NATE Gas Heating Service common mistakes
NATE Gas Heating Service · Understand the traps and how to avoid them.
When computing combustion air for a confined space, candidates add up only the BTU/hr input of the appliance they are focused on and ignore the other appliances sharing the space, or they forget to convert input in BTU/hr to cubic feet of air using the 50 ft³ per 1,000 BTU/hr rule and instead apply the 1 in² per 1,000 BTU/hr opening rule to the volume calculation.
The two rules look similar (both keyed to 1,000 BTU/hr) and candidates memorize the opening sizing without understanding that the dilution-volume rule is about the room's cubic footage while the opening rule is about free area of supply air. They also tend to treat each appliance in isolation rather than summing all input in the space.
What to do instead
Before any calculation, list every fuel-burning appliance in the space and total their input. Compute required volume as total input ÷ 1,000 × 50 ft³. Only if the actual volume is less than required do you go to the opening method, and then use the standard two-opening (high and low) approach with each opening sized at 1 in² free area per 1,000 BTU/hr of total input, corrected for louvers. Practice with a room containing a furnace plus a water heater so the summing step becomes automatic.
On a Category I furnace, candidates install or approve a vent connector with a negative slope, or they allow a single-wall connector to run through an unconditioned attic without treating the resulting condensate and heat loss as a defect.
They focus on whether the flue gas will 'get out' and overlook that Category I appliances rely on a natural draft that depends on the connector rising toward the chimney and staying warm; negative slope and cold runs are the classic causes of spillage and condensation damage.
What to do instead
Apply the rule mechanically: vent connector slope upward toward the chimney at not less than 1/4 in per foot of run, shortest possible route, and single-wall connectors only where the code permits (not in unconditioned or concealed spaces, and with proper clearance to combustibles). When you see a sagging or back-pitched connector, call it as a venting defect, not a cosmetic one.
Candidates treat a manifold pressure reading as a single specification instead of matching it to the gas type and appliance rating plate, and they adjust manifold pressure to 'make the flame look better' rather than to the nameplate value.
Manifold pressure is the easiest knob to turn and the flame appearance is a tempting feedback signal, so candidates conflate the two. They also confuse supply (line) pressure with manifold pressure and may set the regulator to the wrong target.
What to do instead
Always read the rating plate first and record the specified manifold pressure for the gas being supplied (natural gas and LP have different targets, and LP appliances use a different regulator spring or conversion kit). Verify inlet pressure is within the appliance's allowed range with the unit running, then set manifold pressure to the nameplate value with a calibrated manometer, never by eye. Recheck after any gas valve or orifice change.
When a furnace will not ignite, candidates replace the igniter or the gas valve before verifying the safety circuit chain (pressure switch proving, limit continuity, rollout switch, flame sensor signal) with a meter.
Ignition components are visible and easy to swap, and a no-heat call feels like an ignition problem. Candidates skip the step-by-step electrical verification because it takes longer than parts-swapping.
What to do instead
Follow the sequence of operation from the schematic: call for heat, inducer proves, pressure switch closes, ignition trial, flame sensed, blower on. At each step, use a voltmeter to confirm voltage present or absent at the expected terminal. A pressure switch that never closes is usually a blocked flue, water in the tubing, or a failing inducer, not a bad igniter. Confirm flame sensor microamp signal and clean it before condemning it.
Candidates diagnose a flame rollout switch trip as a failed switch and reset or replace it without finding the cause, then declare the repair complete when the unit fires.
The switch is designed to trip and looks like the failed part; resetting it makes the unit run, which feels like a fix. The underlying cause (blocked heat exchanger, restricted flue, cracked exchanger, wrong orifice, overfiring) is less obvious.
What to do instead
Treat a tripped rollout as a symptom, never a cause. Before resetting, inspect the heat exchanger for cracks or blockage, verify venting and combustion air, check manifold pressure and orifice sizing, and confirm the inducer is moving adequate air. Document the cause you found. If you cannot identify a cause, do not leave the appliance operating.
When measuring temperature rise, candidates take a single supply-air reading at one register or measure return air at a distant grille, then compare that number directly to the rating plate rise without accounting for duct losses or blower speed.
Temperature rise is an airflow diagnostic, and candidates treat it as a simple two-thermometer subtraction. They do not realize the measurement must be taken at the appliance (return plenum and supply plenum) with steady-state operation, and that duct gains/losses corrupt readings taken at registers.
What to do instead
Drill or use existing ports in the return and supply plenums close to the unit, run the blower at the speed being evaluated until temperatures stabilize, and compute rise as supply minus return at the unit. Compare to the nameplate rise range. If rise is high, suspect low airflow (dirty filter, restrictive coil, wrong blower speed, undersized duct); if low, suspect excessive airflow or a bypass. Adjust blower speed per the airflow table, not by guess.
Candidates perform combustion analysis with the probe in the wrong location, before the appliance reaches steady state, or without zeroing/calibrating the analyzer in ambient air, and then interpret a single O₂ or CO reading as the whole story.
Analyzers are treated as push-button tools. Candidates skip the warm-up and zeroing steps, sample too soon after startup, or sample in the flue before the unit stabilizes, producing numbers that are not comparable to specifications.
What to do instead
Zero the analyzer in outdoor or fresh ambient air, insert the probe into the flue at the manufacturer's specified location, and let the appliance run at steady state (typically 10–15 minutes) before recording. Read O₂, CO (air-free), CO₂, stack temperature, and draft together, then compute efficiency. A single CO number without O₂ and draft is not diagnostic. Recheck after any adjustment.
Candidates size black iron gas pipe from a table by matching only the appliance input and the longest run, ignoring the additional allowance for other appliances on the same branch, the specific gravity and heating value of the gas, and the pressure drop basis of the table being used.
Pipe sizing tables are dense and candidates grab the first column that looks close. They forget that a table built for a 0.5 in w.c. drop with natural gas at a given specific gravity does not directly apply to LP or to a system with several appliances sharing the run.
What to do instead
Identify the table's basis (pressure drop, gas specific gravity, inlet pressure) before using it. Total the connected load downstream of each pipe segment, measure the longest developed length including fittings, and select pipe size so the cumulative pressure drop stays within the table's allowance. For LP or unusual gas, use the correct specific-gravity column or convert. Verify with a manometer at the appliance inlet under full flow.
Candidates check gas piping for leaks with a flame or by smell, or they pressure-test with the appliance connected and the gas valve open, rather than isolating the appliance and using a manometer or approved leak-detection method.
Flame testing is a habit carried over from informal field practice, and candidates do not think through what a proper test isolates. They also skip the requirement to protect appliance controls from test pressure.
What to do instead
Isolate the appliance (close the manual shutoff, disconnect or cap the appliance connection as required), then pressure-test the piping per the applicable code with air or inert gas at the specified test pressure for the specified duration, using a calibrated gauge or manometer. For operating leaks, use an approved bubble solution on joints, never a flame. Confirm the appliance regulator and valve are not exposed to test pressure beyond their rating.
Candidates verify airflow only by feeling for 'good' supply air at a register, or they set blower speed by trial and error, without measuring static pressure across the blower and comparing to the manufacturer's fan curve.
Static pressure requires gauges and interpretation of a fan performance table, while 'feels strong' is immediate. Candidates avoid the table because it looks complicated and instead swap speeds until the temperature rise looks acceptable.
What to do instead
Measure external static pressure with a manometer at the supply and return plenum taps, add the coil/filter pressure drops per the manufacturer's data, and plot total static against the fan table to read actual CFM at the installed speed. Then compare CFM to the required airflow for the temperature rise and capacity. Adjust speed or correct duct restrictions based on the table, and record the before/after numbers.