CA C-20 HVAC cheat sheet

Heating and cooling loads are calculated room-by-room with ACCA Manual J using construction, window, orientation, infiltration, occupant, and climate inputs; equipment is then selected to meet the calculated load.

Nominal cooling airflow is about 400 cfm per ton (12,000 Btu/h); the design range is roughly 350–450 cfm/ton and must be checked against the equipment airflow table and sensible/latent load.

Sensible airflow: cfm = sensible Btu/h ÷ (1.08 × ΔT). Example: 24,000 Btu/h sensible with a 20°F ΔT requires 1,111 cfm.

Available external static pressure = blower rated external static minus pressure drops of filter, coil, electric heat, registers, grilles, and other devices; the remainder is the duct design budget.

Duct sizing uses ACCA Manual D with a friction rate derived from available static pressure and effective length; do not select duct size from cfm alone.

Total cooling capacity must meet sensible plus latent load; an oversized unit may satisfy sensible load before removing enough moisture.

Takeoffs list duct linear feet, fittings, insulation, sealant, hangers, and equipment; add a documented waste factor for cutoffs and field changes.

California Title 24 requires duct insulation, duct sealing, and duct leakage testing in specified climate zones; verify the applicable compliance form before layout.

Smaller gauge number means thicker sheet metal; gauge is selected from SMACNA pressure-class tables using duct size, span, pressure, and support spacing.

Residential trunk design often uses about 0.1 in. w.c. friction loss per 100 ft of equivalent length; fittings add equivalent length and must be counted.

Seal all joints, seams, and penetrations in unconditioned spaces with mastic or listed foil tape; cloth-backed duct tape is not a permanent sealant.

Flexible duct must be fully extended, supported at manufacturer intervals, and kept free of sharp bends; compression or sag reduces airflow.

Rectangular equivalent diameter: De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25, where a and b are side dimensions in inches.

Use radius elbows or turning vanes instead of square elbows; abrupt turns increase pressure drop and turbulence.

Return air plenums must be sealed and must not use combustible materials or be penetrated by gas flues or electrical devices unless listed for the application.

Duct insulation R-value and vapor barrier placement depend on climate and location; in California attics, ducts are commonly specified at R-8 with a sealed vapor retarder.

Superheat = actual suction temperature minus saturation temperature at suction pressure; typical target is 5–20°F depending on metering device and manufacturer data.

Subcooling = saturation temperature at liquid-line pressure minus actual liquid-line temperature; typical target is 8–15°F for many TXV systems.

A TXV maintains evaporator superheat by modulating flow; a fixed orifice relies on charge and operating conditions, so charge is set by superheat.

Compressors pump vapor; liquid slugging can damage valves and bearings, so low superheat and flooded starts must be corrected.

The evaporator absorbs heat and boils refrigerant; the condenser rejects heat and condenses refrigerant.

Saturation temperature is the boiling or condensing temperature at a given pressure; use a PT chart or gauge for the specific refrigerant.

Compression ratio = discharge absolute pressure ÷ suction absolute pressure; high ratios increase discharge temperature and reduce capacity.

Zeotropic blends have temperature glide; charge and service procedures must follow the refrigerant manufacturer's glide and fractionation guidance.

Furnace airflow: cfm = output Btu/h ÷ (1.08 × temperature rise); measured temperature rise must be within the nameplate range.

AFUE rates seasonal fuel efficiency; condensing furnaces typically exceed 90% AFUE and require a condensate drain and PVC venting.

Category IV furnaces operate with positive vent pressure and condensing flue gas; vent material, slope, and termination must follow the listing and code.

Heat pump COP = heat output in Btu/h ÷ electrical input in Btu/h; higher COP means more heat per unit of electricity.

The balance point is where heat pump capacity equals building load; below it, auxiliary heat stages operate.

Defrost is initiated by coil temperature, pressure, or time-temperature controls; auxiliary heat is usually energized during defrost to temper supply air.

Heat pumps need correct airflow and refrigerant charge; low airflow can cause high discharge temperature, icing, and compressor overload.

Combustion air and venting must be sized for all appliances in the space; backdrafting or spillage of flue gases is a safety hazard.

Ohm's law: V = I × R. Single-phase power: P = V × I × power factor; resistive loads use P = V × I.

MCA is minimum circuit ampacity; MOP is maximum overcurrent protection. Wire ampacity must be at least MCA, and the breaker must not exceed MOP.

Voltage drop should generally not exceed 3% on a branch circuit and 5% total from service to load.

A disconnect must be within sight of and readily accessible from the equipment; grounding and bonding are required for safety.

Thermostat signals: R to W = heat, R to Y = cool, R to G = fan, R to O/B = reversing valve on heat pumps.

A run capacitor stays in circuit during operation; a start capacitor is switched out by a potential relay or centrifugal switch.

Capacitor microfarad reading should be within the manufacturer's tolerance, commonly about ±6%; a bulged capacitor should be replaced.

Contactors and relays use a low-voltage coil to control high-voltage contacts; pitted contacts cause voltage drop and intermittent operation.

Flow dry nitrogen through tubing while brazing to prevent internal oxidation; use a regulator and avoid excessive pressure.

Copper-to-copper joints can use silver-phosphorus brazing alloy without flux; copper-to-steel or brass joints require flux and proper cleaning.

Never use oxygen, air, or flammable gas to pressure-test a refrigerant system; use dry nitrogen and a pressure regulator.

Evacuate to 500 microns or lower, isolate the pump, and hold; a rising micron gauge indicates leak or moisture.

EPA Section 608 certification is required to service, repair, or dispose of equipment containing regulated refrigerants.

Install filter driers in the liquid line; use a suction-line filter drier after a burnout, then remove or replace it per manufacturer instructions.

Line-set size must follow the equipment manufacturer's table; undersized lines increase pressure drop and reduce capacity.

Insulate the suction line to prevent condensation and heat gain; do not insulate a liquid line unless required for freeze protection.

Before charging, verify airflow, compressor rotation, electrical voltage, and stable operating conditions; charge adjustments require steady-state readings.

Fixed-orifice systems are charged by superheat using indoor wet-bulb and outdoor dry-bulb tables from the manufacturer.

TXV systems are charged by subcooling using liquid-line pressure and temperature, following the manufacturer's target.

Weigh-in charging is preferred when the total charge and line-set adjustment are known; use a scale accurate to the required tolerance.

Evacuation target is 500 microns or less for a tight system; hold the vacuum and watch for decay before opening valves.

External static pressure is measured with a manometer at the blower; total external static includes supply and return pressures.

Combustion analysis measures O2, CO2, CO, and flue temperature; use it to verify safe combustion and venting.

Startup records should include voltage, amp draw, capacitor microfarads, pressures, temperatures, superheat/subcooling, and temperature split.

Low suction pressure with high superheat suggests undercharge, a restriction, or low evaporator airflow; verify airflow and pressure drop before adding refrigerant.

High suction pressure with low superheat suggests overcharge, oversized orifice, or excessive evaporator airflow; check metering device and charge.

High head pressure suggests dirty condenser, high ambient, noncondensables, or overcharge; check condenser airflow and subcooling.

Low head pressure suggests undercharge, low ambient, or inefficient compressor; compare suction and discharge pressures to expected values.

Ice on the evaporator coil usually indicates low airflow, low charge, or a restriction; correct the cause rather than only defrosting.

A compressor that will not start requires checking thermostat call, disconnect, breaker, contactor, capacitor, overload, and control lockout.

Excessive filter pressure drop reduces airflow; replace filters based on measured pressure drop or manufacturer schedule.

Preventive maintenance includes coil cleaning, belt and bearing inspection, electrical connection checks, condensate drain clearing, and charge verification.

Lockout/tagout isolates electrical and other hazardous energy; verify de-energization with a meter before touching conductors.

Refrigerant vapor can displace oxygen in low or confined spaces; ventilate and use a refrigerant monitor.

Never heat a refrigerant cylinder with an open flame; use warm water or a listed heater and follow cylinder pressure limits.

Recovery cylinders must not be filled above 80% of capacity by weight; use a scale and a cylinder rated for the refrigerant.

Use a nitrogen regulator and keep oil and grease away from oxygen fittings; oxygen can ignite hydrocarbons.

Combustion appliances require adequate combustion air and proper venting; use a CO monitor and check for spillage.

Arc flash and shock hazards require appropriate PPE and safe work practices; treat all conductors as energized until tested.

HVAC replacement and duct alterations may require permits and California Title 24 compliance documentation; verify before work begins.