AK HVAC cheat sheet

Sensible heat formula: Btu/h = 1.08 × CFM × ΔT for standard air (0.075 lb/ft³, 0.24 Btu/lb·°F).

Latent heat formula: Btu/h = 0.68 × CFM × Δgrains (grains of moisture removed per pound of dry air).

Total heat = sensible heat + latent heat; sensible heat ratio (SHR) = sensible/total, typically 0.70–0.80 for comfort cooling.

Standard air density is 0.075 lb/ft³ at 70°F sea level; density decreases roughly 3.5% per 1,000 ft of elevation.

1 ton of refrigeration = 12,000 Btu/h = 200 Btu/min = heat absorbed melting 1 ton of ice in 24 hours.

Atmospheric pressure drops about 0.5 psi per 1,000 ft; at 4,000 ft it is roughly 12.7 psia versus 14.7 psia at sea level.

1 CFM = 60 cubic feet per hour; 1 watt = 3.412 Btu/h; 1 boiler horsepower = 33,475 Btu/h.

Airflow rule of thumb: 400–450 CFM per ton for standard cooling coils; higher altitudes require more CFM for the same mass flow.

AFUE measures seasonal efficiency of furnaces and boilers; condensing units typically 90%+, non-condensing 80–83%.

Natural gas heating value is about 1,000–1,050 Btu per cubic foot; propane about 2,500 Btu per cubic foot.

Combustion air: 50 ft³ per 1,000 Btu/h input when using indoor air from an unconfined space.

A 1 psi pressure drop equals about 2.31 feet of water column head.

Hydronic system: Btu/h = 500 × GPM × ΔT (°F); 1 GPM = 8.33 lb/min of water.

Steam at sea level occupies about 1,600 times the volume of the water that produced it; at altitude the ratio increases.

Furnace temperature rise = supply air temp minus return air temp; must fall within the nameplate-rated rise range.

Draft hood or induced draft fan maintains negative pressure in the flue to prevent spillage of combustion products.

ASHRAE 62.1 uses ventilation rate procedure: Vbz = Rp × Pz + Ra × Az (people component plus area component).

Velocity pressure VP = (V/4005)² in inches of water, where V is fpm and 4005 is the standard air constant.

Static pressure losses: supply ducts 0.1 in. w.c. per 100 ft is a common design target; fittings add equivalent length.

Return air paths must be sized equal to or larger than supply; undersized returns cause pressurization and noise.

Friction chart: round duct friction loss rises roughly with the square of velocity; doubling velocity quadruples friction.

Duct velocity guidelines: 600–900 fpm trunk supply, 400–700 fpm branch, 300–500 fpm return.

Alaska buildings often need balanced ventilation with heat recovery (HRV/ERV) because envelope tightness limits natural infiltration.

A 90° elbow has an equivalent length of roughly 30–60 ft of straight round duct depending on radius ratio.

Superheat = actual suction temperature minus saturation temperature at suction pressure; target 8–12°F at evaporator outlet for TXV systems.

Subcooling = saturation temperature at liquid pressure minus liquid line temperature; target 8–12°F for TXV systems.

Refrigerant glide occurs in zeotropic blends (e.g., R-407C, R-410A); use dew point for superheat and bubble point for subcooling.

R-410A operates at roughly 50–70% higher pressure than R-22 and requires POE oil, not mineral oil.

The compressor raises refrigerant pressure and temperature; the condenser rejects heat; the metering device drops pressure; the evaporator absorbs heat.

Evaporator coil temperature must stay above 32°F to prevent frost on comfort cooling systems; low airflow causes icing.

Refrigerant charging by weight is required when the system has no subcooling/superheat chart or when lines exceed 15 ft.

Recovery cylinders must never be filled beyond 80% of their water capacity by weight.

Duct sizing: equal friction method uses a constant pressure drop per foot; static regain method uses velocity reduction at each branch.

Sheet metal gage: 26 gage for ducts up to 12 in. width, 24 gage up to 18 in., 22 gage up to 24 in. (SMACNA).

Copper tubing types: K (heavy, underground), L (standard, above ground), M (light, interior only).

Pipe expansion: ΔL = 12 × L × α × ΔT, where α for copper is about 9.3 × 10⁻⁶ in/in·°F.

Insulation R-value per inch: fiberglass ~3.7, closed-cell polyiso ~6.0, spray foam ~6.5.

Duct leakage testing per SMACNA: seal all joints and seams on medium- and high-pressure ducts; leakage class depends on pressure class.

Refrigerant line insulation must cover suction lines; liquid lines are insulated in high-ambient or long-run applications.

Piping slope for condensate drains: at least 1/8 in. per foot toward the drain, with a trap at the air handler.

Ohm's law: E = I × R; power P = E × I; for single-phase, watts = volts × amps × power factor.

Three-phase power: watts = 1.732 × volts × amps × power factor.

Motor full-load amps appear on the nameplate; minimum circuit ampacity (MCA) and maximum overcurrent protection (MOCP) govern wire and breaker sizing.

Locked rotor amps (LRA) are typically 5–6 times running load amps; hard-start kits reduce starting current.

Thermostat wiring: R = 24V power, W = heat, Y = cooling, G = fan, C = common, O/B = reversing valve.

A contactor coil is energized by the thermostat; the contacts switch line voltage to the compressor and condenser fan.

24V control circuits are Class 2; line-voltage circuits are Class 1 and require different conductor and enclosure rules.

A capacitor's microfarad rating should be within ±10% of nameplate; a failed run capacitor causes high current and no-start conditions.

Recover refrigerant before opening a system; venting CFCs, HCFCs, and HFCs is prohibited under Section 608 of the Clean Air Act.

Technicians handling refrigerant must hold EPA Section 608 certification; Type I (small appliances), Type II (high pressure), Type III (low pressure), or Universal.

Carbon monoxide alarm placement: outside sleeping areas and on each level; CO is colorless, odorless, and slightly lighter than air.

Manifold gauge set: blue = low side, red = high side, yellow = center/charging.

Combustion safety: measure CO in flue gas; draft must be verified before and after service on gas appliances.

Lockout/tagout is required before servicing electrical components; verify with a meter, never assume the disconnect is off.

Leak detection: electronic detectors, UV dye, and nitrogen pressure testing; never use oxygen or acetylene as a trace gas.

OSHA requires fall protection above 6 ft in construction and 4 ft in general industry; Alaska adds cold-stress and ice hazards.

Alaska mechanical contractor work requires a state Mechanical Administrator license; the administrator must pass an exam and meet experience requirements.

Alaska has no statewide HVAC journeyman license; local municipalities (e.g., Anchorage, Fairbanks) may impose their own licensing.

Alaska Business License is required for any person or entity doing business in the state; renewed every two years.

Alaska has no state general sales tax; local boroughs and cities may levy sales tax (e.g., Anchorage 0%, some boroughs up to 7%).

State procurement and contract law follow Alaska Statute Title 36 (public works) and Title 45 (trade and commerce).

Alaska OSHA (AKOSH) enforces workplace safety under 29 CFR 1910/1926 as adopted by the state.

Contractors must carry workers' compensation insurance for employees under Alaska Statute Title 23; independent-contractor status is narrowly defined.

Bonding: contractors handling funds or public work may be required to post performance and payment bonds; Alaska Statute 36.25 governs public works bonds.