10.5 System Charging, Combustion Venting & Startup/Commissioning

System Charging, Combustion Venting & Startup/Commissioning

The final step in any HVAC installation — and a heavily tested topic on the C-20 trade exam — is bringing the system to life correctly: charging it with the right amount of refrigerant, venting a gas furnace's combustion byproducts safely, and running a documented startup sequence that proves the system performs the way it was designed to. Getting this stage wrong hides itself for months (a slightly undercharged system still cools, just less efficiently) until it shows up as a comfort complaint, a compressor failure, or a carbon monoxide incident.

Refrigerant Charging Methods

Three charging approaches show up on the exam, and choosing the right one depends on the equipment's metering device:

• Weigh-in charging. The equipment's installation manual specifies a factory charge for a stated line-set length (for example, a base charge sized for a 15- or 25-foot line set) plus an additional charge per foot for any length beyond that. The installer weighs refrigerant into the system using a calibrated scale to hit that exact target rather than charging to a gauge feel. Weigh-in accuracy matters even more with the low-global-warming-potential (GWP) refrigerant blends now used in new equipment, where correct charge weight directly affects both performance and the refrigerant's flammability classification margins.
• Superheat method — used on systems with a fixed-orifice metering device (not a thermostatic or electronic expansion valve). Because a fixed orifice doesn't actively regulate refrigerant flow, superheat rises and falls predictably with the amount of charge in the system, making it a reliable charge indicator. Target superheat is read from a manufacturer's target-superheat chart based on outdoor and indoor conditions rather than a single fixed number; overcharging lowers superheat (and can flood the compressor with liquid), while undercharging raises superheat (and starves capacity).
• Subcooling method — used on systems with a thermostatic expansion valve (TXV) or electronic expansion valve (EXV). Because a TXV or EXV actively modulates its opening to hold evaporator superheat within a narrow target range on its own, superheat stops being a useful charge indicator on these systems — subcooling at the condenser outlet becomes the reliable measurement instead, typically targeted in a manufacturer-specified range.

As of January 1, 2025, new residential and light-commercial air-conditioning and heat-pump equipment manufactured for the U.S. market moved away from R-410A toward lower-GWP refrigerants such as R-454B and R-32 under the U.S. Environmental Protection Agency's (EPA) AIM Act (American Innovation and Manufacturing Act) Technology Transitions Rule. Both replacement refrigerants carry an A2L safety classification under ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) Standard 34 — A for lower toxicity and 2L for lower flammability, meaning a mildly flammable refrigerant with a specified maximum burning velocity — which means installers must follow manufacturer-specific handling, leak-detection, and brazing procedures for A2L systems rather than treating them identically to R-410A. Existing R-410A equipment already in service is not affected, and R-410A remains available for servicing those systems.

Combustion Venting for Gas Furnaces

Gas furnace venting is classified into four categories based on the vent's internal pressure (negative or positive) and whether the appliance is condensing, meaning it produces excessive, corrosive condensate in the vent:

Matching the vent material to the correct category is not optional: a Category IV condensing furnace's exhaust is both cool and acidic, which corrodes standard metal vent pipe over time, while a Category I appliance's hot, non-condensing exhaust would damage plastic vent pipe. Vent terminations also carry required clearances from windows, doors, soffits, mechanical air intakes, and property lines that scale with the appliance's British thermal units per hour (BTU/hr) input — always verify the specific clearance table in the current adopted fuel gas code and the equipment's own venting instructions rather than assuming one clearance fits every appliance.

Startup Checklist & Commissioning Verification

A complete startup and commissioning sequence documents that the installation performs as designed, not just that it powers on. A typical sequence includes:

1. Pre-power verification — confirm the refrigerant piping evacuated to the target vacuum level (commonly 500 microns or below, per Section 10.3) and the refrigerant charge weighed in per the manufacturer's specification before energizing the compressor.
2. Electrical verification — confirm supply voltage is within the equipment's rated tolerance, correct phasing on three-phase equipment, and operating amp draw within the nameplate's rated values; confirm the disconnect and breaker were sized per Section 10.4.
3. Airflow verification — measure external static pressure against the blower's rated operating range, confirm the temperature split across the evaporator coil falls in the expected range for the outdoor and indoor conditions, and confirm delivered airflow in cubic feet per minute (CFM) is reasonably close to the design airflow used in the Manual D duct design.
4. Combustion verification (gas furnaces) — use a combustion/flue-gas analyzer to check carbon monoxide (CO) levels, verify adequate draft, and confirm the measured gas input rate, commonly checked by timing, or clocking, the gas meter, matches the furnace's nameplate BTU/hr input rating; any CO reading outside the manufacturer's and analyzer's safe operating threshold means the appliance does not get commissioned until the cause is corrected.
5. Documentation — record the final refrigerant charge, measured superheat/subcooling, static pressure, temperature split, and combustion readings on a startup/commissioning report. On a California installation, this is also where the HERS rater's field verification of duct leakage and refrigerant charge or airflow, as applicable under Title 24, gets scheduled and completed, tying this section back to the Title 24 compliance obligations covered in Chapter 9.

A system that passes every step above is one a C-20 contractor can stand behind — and one that won't generate a callback for a problem that should have been caught before the customer ever felt a difference in comfort.