Ductwork, Gas Piping, Electrical, and Startup
The official CSLB outline calls this content area Fabrication, Installation, and Startup, and startup is the word candidates skip. A system is not installed when the last screw goes in; it is installed when the temperature rise is inside the range on the rating plate, the manifold pressure is where the manufacturer wants it, the static pressure is survivable, and the charge has been verified. This chapter, the last of four on installation, covers the three trades that meet at the equipment, sheet metal, gas piping, and electrical, and then the startup readings that prove the job.
Duct Fabrication, Sealing, and Support
Duct leakage is not a small loss. Air that escapes a supply trunk in a vented attic is air you paid to condition and then threw into the attic, and the return side is worse because it pulls hot attic air into the system. Seal every joint and seam with mastic, reinforced with fibrous mesh where the gap is wide, or with a listed tape rated for the application; ordinary cloth-backed duct tape is not a listed duct sealant and fails in attic heat. Flexible duct must be pulled tight and supported with wide straps at the manufacturer's spacing, because sag is not cosmetic: a sagging flex run behaves as though its friction rate were far higher than the design tables assume, so the delivered airflow drops below the design value and the room it serves never satisfies. Narrow straps that pinch the liner do the same thing in a smaller area. A flexible connector between the air handler and the supply plenum exists for one purpose, to break the transmission of blower vibration into the duct system, which is why the correct answer to a question about its function is noise and vibration and not expansion or airflow. Building cavities such as stud bays and panned floor joists are no longer acceptable as return ducts in new work, because they leak to unconditioned space and there is no way to seal or verify them.
Return Air: Where It May and May Not Come From
The prohibited return air locations form a short list that shows up on the exam in several disguises. Return air may not be taken from a garage, because vehicle exhaust and stored chemicals would be distributed through the dwelling. It may not be taken from a closet or room containing a fuel-burning appliance, because the negative pressure the return creates can pull flue gas backward out of the appliance and into the airstream. It may not be taken from a bathroom, because moisture and contaminants would be spread through the house. And a supply or return opening may not connect a garage to the dwelling's duct system at all. Where a supply duct must pass through the garage ceiling or wall that separates the garage from the dwelling, the duct has to be of a material and construction that maintains the required separation, so the assembly is not compromised by the penetration. In a small commercial building, a supply duct crossing a rated fire wall generally requires a listed fire damper installed in accordance with its listing. A related design problem is the bedroom with a closed door and no dedicated return. The objective is to limit the pressure difference the closed door creates between the room and the hall, using transfer grilles, jump ducts, or undercut doors, because a pressurized bedroom pushes conditioned air out through the envelope and drags unconditioned air in somewhere else.
Gas Piping, Sediment Traps, and Connectors
Gas pipe is sized to deliver the required volume of fuel at the required pressure to every appliance running at once. The three inputs to the code sizing tables are the total connected input in BTU per hour, the developed length of the pipe run, and the gas pressure and allowable pressure drop for the system. Undersized pipe does not fail visibly; it starves the appliance at full fire, which shows up as low manifold pressure, poor combustion, and nuisance lockouts on the coldest day. A sediment trap, sometimes called a drip leg, is a capped vertical leg installed downstream of the appliance shutoff valve and ahead of the appliance control, positioned so that moisture and scale carried by the gas fall into the leg instead of entering the gas valve. Its arrangement is the tested detail: downstream of the shutoff, ahead of the appliance, with a capped drop. A manual shutoff valve belongs in the same room as the appliance, ahead of the connector and within reach. Flexible appliance connectors are single-length, adequately sized, and must not pass through a wall, floor, or partition. Corrugated stainless steel tubing has an additional requirement that catches people out: the CSST system generally must be bonded in accordance with the manufacturer's instructions and the electrical code, which reduces the risk of arcing damage from a nearby lightning strike. After any gas connection, check the joints with an approved leak detection solution or an electronic detector, never with a flame.
Electrical: Nameplate Ratings, Disconnects, and Control Wiring
The nameplate on a condensing unit gives you two numbers that build the branch circuit. The minimum circuit ampacity, or MCA, is the smallest conductor ampacity permitted. The maximum overcurrent protection, or MOCP, is the largest breaker or fuse permitted. They are not the same number and they are not interchangeable. If a nameplate lists MCA 22 amps and MOCP 35 amps, the correct circuit uses conductors rated at least 22 amps protected at no more than 35 amps, and a candidate who sizes the conductor to 35 amps has misread the plate. Before energizing anything, verify voltage, polarity, and grounding. The equipment grounding conductor run with the branch circuit exists to provide a low-impedance fault path so the overcurrent device opens quickly on a ground fault, and that is the answer to why it is there rather than any story about static or lightning. The disconnecting means for the outdoor unit is located within sight of and readily accessible from the equipment. On the low-voltage side, residential control circuits run at 24 volts from a control transformer, and the standard terminal that energizes the compressor contactor on a cooling call is Y. Keep low-voltage cable separated from line-voltage conductors and support it independently rather than lashing it to the feeder. On three-phase commercial equipment, a blower running backwards after wiring is a phase rotation problem, corrected by swapping any two of the three line conductors at the disconnect.
Thermostat Placement, Staging, and Auxiliary Heat
A thermostat measures one point and controls the whole house, so where it goes decides how the house feels. Mount it on an interior wall in a frequently used room, away from supply registers, direct sun, exterior doors, kitchen appliances, and the warm air rising inside a wall cavity from an uncapped stud bay. A thermostat in a hallway that never gets sun will keep the hall comfortable and let the west bedroom cook. On a heat pump with electric strip heat, the strip heat is the most expensive heat in the building, and the control strategy is to stage it on only when the heat pump cannot satisfy the call, either because the outdoor temperature is below the balance point or because the room temperature is falling faster than the compressor can recover it. Wiring the strips to run alongside the compressor at every call turns an efficient machine into an electric furnace. Smart thermostats introduce a wiring issue worth knowing: many of them need a common conductor to power themselves continuously, and a stat installed on a system with no C wire will reset intermittently as it steals power through the control circuit. The standard fix is to pull a common conductor from the equipment or install a listed adapter, not to keep replacing thermostats.
Startup: The Readings That Close the Job
Three measurements turn an installation into a commissioned system. First, temperature rise on a furnace. Measure supply and return air temperature in the correct locations and compare the difference against the range printed on the rating plate. If a new furnace measures a 30 degree rise against a plate range of 40 to 70 degrees, the airflow is too high for the fire rate, and the correction is to reduce blower speed until the rise falls inside the listed range. A rise above the range means the airflow is too low, and the furnace will eventually trip its high limit. Second, manifold gas pressure. Connect a manometer to the outlet tap on the gas valve with the burner firing and compare against the manufacturer's specified pressure for the fuel; do not adjust the regulator to make a number you like without checking the supply pressure and the appliance input. Third, total external static pressure, measured as the sum of the supply plenum pressure and the return pressure at the equipment. If the blower is rated around half an inch of water column and you measure nearly an inch, the airflow will be well below design and the capacity goes with it. On the refrigerant side, verify the charge by subcooling on a TXV system or by target superheat on a fixed-orifice system, and record the numbers. A written record of the commissioning readings is also your defense if the system is questioned later.
Last updated: September 2026