HVAC Loads and Psychrometrics

Sizing heating and cooling loads and working the air-property processes: sensible vs. latent heat, the psychrometric chart, mixing, humidification and dehumidification — at sea level and at altitude. One of the heaviest areas on the HVAC & Refrigeration depth, and the one to make automatic first: the chart has to be reflexive under time pressure.

What the problems look like

Heavily psychrometric and calculation-based: fix an air state, then run a process. Recurring shapes — split a load into sensible and latent and find the sensible heat ratio; mix two airstreams; find leaving conditions across a coil; a humidification/dehumidification step; a design heating or cooling load. Chart-adjacent (you're reading or reasoning about the psych chart) and often multi-step (one state feeds the next). Altitude (sea-level vs. ~5,000-ft property values) shows up.

By exam day you should be able to —

• move fluently between dry-bulb, wet-bulb, humidity ratio, enthalpy, dew point, and RH for a given state;
• split a load into sensible and latent and compute the SHR;
• work an adiabatic mixing of two airstreams (mass/flow-weighted, not a temperature average);
• find leaving conditions and the load across a cooling or heating coil;
• handle humidification (steam vs. adiabatic) and dehumidification;
• adjust property values for altitude.

Where people lose points

• Confusing humidity ratio with relative humidity.
• Humidity-ratio units — using ΔW in grains (7,000 gr/lb) in the 4,840 relation inflates the latent load 7,000×.
• Dropping the latent term on a dehumidifying coil; using a dry-bulb-only ΔT.
• Mixing by averaging temperatures instead of mass/flow-weighting (and slipping between cfm and mass flow when densities differ).
• Setting SHR from total instead of sensible load.
• Altitude scales the constants, not just the chart — at ~5,000 ft air density ≈ 0.83× sea level, so 1.10/4.5/4,840 drop to ≈ 0.91/3.7/4,000; sea-level constants over-predict loads at altitude.

Have these at your fingertips

The psychrometric chart, and the standard sea-level air relations — q_s = 1.10 × cfm × ΔT, q_l = 4,840 × cfm × ΔW (ΔW in lb/lb), q_t = 4.5 × cfm × Δh (Δh in Btu/lb dry air) — plus where the psychrometric relations sit in the Handbook so you're navigating, not hunting.

Going deeper

• Psychrometrics — the chart and the standard processes
• Cooling & heating load calculations

Part of the HVAC & Refrigeration depth. Practice this area with exam-real problems — the tutor walks any one you miss. → Start your 7-day free trial — no card. Or take the free practice exam first.