Certified Mechanical Contractor/Trade Knowledge

Ductwork & Ventilation

20% of this exam

Moving the right amount of air and keeping it in the duct: sizing methods, airflow rules of thumb, approved sealing materials, and ventilation equipment. Sealing and ventilation-rate questions are code-flavored lookups; the airflow math and ERV/HRV distinction are concepts to know cold.

Core concepts

Equal friction is the default sizing method

The equal friction method holds the friction rate — pressure drop per unit length, in inches of water column per 100 feet — constant for every duct run, and lets the duct size vary with the airflow each section carries. It's the standard approach for residential and light commercial design.

Airflow scales with tonnage

Design cooling airflow runs about 350–400 CFM per ton. A 3-ton system at 400 CFM/ton needs roughly 1,200 CFM. In humid climates designers lean toward the 350 end — slower air over a colder coil removes more moisture.

Mastic, not duct tape

Duct joints get sealed with mastic or tapes listed and labeled for duct sealing (UL 181). Ordinary cloth-backed 'duct' tape is never the answer — its adhesive dries out and fails. Leaky ducts waste capacity and can pull humid unconditioned air into the system.

ERV = heat + moisture, HRV = heat only

A heat recovery ventilator exchanges only sensible heat between exhaust and incoming air; an energy recovery ventilator transfers moisture too. In Florida the ERV wins, because it sheds moisture from the incoming humid air and cuts the latent load.

Key facts to know cold

Equal friction methodSame friction rate (pressure drop per 100 ft) for all runs; sizes vary with airflow

Cooling airflow rule of thumb350–400 CFM per ton (3 tons ≈ 1,200 CFM at 400)

Approved duct sealingMastic or UL 181-listed tape — never cloth-backed duct tape

ERV vs HRVERV transfers heat and moisture; HRV transfers heat only

Condensate trapKeeps air handler suction from blocking the drain while letting water flow by gravity

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

code

Florida Building Code

Code questions give themselves away with words like 'minimum', 'required', or 'permitted'. Go to the code — never answer a code question from memory when the book is on the desk.

9 recommended tabs

Lookup strategy

· Ventilation rates (outside air per person, by occupancy) are tables in the FBC Mechanical ventilation chapter — tab it and read the number off the page rather than memorizing it.
· Duct construction and sealing detail is SMACNA territory; the sizing method itself is Manual D. Match the question's verb: 'build/seal' → SMACNA, 'size' → Manual D.

Reading isn't learning — retrieval is.

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