General HVAC Knowledge and Fundamentals
Start with the physics that every other domain rests on: heat flows from warmer to cooler bodies by conduction, convection, and radiation, and the job of HVAC equipment is to manage that flow. Candidates must be fluent in sensible versus latent heat, the difference between temperature and heat content, and the fact that a Btu raises one pound of water one degree Fahrenheit. Understand psychrometrics conceptually - dry-bulb, wet-bulb, dew point, relative humidity, and enthalpy - because questions often hinge on which property changes when air is cooled below its dew point (latent removal begins, sensible cooling continues).
Know the standard air constants and be able to use them: roughly 1.08 × cfm × ΔT for sensible Btu/h and 4.5 × cfm × Δenthalpy (or 0.68 × cfm × Δgrains) for total load. These are the workhorses of quick estimates. Pressure basics matter too: static pressure, velocity pressure, and the idea that a fan curve trades flow against pressure.
Traps to watch: confusing tonnage (12,000 Btu/h per ton) with airflow; assuming relative humidity stays constant as air is heated (it falls); mixing up wet-bulb and dew point; and forgetting that altitude changes air density and therefore the sensible-heat multiplier. Alaska elevations and cold outdoor design temperatures make density corrections more than academic.
Worked example: a 400-cfm airstream is heated from 60°F to 100°F. Sensible load = 1.08 × 400 × 40 ≈ 17,280 Btu/h, about 1.4 tons of heating equivalent. Rule of thumb: memorize 1.08 and 4.5 and you can solve most load-check questions without a chart.