MI HVAC common mistakes
MI HVAC · Understand the traps and how to avoid them.
Sizing heating and cooling equipment from square footage alone, ignoring the full Manual J-style load inputs such as window area, orientation, insulation values, infiltration, and internal gains.
Square-foot rules of thumb are widely repeated in the field and feel fast under time pressure, so candidates skip the component-based reasoning the load questions actually test.
What to do instead
For every sizing question, list the load components named in the stem (envelope, glazing orientation, infiltration, occupants, appliances) and reason about which direction each shifts the load before selecting an answer.
Selecting a furnace or air handler without checking that available static pressure covers the coil, filter, and duct resistance, then missing questions about airflow or blower performance.
Candidates treat equipment selection as a capacity-only decision and overlook the pressure budget concept, which is tested indirectly through scenario questions.
What to do instead
Practice computing total external static pressure as the sum of filter, coil, and duct drops, and compare it to the blower rating; when a stem gives those values, add them before answering.
Conflating combustion air requirements with venting requirements, or applying the wrong category of venting (Category I vs. III/IV) to a given appliance.
The two topics are taught together and use overlapping vocabulary, so candidates default to a single remembered rule instead of matching the appliance type to the correct venting method.
What to do instead
Build a two-column mental table: combustion air (volume method, opening sizing, indoor vs. outdoor source) versus venting (appliance category, material, slope, termination); classify the appliance first, then apply the matching rule.
Misapplying gas pipe sizing tables by using the wrong length (developed length including fittings versus straight run) or ignoring the pressure drop and specific gravity assumptions built into the table.
Candidates memorize a single table entry and forget the conditions the table is based on, and developed length is easy to underestimate.
What to do instead
For each gas piping item, write down developed length, the table's pressure-drop and specific-gravity basis, and the connected load in Btu/h before choosing a pipe size; if a stem changes any basis, discard the default table.
Mixing up the roles of limit switches, rollout switches, flame sensors, and pressure switches when diagnosing a described fault sequence.
All are safety devices in the same circuit with similar-sounding functions, and candidates answer from memory of a component name rather than tracing the described sequence.
What to do instead
Sketch the safety circuit in the order the stem describes and label what each device proves (temperature, flame presence, draft, spillage); eliminate answers that assign a device a function outside its proof role.
Treating electrical questions as pure memorization of wire ampacity or breaker size without applying the 80 percent continuous-load rule, conductor derating, or motor branch-circuit rules.
Candidates recall a single ampacity number and skip the adjustment factors the stem implies, especially for continuous duty and motors.
What to do instead
On every electrical item, identify load type (continuous vs. noncontinuous), ambient/temperature correction, and conductor count before selecting; show the derating step explicitly in scratch work.
Applying generic national code knowledge to Michigan-specific permit, licensing, and inspection questions instead of the state's requirements.
Most study material is national in scope, so candidates answer with the model code default and miss Michigan-specific administrative rules.
What to do instead
Separate administrative items (who may pull permits, licensing classifications, inspection sequencing) from technical code items, and treat any Michigan-specific stem as a signal to recall state rules rather than model-code defaults.
Ignoring duct design constraints in air distribution questions, such as friction rate, equivalent length of fittings, and balancing damper placement, and answering from rules of thumb.
Duct sizing is often learned as a target velocity or friction rate without connecting it to the fittings and equivalent lengths the question provides.
What to do instead
When a stem supplies fittings or equivalent lengths, convert them to equivalent length and add to the straight run before applying the friction chart; check that the answer is consistent with the total effective length.
Confusing ventilation-for-indoor-air-quality requirements with exhaust-only or balanced ventilation strategies, and misjudging which spaces require exhaust versus supply.
Ventilation terminology overlaps across residential and commercial contexts, and candidates answer from a remembered label rather than the space's occupancy and use.
What to do instead
Classify the space first (occupancy type, contaminants, moisture source), then decide exhaust, supply, or balanced; eliminate answers that specify a strategy inconsistent with the stated contaminant or occupancy.
Skipping the estimating and business-practice questions or answering them from instinct, missing concepts like overhead recovery, markup versus margin, and change-order documentation.
Candidates focus study time on technical domains and treat business items as common sense, but the terms have precise definitions that differ from everyday usage.
What to do instead
Drill the arithmetic distinctions: margin is profit divided by price while markup is profit divided by cost, and overhead must be recovered before profit; compute rather than estimate on these items.