Rough-In Installation: Mechanical, Electrical, and Refrigerant Piping

Rough-In Installation: Mechanical, Electrical, and Refrigerant Piping

The rough-in phase is where your approved drawings become a real system. Every measurement, connection, and label you make now determines whether commissioning goes smoothly or turns into a troubleshooting marathon. Work methodically through the project checklist—never skip a step to save time.

Safety Before You Touch Anything

Lockout/tagout (LOTO) is mandatory any time you open a panel, connect or disconnect wiring, or work near energized conductors. Verify the circuit is de-energized with a multimeter before touching any terminal. Wear safety glasses throughout the lab—brazing, drilling, and cutting all create projectile hazards. Add leather gloves when handling sheet metal edges and brazing equipment. Use hearing protection when running reciprocating saws or hole saws. When brazing, ensure adequate ventilation and keep a fire extinguisher within reach. Nitrogen purge gas is non-toxic but displaces oxygen in confined spaces; keep the area ventilated.

Setting the Air-Handling Unit

Begin with the air-handling unit (AHU). If the unit is floor-mounted, build or verify a level platform—check level in two directions with a spirit level. If the unit hangs from structure, install the manufacturer-specified hanging brackets and vibration isolators so the unit sits plumb and level. An out-of-level AHU causes condensate to drain toward the blower instead of the drain pan outlet, leading to overflow and water damage.

Fabricate the supply plenum and return-air box to the dimensions on your drawings. Use sheet-metal screws on every joint, then seal every seam with mastic sealant and mesh tape—never use duct tape alone for final sealing. Connect branch duct runs and register boxes, maintaining the duct sizes shown on your drawings to preserve the design airflow.

For the condensate drain, install a trap sized to the manufacturer's specification. A properly dimensioned trap prevents air from being drawn back through the drain line, which would break the seal and allow drain pan overflow. Slope the horizontal drain line at least one-quarter inch per foot toward the discharge point. Glue all PVC fittings with the appropriate primer and cement, following the product's cure time before testing.

Refrigerant Piping (Lineset)

The lineset connects the indoor and outdoor units. Use only clean, dry refrigerant-grade copper tubing. Before brazing, flow dry nitrogen through the line at low pressure to purge oxygen; this prevents copper oxide scale from forming inside the tubing, which would contaminate the refrigeration system.

For flared connections, cut the tubing square, deburr the end, slide the flare nut on before flaring, and make the flare with the correct flaring tool for the tubing size. Inspect each flare for cracks or uneven thickness before assembling. Torque flare nuts to the manufacturer's specification using a flare-nut wrench, not an open-end wrench.

For brazed joints, clean the tubing with emery cloth, apply the appropriate flux if required, heat the joint evenly, and let the braze alloy flow by capillary action—do not push the rod into the flame. Overheating burns flux and weakens joints. All braze joints must be visually inspected for full penetration and smooth fillets.

Support the lineset with hangers or strapping at intervals required by the manufacturer or applicable mechanical code, and insulate the suction line continuously from indoor to outdoor unit to prevent condensation and heat gain.

Electrical Rough-In

With LOTO applied at the panel, pull the power circuit for the condensing unit and for the AHU. Use the wire gauge and breaker size specified on the equipment nameplate and your wiring diagram—never upsize a breaker beyond what the equipment requires. Install a properly rated disconnect within sight of the condensing unit as required by the National Electrical Code. Connect power wiring to the unit terminals per the manufacturer's wiring diagram, confirming wire colors and terminal labels match.

Run low-voltage thermostat cable between the AHU control board, the outdoor unit, and the thermostat. Most residential and light-commercial systems use 18-gauge, multi-conductor thermostat wire. Label each conductor at both ends using the letter designations shown on the wiring diagram (R, C, Y, G, W, etc.) before making connections.

Install the thermostat on the mock wall assembly at approximately 5 feet from the floor, away from direct sunlight, supply registers, and exterior walls—locations that cause false temperature readings.

Labeling and Final Checks Before Inspection

Label every circuit breaker, disconnect, and major component with a durable label that matches your equipment schedule. Label thermostat wire conductors at both the AHU board and the thermostat sub-base. Use your project checklist to verify each item before calling for the rough-in inspection.

The instructor will evaluate your work against a 30-point punch list covering mechanical mounting, refrigerant piping quality, electrical wiring, duct connections, and condensate drain slope. You may have no more than three minor deficiencies and zero major deficiencies (safety or code violations) to proceed. Correct all major deficiencies and request re-inspection before the next lesson.