Lesson 2.1

The sizing chain

Air: CFM from sensible load and design ΔT; duct sizing balances velocity/friction against noise and fan energy — designing the static pressure story you learned to read in 304.
Water: GPM from load and ΔT (≈500 BTU/hr per GPM per °F); pump head sums friction through the index circuit; valve Cv from flow and available ΔP — sizing the throat so authority stays decent (104/CE, math shown).
Equipment selection: capacity meeting load with sane turndown — equipment that can't run low forces cycling no sequence can fix. Diversity sizes central plant below the sum of zone peaks (not everything peaks at once).
The compounding-margin disease: safety factors stacked at every step = chronic oversize. One reasoned margin beats five hidden ones.
Your payoff: reading the calcs lets you sanity-check sequences and answer 'the unit is undersized!' complaints with math instead of shrugs.

LAB — size a small system

Run the simplified load calc (worksheet) for a 4-zone office suite: envelope, internal, ventilation components; sensible/latent split.
Size the chain: zone CFMs, box selections from the schedule, coil GPM, pump head from the supplied circuit, valve Cv choices with authority check.
Diagnose the trap case: an ‘undersized cooling’ complaint whose real cause is stacked oversizing short-cycling the compressor — prove it from the trends + your math.

Pass: calc components correct; chain consistent; trap diagnosed with numbers.

LAB DEMO VIDEO

Instructor walkthrough — the sizing chain end to end

[ ~12 min ]

The sizing chain — flow: load components → CFM/GPM math → duct/pipe/pump/valve sizing → equipment selection w/ turndown badge; diversity funnel at the plant; compounding-margins ember spiral as the anti-pattern