HVAC Excellence Gas Heat Certification Exam common mistakes
HVAC Excellence Gas Heat · Understand the traps and how to avoid them.
Treating manifold pressure as a fixed number instead of looking it up for the specific gas and appliance rating plate, then 'correcting' a furnace that is actually within spec.
Candidates memorize a single value (often 3.5 in. w.c. for natural gas) and apply it to propane, derated appliances, or two-stage/Modulating units where the rating plate specifies different high-fire and low-fire pressures.
What to do instead
Always set manifold pressure from the rating plate for the installed fuel and stage. Know typical natural gas values (about 3.5 in. w.c.) versus propane (about 10-11 in. w.c.), and verify with a manometer at high fire and low fire with the burner box closed.
Diagnosing a no-heat call by swapping parts (ignitor, gas valve, board) before confirming the sequence of operation stalls at a specific step.
Time pressure and pattern-matching to the last similar job; candidates skip the observe-the-sequence step and never identify whether the failure is at call-for-heat, inducer proving, ignition, flame proving, or blower delay.
What to do instead
Run the sequence of operation step by step: thermostat call, inducer start, pressure switch prove, ignition trial, flame sense, blower on. Note the exact step where it stops and the fault code, then test that component with a meter rather than replacing parts.
Using a standard multimeter on the flame sensor or ignitor circuit and misreading microamp or voltage readings, or testing flame sense with the burner compartment open.
Microamp flame signals are tiny (often under 5 microamps) and require a meter on the microamp scale in series; testing with the door off changes airflow and flame shape, producing readings that do not reflect normal operation.
What to do instead
Measure flame current in series on the microamp scale with the unit assembled and running; know the manufacturer's minimum (commonly around 1-2 microamps, with higher being healthier). Clean the sensor with steel wool, never sandpaper, and confirm the sensor is in the flame.
Sizing or evaluating gas piping by pipe diameter alone, ignoring equivalent length for fittings, appliance load in BTU/h, and the pressure drop allowance.
Candidates remember a single 'max feet' table row and forget that elbows, tees, and valves add equivalent length, and that the table used depends on inlet pressure, pressure drop, and gas specific gravity.
What to do instead
Compute total load in BTU/h, convert fittings to equivalent length, select the correct table for the pressure and specific gravity, and confirm the delivered pressure at the appliance under load with a manometer. Remember sediment trap and drip leg requirements at the appliance connection.
Assuming combustion air is adequate because the closet 'looks ventilated', without calculating free area for the input rating and the opening type.
Candidates confuse louvered opening dimensions with free area, forget that grilles reduce free area, and do not distinguish confined versus unconfined space or indoor versus outdoor air methods.
What to do instead
Calculate required volume and compare to actual space; when using openings, apply the free-area rules for the method (for example two openings sized per 1,000 BTU/h with the correct free-area percentage for louvers) and verify no obstruction or negative pressure from exhaust fans.
Calling a heat exchanger 'bad' from a visual glance or a CO reading alone, or conversely passing a cracked exchanger because the flame looked fine.
Cracks hide at the transition, cell seams, and behind the blower shelf; CO can be elevated from other causes (dirty burners, wrong orifice, venting), and a steady flame does not rule out a breach.
What to do instead
Use multiple methods: visual inspection of the accessible sections, a combustion analyzer before and after the blower starts, and manufacturer-approved tests. Confirm with the analyzer that CO and O2 shift when the blower energizes, and follow the manufacturer's acceptance criteria before condemning or passing.
Verifying temperature rise with an inaccurate thermometer or with the wrong measurement points, then adjusting blower speed to 'fix' a rise that was actually in range.
Candidates use clip-on dial thermometers, measure in the wrong duct location, or take the reading before the system stabilizes, and they do not convert rise to expected CFM using the sensible heat equation.
What to do instead
Use calibrated digital probes in the return and supply plenums out of the line of sight of the heat exchanger, let the unit stabilize, and compare measured rise to the rating plate range. Use CFM equals output BTU/h divided by (1.08 times temperature rise) to sanity-check airflow before changing speed taps.
Miswiring or misreading low-voltage schematics, especially around the common terminal, the C-wire, and safety switch strings in series.
Candidates trace color codes instead of the schematic, forget that limits, rollout switches, and pressure switches are wired in series in the 24V circuit, and overlook that some boards need a common wire for the thermostat.
What to do instead
Read the wiring diagram legend and trace from transformer through each switch to the gas valve operator. Confirm 24V at each point in the string with the meter, and remember that an open safety anywhere in series removes the call to the gas valve.
Ignoring combustion analyzer results and relying only on flame appearance, or taking a single reading at one firing rate.
A yellow-tipped flame is a crude indicator; proper diagnosis needs O2, CO, CO2, and stack temperature, and two-stage or modulating equipment must be checked at each rate with the correct steady-state conditions.
What to do instead
Take steady-state readings after several minutes, record O2, CO (air-free), CO2, and stack temperature, and compare against the manufacturer's range. Check for draft and spillage at the vent and dilution air openings, and recheck after any adjustment.
Skipping the safety check on ignition components: not verifying hot surface ignitor resistance, not checking for a proper ground, and not confirming flame rollout and limit switches actually open.
Candidates treat these as pass/fail by eye, assume a glowing ignitor is good, and never test safety devices for actual function, so intermittent failures and latent hazards go undiagnosed.
What to do instead
Measure ignitor resistance and compare to the manufacturer's specification; verify the burner and board ground; and functionally test rollout, limit, and pressure switches (jumper-free, using the manufacturer's procedure) to confirm they open and close the circuit as designed.