Equipment Selection, Duct Design, Estimating, and Title 24
Once the load is known, three things follow: which piece of equipment to buy, how to get the air where it has to go, and what to charge. This chapter covers Manual S selection, Manual D duct design with the arithmetic worked, ventilation and air-change calculations of the kind CSLB prints as a sample question, job cost estimating, and the California Energy Code requirements that attach to duct systems. Together with the previous chapter this is the Planning and Estimating content area, roughly 22 percent of the official CSLB outline.
Manual S: Selecting the Equipment
Manual J tells you the load. Manual S tells you which model actually meets it. The distinction that gets tested is that a nominal tonnage on a box is not a capacity; the capacity you must compare against the calculated load is the expanded performance data for the matched coil and condenser at your local design conditions, and specifically the sensible capacity at those conditions compared against the calculated sensible load. Manufacturers publish expanded tables giving total and sensible capacity as functions of outdoor dry-bulb, indoor entering wet-bulb, and airflow, and a nominal 3-ton unit will not produce 36,000 BTU per hour at a 105 degree outdoor design temperature. Capacity falls as outdoor temperature rises, because a hotter condenser means a higher condensing pressure and temperature, a smaller pressure difference is available across the metering device relative to the lift, and the compressor works harder for less mass flow. So for a house with a 22,000 BTU per hour design cooling load and equipment available in 1.5, 2, 2.5, and 3 ton sizes, the Manual S answer is a 2 ton unit verified against the expanded performance data at design conditions, not the 2.5 ton chosen as a safety margin. Where the load swings widely across the season, the comfort argument for two-stage or variable-capacity equipment is run time: it operates longer at low capacity, which improves humidity control and evens out temperature, instead of blasting and stopping.
Manual D: Friction Rate, Worked
A duct design begins with the air the blower can move and the pressure it has to move it against. The available static pressure is the blower's rated external static pressure minus the pressure drops of the components in the airstream that are not duct: the filter, the coil, a balancing damper, a register, a grille. What is left is what the duct system may consume. Total effective length is the measured length of the longest supply run plus the longest return run, plus the equivalent lengths assigned to every fitting in those runs, because an elbow or a boot behaves like some length of straight duct. The friction rate is then the available static pressure divided by the total effective length, expressed per 100 feet of duct, which is the unit the friction chart and the ductulator use. Work it. If 0.50 inches of water column is available and the total effective length is 250 feet, the friction rate is 0.50 divided by 250, times 100, which is 0.20 inches of water column per 100 feet. That number, and not a habit, is what you carry into the sizing chart for every run. Two consequences follow. Undersizing branch ducts to save material raises velocity, and high velocity at a register is heard as noise, which is the usual cause of a complaint about noisy registers on somebody else's installation. And the most effective way to reduce friction loss on a long branch without changing its diameter is to replace sharp fittings with long-radius elbows and smooth boots, because fittings are where the equivalent length lives.
Velocity, Area, and Room Airflow, Worked
Velocity in a duct is airflow divided by cross-sectional area: feet per minute equals CFM divided by square feet. Rearranged, the area needed is CFM divided by velocity. A branch that must deliver 200 CFM at about 600 feet per minute needs 200 divided by 600, which is 0.33 square feet of free area. A round duct carrying 600 CFM at about 800 feet per minute needs 600 divided by 800, which is 0.75 square feet. Keeping velocity in a sensible band is how you control both noise and friction; a return grille sized so that its face velocity reaches about 800 feet per minute will be audible and will raise return static. Room airflow is proportioned from the room-by-room load. A bedroom with a 3,000 BTU per hour cooling load, in a house whose total cooling load is 24,000 BTU per hour and whose blower moves 1,200 CFM, should receive 3,000 divided by 24,000, which is one eighth, of 1,200 CFM, that is 150 CFM. That single calculation is why the room-by-room load has to exist before the ducts are drawn. Nominal system airflow itself comes from a rule of thumb that the exam does use: approximately 400 CFM per ton for residential cooling. A 4-ton system at 400 CFM per ton needs 1,600 CFM; a 3-ton system needs 1,200 CFM. Treat it as a starting target to be confirmed against the manufacturer's data, not as a law.
Ventilation, Air Changes, and Makeup Air
CSLB publishes an air-change problem as a sample question in its official C-20 study guide, so work the method. A storage garage of 180,000 cubic feet requires four air changes per hour. Four air changes means moving 4 times 180,000, which is 720,000 cubic feet, every hour. Divide by 60 minutes to get 12,000 cubic feet per minute. If each ventilator exhausts 1,000 CFM, you need 12,000 divided by 1,000, which is 12 ventilators. The general form is: required CFM equals volume in cubic feet times air changes per hour, divided by 60. In occupied commercial spaces, the required outdoor air rate is normally set by two factors together, the floor area of the space and the design number of occupants, with the ventilation standard giving a rate per person plus a rate per square foot. Exhaust creates its own obligation. A restaurant kitchen with a 3,000 CFM exhaust hood must be provided with makeup air roughly equal to the exhausted volume, or the building will go negative, doors will be hard to open, and natural-draft appliances elsewhere will backdraft. The same physics on a residential scale is what causes a tight new house to spill flue gas at the water heater when the dryer and range hood run together: the exhaust appliances depressurize the house below what the vent can overcome. In a light commercial building that uses a ceiling plenum as a return path, materials exposed in that plenum must meet flame-spread and smoke-development limits, which is a code question, not an airflow question.
Zoning, Bypass, and Design Traps
Zoning a residential system with motorized dampers solves a comfort problem and creates an airflow problem, because when only one small zone calls, the blower is still trying to move most of its design airflow through a fraction of the duct system. The traditional fix, a bypass duct that dumps excess supply air back into the return, is the wrong fix and the exam asks why: it recirculates cold supply air to the coil, so the coil sees progressively colder entering air, the evaporator temperature falls, and the coil can freeze during a small zone call. Better answers are a variable-capacity or staged system, a properly sized dump zone, or a static-pressure-controlled variable-speed blower. Two more design traps are worth naming. Flexible duct installed with visible sag and slack behaves as though its friction rate were far above the tables, so the delivered CFM falls below design even though the drawing was correct. And an oversized air conditioner in a coastal climate is the classic clammy-house complaint, because the short run times never remove the latent load. On the measurement side, if you find total external static pressure of 1.1 inches of water column on a system whose blower is rated for 0.5, the first consequence for the design is that airflow will fall below the design CFM and capacity goes with it; the productive next step is to measure the pressure drop across the filter, the coil, and each duct section to find where the pressure is being spent.
Title 24 Part 6, HERS Verification, and Job Cost Estimating
California's Building Energy Efficiency Standards live in Title 24, Part 6, and new and altered HVAC systems must demonstrate compliance with them. Two field-verified measures come up most often on residential work: duct leakage, tested by pressurizing the duct system with a calibrated fan, and refrigerant charge or airflow verification. Where the standards require field verification, it is performed by an independent third-party HERS rater, not by the installing contractor, and a change-out that replaces a furnace and a substantial length of duct is the scenario most likely to trigger duct leakage testing. Ducts in unconditioned space must also meet a minimum insulation R-value, which is one more reason to bring them inside conditioned space where the design allows. Because the specific leakage thresholds, R-values, and verification triggers differ by climate zone and have changed between code cycles, confirm the current requirement in the edition of Part 6 in force for your permit rather than relying on a remembered figure. A mechanical permit is generally required for installing, altering, or replacing regulated equipment and duct systems, and it is required even when the new equipment matches the old, because the work is regulated construction affecting life safety and energy compliance. On estimating, the official outline lists job cost estimation explicitly. A defensible estimate separates material, labor, equipment, subcontracts, permit and HERS fees, overhead, and profit; prices labor as crew hours times a burdened rate that includes payroll taxes, workers compensation, and insurance; and carries a contingency for the conditions you cannot see until demolition.
Last updated: September 2026