Table used: 6.2.1(b) — natural gas, 0.60 SG, less than 2 psi inlet, 0.5 in. w.c. drop, Schedule 40. Other conditions use a different table. Verify against your current NFPA 54 (2024) tables before final sizing.
Combustion Air
Confined vs unconfined space, and the opening sizes if the space is confined.
Standard method shown (50 ft³ per 1,000 Btu/hr; indoor openings 1 in² per 1,000 Btu/hr, min 100 in²; outdoor 1 in² per 4,000 direct/vertical or 1 in² per 2,000 horizontal). Louvers reduce free area (~75% metal / ~25% wood). Confirm the method and figures in NFPA 54 (2024) §9.3.
Meter Clocking
Clock the meter with everything else off to confirm an appliance's actual input.
Formula: cfh = 3600 × dial size ÷ seconds. Btu/hr = cfh × heating value. Local heating value varies — get the actual value from the utility for billing-grade numbers.
Pressure ↔ Water Column
Convert between psi and inches of water column. 1 psi = 27.7 in. w.c.
Type the value you know into either box — the other updates. Nameplate manifold pressure always governs the appliance.
Vent Size — Category I (Type B, single appliance)
NFPA 54 Tables 13.1(a)/(b) — one Category I appliance into a Type B double-wall vent, through a Type B double-wall or single-wall metal connector. Enter the input, connector & draft type, vent height and lateral; see which vent diameters carry it.
How to read it: a draft-hood appliance uses the NAT Max column (input must be ≤ NAT Max). A fan-assisted appliance must land between FAN Min and FAN Max. NA = not allowed. Lateral rows already include two 90° elbows; each extra elbow cuts max ~5% (≤45°) or ~10% (>45°). A vertical vent larger than the connector uses the vertical vent diameter for FAN Min and the connector diameter for the max (§13.1.9), area ≤ 7× the appliance outlet. For vents not exposed to the outdoors below the roof line. Table 13.1(a) = Type B connector; 13.1(b) = single-wall connector (up to 50 ft, 3–8″). Confirm against your current NFPA 54 (2024) tables.
Combustion Analysis
From a flue-gas sample: excess air, CO₂, air-free CO, net stack temp, and a steady-state efficiency estimate. Enter O₂, CO, and the flue & ambient temperatures.
Formulas: excess air = O₂ ÷ (20.9 − O₂) × 100; air-free CO = CO × 20.9 ÷ (20.9 − O₂) (NFPA 54 Annex G, Table G.6a); CO₂ ≈ CO₂max × (1 − O₂/20.9), CO₂max ≈ 11.8% natural gas / 13.8% propane; steady-state efficiency ≈ 100 − K × (flue − ambient °F) ÷ CO₂, K ≈ 0.21 NG / 0.23 LP (Siegert; sensible/dry — a training estimate, your analyzer's algorithm and any condensing/latent credit govern). Air-free CO over ~400 ppm (furnace, Table G.6) or a rising trend = investigate. Confirm against your analyzer and the manufacturer's spec.
High-Altitude Derate
Appliance input ratings are for sea level. Above 2,000 ft, NFPA 54 §11.1.2 reduces input 4% per 1,000 ft above sea level (traditional above-2,000-ft variant shown too). Enter the nameplate input and the installation elevation.
Rule: no derate required at or below 2,000 ft; above 2,000 ft, the code text reduces input 4% per 1,000 ft above sea level (NFPA 54 §11.1.2(1); the Annex E equivalent-orifice tables bake the same 4% in). The looser "4% per 1,000 ft above 2,000 ft" you may have learned is traditional practice — shown here as the secondary number. Many modern appliances are certified to 4,500–10,000 ft without derating — the manufacturer's high-altitude instructions and orifice/derate kit govern, along with your AHJ. Re-check manifold pressure and combustion after any orifice change.
Training tools — verify before you cut pipe or set pressure. Results are for study and field estimation only. Always size and set from the current NFPA 54 (2024) tables and text, the appliance manufacturer's instructions, your AHJ, and your employer's procedures. Independent; not affiliated with any utility.