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Certified Mechanical Contractor/Trade Knowledge

Load Calculation & System Design

25% of this exam

The heaviest domain on the trade exam, built around the ACCA design sequence: Manual J calculates the load, Manual S selects the equipment, Manual D sizes the ducts. Expect tonnage conversions, sensible-vs-latent questions, and scenarios that punish rule-of-thumb sizing — Florida's humidity makes the latent load the star of this domain.

Core concepts

J, then S, then D — in that order

Manual J computes the heating and cooling load from the building itself: envelope, windows and orientation, infiltration, internal gains, occupants. Manual S picks equipment whose actual rated performance covers that load at design conditions. Manual D sizes the ducts to deliver the airflow. Questions love to test which manual does which job.

The old unit's nameplate is not an input

Loads come from the building, never from the equipment being replaced — the original unit may have been sized wrong, and the house may have changed. 'Match the existing tonnage' is always the wrong answer.

Oversizing is the classic trap

An oversized unit satisfies the thermostat fast, short cycles, and never runs long enough to wring moisture out of the air. Cold-but-clammy complaints on a new install point to oversizing, not undersizing.

Sensible vs latent, dry bulb vs dew point

Sensible load lowers air temperature; latent load condenses moisture out of the air without changing temperature. Any surface colder than the air's dew point will sweat — that's the physics behind condensation on grilles and uninsulated ducts.

Key facts to know cold

One ton of cooling12,000 Btu/h — divide total Btu/h by 12,000 to get tons (42,000 Btu/h = 3.5 tons)
Manual JResidential heating/cooling load calculation
Manual SEquipment selection — verify sensible AND latent capacity from expanded performance data, not nominal tonnage
Manual DDuct system design and sizing from the Manual J airflows
Latent loadHeat removed to condense water vapor — large in Florida; oversized units fail it
Dew pointTemperature where air saturates — surfaces below it condense moisture

See it drawn out

Fig — The ton ↔ Btu/h conversion
  1. Rule
    1 ton of cooling = 12,000 Btu/h
  2. Load → equipment
    42,000 Btu/h ÷ 12,000 = 3.5 tons
  3. Equipment → capacity
    5 tons × 12,000 = 60,000 Btu/h

    The conversion works in both directions — memorize 12,000.

Fig — Sensible heat ratio
  1. Formula
    SHR = sensible load ÷ total load
  2. Example
    30,000 ÷ 40,000 = 0.75

    The rest is latent (moisture) load — equipment must handle both.

Where it lives in your books

The real exam is open book. Knowing which book — and which tab — answers this domain is worth as much as memorizing it.

manual

HVAC References

RACT is a 1,700-page textbook — it answers almost any fundamentals, electrical, or troubleshooting question, but only if you tab it by unit. Build the tab set below and trust the symptom tables over memory.

9 recommended tabs

Lookup strategy

  • · The J = load / S = equipment / D = duct mapping must be memorized — it's a 5-second question if you know it and unfindable if you don't.
  • · Tonnage math is arithmetic, not lookup: write '12,000 Btu/h per ton' on your scratch paper at the start of the exam and convert mechanically.

Reading isn't learning — retrieval is.

65 questions in this domain, each with an explanation and source.