AC Installation Done Right: Evacuation, Charging and Commissioning

Installation day: the condenser and lines you can see, and the work you cannot

The visible part of the job is the old equipment coming out, the new outdoor condenser set on a level pad, the indoor coil or air handler placed, copper lines run between them, and the wiring and condensate drain connected. The permit and inspection that normally accompany that work are covered in HVAC permits in Florida, and whether the equipment is the right size is a separate question, covered in how to choose the right AC size.

The work that decides how the system performs happens after the lines are connected, and it is invisible once the crew leaves. A manufacturer's manual, such as Bosch's installation instructions for its split heat pumps, has the technician braze the lines while dry nitrogen flows through them, pressurize the lines and indoor coil with dry nitrogen to check for leaks, and then evacuate them. The manual's rule is explicit: do not open the service valves until the leak check and evacuation are complete. Only then is the charge set, and the system started and measured.

None of that is visible on installation day, and a system with moisture in the circuit, a slow leak, a wrong charge or a starved blower can still cool. A guideline written for the Department of Energy's Building America program notes that defects common in existing systems can appear in newly installed ones when airflow and charge procedures are not followed. Three readings turn the hidden work into something a homeowner can ask about: the evacuation reading and its hold, the charge verification, and the static pressure and airflow.

Evacuation: the micron reading and the hold

Evacuation pulls the air, nitrogen and moisture out of the lines and indoor coil before refrigerant goes in. Moisture does lasting harm: the DOE-sponsored guideline explains that the heat of compression makes moisture react with refrigerant and form hydrochloric and hydrofluoric acid, which erodes machine surfaces and can lead to compressor failure. The depth of the vacuum is measured in microns of mercury with a dedicated micron gauge.

The photo on this page shows a digital vacuum gauge on a Blume job reading 400 microns. It shows what the instrument looks like. It does not prove a finished evacuation: a single frame shows neither how long the reading was held nor which manufacturer's target applied, and it cannot say whether the number was a stopping point or a moment during the pull-down. The target itself is not one universal figure. Four documents read for this page give three different figures:

• Bosch, IDS BOVA20 and BOVC20 split heat pump installation instructions (12.2023): evacuate until the gauge reads no higher than 350 microns, close the valve to the pump, and the evacuation is complete if the gauge does not rise above 500 microns in one minute.
• Trane, 4TTR3 condensing unit installer's guide (18-AC79D1-8-EN, 07/16): the same 350-micron target and one-minute check against 500 microns.
• Rheem, RP13, RP14 and RP15 heat pump installation instructions (8/15): evacuate to 500 microns and hold 500 microns or less for at least 15 minutes.
• The DOE-sponsored guideline: evacuate to 240 microns and hold for 10 minutes before opening the service valves.

Each one pairs a number to reach with a hold to prove it, and the targets run from 240 to 500 microns. A reading means something only beside the target in the manual for the equipment being installed. So the first request is for three items: the final gauge reading, the target in that manual, and the hold result with the time it was watched.

Refrigerant charge: measured against the manufacturer goal

After evacuation the service valves are opened and the system takes its charge. An outdoor unit ships with a factory charge for a stated length of line set. The Bosch manual's factory charge, for example, is sufficient for 15 feet of standard-size liquid line, and the manual gives a per-foot adjustment beyond that, so the length of the run changes how much refrigerant belongs in the system. The manual has the installer weigh in the charge on an initial installation and adjust it by subcooling when the outdoor temperature allows.

A charge set by weight is a starting point; the check is a measurement. According to the DOE-sponsored guideline, the most accurate way to verify proper refrigerant charge is to measure subcooling and superheat using manifold gauges. Subcooling is how far the liquid refrigerant leaving the outdoor coil sits below its condensing temperature. Superheat is how far the vapor returning to the compressor sits above its boiling temperature. Which one is used depends on the metering device: the ENERGY STAR commissioning checklist uses subcooling for a system with a thermal expansion valve and superheat for a fixed-orifice system, compares each with the manufacturer's goal, and records the deviation.

That checklist's pass item is a deviation within 3°F for subcooling or 5°F for superheat. The band belongs to ENERGY STAR's new-homes program rather than to every installation, and the manufacturer's goal is what the reading is measured against. So the charge reading to ask for is three numbers: the manufacturer's goal, the measured subcooling or superheat, and the outdoor air temperature at the time.

A charge check also depends on airflow being right. The same guideline says airflows below 350 cfm/ton (cubic feet of air per minute for each ton of cooling capacity) render standard tests for determining refrigerant charge invalid and can lead to improper charging.

Airflow and static pressure: the reading behind the charge

Static pressure is the resistance the blower works against as it pushes air through the filter, the grilles and the ducts, measured in inches of water column at small test holes in the duct system. The ENERGY STAR checklist asks the contractor to confirm that the test holes exist and are well marked and accessible, to measure return and supply external static pressure and add them into a total, and to use that total with the fan-speed setting to find the measured airflow, which is then compared with the design airflow.

Low airflow is the problem the reading is meant to catch. The DOE-sponsored guideline says airflows below 200 cfm/ton can lead to coil icing and greatly shorten compressor life. It also summarizes field data from 62 California homes built around 2007, where about two-thirds had airflow below the recommended minimum of 350 cfm per ton and filters were responsible for the majority of the return-side pressure loss. Those are California homes, not Tampa Bay ones. They show how common low airflow was in that sample, not what any Florida house will read.

The reading to ask for is the measured total external static pressure next to the limit the equipment's manufacturer publishes, with the measured airflow. High static pressure can be a duct or filter problem, and two guides cover that ground: duct sealing versus duct repair and a return air vent that is too small.

What the permit and the checklists do and do not cover

A permit inspection and a checklist are not the same thing as the installer's own readings. The ENERGY STAR checklist discussed here belongs to the program for new homes, applies to ducted air conditioners and heat pumps up to 65,000 Btu per hour, and exempts mini-split and multi-split systems. ENERGY STAR itself notes that the checklist does not promise proper HVAC performance. It is useful as a neutral list of what a commissioning record can contain, not as a rule for your job.

For installations in existing homes, EPA ended the ENERGY STAR Verified HVAC Installation program effective August 31, 2025. That makes the installer's own readings the record to ask for.

Ask the installer and the building department whether the permit for your job calls for a duct-leakage test. Check the contractor's license as well: the Florida Department of Business and Professional Regulation explains on its construction pages how to verify one.

What to ask for before you sign off

Five items cover the hidden work. Ask for them in writing, with the date, before you sign off:

1. The micron-gauge reading at the end of evacuation, the target in the manual for this model, and the hold: any rise, and the time it was watched.
2. How the charge was set and verified: the manufacturer's goal, the measured subcooling or superheat, and the outdoor temperature.
3. The measured total external static pressure and airflow, beside the equipment's published limit.
4. The make, model and serial number of the equipment installed, and the installation manual followed.
5. The permit number and the inspection result.

These are readings a technician takes in the course of the work, so asking for them is an ordinary request; if one was not taken, you know which one is missing. Taking the readings and correcting what they show is a technician's job. For the refrigerant circuit, EPA requires technicians who maintain, service, repair or dispose of equipment that could release refrigerant to be certified under Section 608. Its venting rules prohibit intentionally venting refrigerant while maintaining, servicing, repairing or disposing of air-conditioning equipment. The nitrogen pressure test, the brazing and the charging all carry risk, so do not attempt any of it yourself.