions define as saturated liquid temperature minus liquid line temperature.
In practice, the same instructions walk through the steps: read the liquid pressure, convert it to temperature with a pressure/temperature chart, measure the liquid line temperature at the liquid service valve, and subtract to get subcooling.
That model's target is 8°F ± 1°F with a TXV indoor coil — 10°F ± 1°F on 5-ton units — and it is exactly that: that model's target.
The number for the unit in front of you comes from its own documentation, and ACCA 9 QIvp passes a charge when measured subcooling lands within ±3°F of the OEM-specified value, or measured superheat within ±5°F.
The third method takes the gauge reading out of the decision.
Goodman's charging table for the GVXC20 says that below 65°F outdoor ambient the charge should be weighed in rather than set by subcooling — the charge is set by measured weight instead of by a pressure–temperature reading.
Before any charge goes in on a new install, Goodman's GVXC20 instructions describe evacuating to 500 microns or less through both the gas and liquid service valves with a vacuum pump rated for 500 microns, and also recommend a "triple evacuation" method with dry-nitrogen breaks.
That is Goodman's procedure — hold and decay criteria differ by OEM — so the equipment's own instructions govern.
| System or condition | Method | Named by |
|---|---|---|
| TXV system | Subcooling (required test; superheat optional) | ACCA 9 QIvp · Goodman's expansion-valve instructions |
| Fixed orifice · capillary tube · piston | Superheat (required test; subcooling optional) | ACCA 9 QIvp · Goodman's fixed-orifice instructions |
| Below 65°F outdoor ambient | Weigh the charge in rather than set by subcooling | Goodman's GVXC20 charging table |
The two gauge methods lean on readings with their own definitions and measurement steps — our superheat and subcooling guide covers the formulas.
Charging is also one skill inside the trade's technical set: the HVAC technician skills guide maps the rest, from electrical troubleshooting to airflow.
When is charging the fix — and when is it not?
A charge measurement is a confirmation, not an instruction to add refrigerant.
ACCA's residential maintenance standard, ANSI/ACCA 4 QM, calls measuring superheat or subcooling "a good diagnostic field practice" for confirming refrigerant charge — the reading confirms what the charge is doing; it does not by itself say what to fix.
The verification protocol keeps the distinction formal.
ACCA 9 QIvp also checks airflow: contractor-recorded airflow must be within 15% of design airflow — a separate check from the charge readings, so the protocol makes you account for the air side alongside the charge tests.
A system that misses its subcooling or superheat target is not automatically a system that needs refrigerant.
Conditions decide whether a reading can be trusted at all.
ACCA 9 QIvp's form says to run the system at least 10 minutes — or until operation is stable — before measuring, and not to test cooling mode below 60°F outdoor air for the subcooling test or below 55°F for the superheat test.
Its footnotes let heat pumps below those outdoor temperatures be checked in heating mode, per the OEM's instructions.
Below 65°F, Goodman's answer for the GVXC20 is not a corrected reading at all but a weighed-in charge.
When a maintenance visit turns up a fix, ACCA 4 QM puts a process around it: the contractor must tell the homeowner about the recommended corrective actions and get prior approval for remedies not included in the inspection, and homeowner-authorized repairs must follow the OEM's instructions.
Where a charge reading sits in the wider sequence of checks is the diagnostic method itself — our HVAC troubleshooting guide covers it step by step.
Recovery and venting: the EPA 608 lines around charging
Charging work happens inside a federal rule.
Clean Air Act §608(c)(2) prohibits knowingly venting or otherwise knowingly releasing substitute refrigerants — EPA restates the prohibition in its A2L SNAP listings, so it covers HFCs and the newer mildly flammable refrigerants alike.
So a charge that comes out of a system is recovered, not released.
Recovery has its own requirements: EPA kept the certification, sales restriction, evacuation, certified-recovery-equipment, safe-disposal and reclamation requirements for appliances using only substitute refrigerants — so recovery runs through certified recovery equipment under EPA's rule.
Who may buy refrigerant is restricted too: only EPA-certified technicians — Section 608 for stationary refrigerants, 609 for motor-vehicle AC — may purchase ozone-depleting or substitute refrigerants such as HFCs, with limited exceptions.
One exception matters at the shop level: an employer may buy if it gives the wholesaler written evidence that it employs at least one certified technician.
Apprentices are a different exception, and it is narrow: EPA's rule exempts an apprentice from 608 certification — the certification requirement, not the sales restriction — only while the apprentice is closely and continually supervised by a certified technician.
The supervising technician and the apprentice share responsibility for compliance, and the rule sets no time limit on the exemption.
Our guides for HVAC apprentices and on EPA 608 for apprentices cover the supervised route.
Federal rules, not shop preferences
Charging an A2L system: what changes
The first rule of A2L charging is that there is no drop-in retrofit.
EPA says its Significant New Alternatives Policy (SNAP) program and industry standards prohibit putting flammable or mildly flammable refrigerants such as HFC-32 or R-454B into systems that were not designed to use them.
What our sources state about the method is the required-test rule: ACCA 9 QIvp keys it to the metering device — subcooling for systems with thermal expansion valves, superheat for systems with fixed metering devices — and the target the readings are judged against is the OEM's value for the model.
Our research captured no A2L-specific manufacturer charging procedure, so treat the unit's own instructions as the procedure.
What changes is the equipment the charge goes into, and the conditions SNAP attaches to it.
R-452B, R-454A, R-454B, R-454C and R-457A are acceptable, under EPA's SNAP rules, in new residential and light-commercial AC and heat pumps only, subject to use conditions: meeting UL 60335-2-40 (3rd edition), charge limits by room size, and red — PMS #185 — marked service ports.
The red service ports are the part you see at the gauges: equipment under these listings marks the connection point.
Recovery changes too.
EPA's SNAP use conditions call for recovery equipment designed for flammable refrigerants, and note that recovered flammable refrigerant from residential and light-commercial AC is likely hazardous waste under RCRA.
On training, the federal position is a recommendation, not a certification: EPA's SNAP use conditions say personnel who commission, maintain, repair, decommission or dispose of these A2L appliances should obtain training and follow practices consistent with Annex HH of UL 60335-2-40.
EPA's AIM Act FAQ — updated September 14, 2026 — answers no when asked whether technicians already certified under Section 608 or 609 need to be re-certified.
What the A2L transition means for the job beyond the charge is its own topic: our A2L refrigerant transition guide covers it.
Common charging mistakes to avoid
None of the mistakes below are exotic — each one traces to a requirement a standard or an OEM instruction actually states, which is what makes them worth self-auditing on every charge call.
- Charging to a number learned on the last job. The target is the OEM's specified value for the model in front of you — ACCA 9 QIvp passes within ±5°F of the OEM-specified superheat or ±3°F of the OEM-specified subcooling, and Goodman's GVXC20, for one example, targets 8°F ± 1°F subcooling (10°F ± 1°F on its 5-ton units). Look it up in the OEM documentation before you connect gauges.
- Setting a subcooling charge in the cold. Goodman's GVXC20 charging table says that below 65°F outdoor ambient the charge is weighed in rather than set by subcooling.
- Measuring before the system settles. ACCA 9 QIvp's form says to run the system at least 10 minutes — or until operation is stable — before measuring.
- Testing cooling mode below the form's outdoor limits. ACCA 9 QIvp's form says not below 60°F outdoor air for the subcooling test or 55°F for the superheat test; heat pumps below those temperatures may be checked in heating mode per the OEM's instructions.
- Trusting a partial-capacity reading on inverter equipment. Goodman notes GVXC20 subcooling readings are only valid while the unit runs at 100% capacity — 100% of compressor speed in its charge mode.
- Chasing the charge target while the air side is unchecked. ACCA 9 QIvp also checks airflow: contractor-recorded airflow must be within 15% of design airflow — separate from the charge tests.
- Venting the charge you are removing. Clean Air Act §608(c)(2) prohibits knowingly venting or releasing substitute refrigerants, and removal goes through certified recovery equipment under EPA's rules.
- Refrigerant from a different system, straight in. EPA says SNAP and industry standards prohibit flammable or mildly flammable refrigerants such as HFC-32 or R-454B in systems not designed for them.
- Opening the refrigerant circuit without certification or supervision. Only EPA-certified technicians may buy refrigerant, with limited exceptions, and an apprentice is exempt from 608 certification only while closely and continually supervised by a certified technician.
The OEM documentation, the outdoor temperature, the unit's capacity mode, the certification status of the person holding the hose — know those four before the charge starts, and the list above takes care of itself.
Build the habit early and it carries through every charge call you run as a working HVAC technician.
Trade information, not legal advice: charging targets come from each equipment manufacturer's instructions for the specific model, and the refrigerant rules come from EPA's Section 608 — confirm both against the current documents before you act on them.