NATE Oil Service cheat sheet

Heat content of No. 2 fuel oil is about 138,000–140,000 Btu per gallon; use 140,000 Btu/gal for rough load estimates.

1 gallon of No. 2 oil weighs about 7.0 lb at 60°F; specific gravity is roughly 0.82–0.85 relative to water.

Heat output Btu/h = nozzle firing rate (gal/h) × 140,000 × steady-state efficiency. A 1.00 gph nozzle at 80% efficiency delivers about 112,000 Btu/h.

Degree-day method: annual Btu ≈ (design heat loss Btu/h) × 24 × heating degree days ÷ (design indoor–outdoor temperature difference), then divide by seasonal efficiency.

Heat loss by conduction: Q = U × A × ΔT, where U is in Btu/(h·ft²·°F); add infiltration and ventilation loads separately.

Hydronic systems use 1 Btu per pound per °F; water weighs 8.33 lb/gal, so 1 gal raised 1°F absorbs about 8.33 Btu.

No. 2 oil pour point is typically around 0°F to 15°F; gelling and waxing below that range starve the pump and nozzle.

Steady-state efficiency = (stack temperature rise ÷ CO₂) × a constant; higher stack rise or lower CO₂ signals lower combustion efficiency.

Stack dew point of oil flue gas is roughly 250°F–300°F; keeping net stack temperature below that range risks acidic condensation and corrosion.

Stoichiometric air for No. 2 oil is about 14.4 lb air per lb fuel, roughly 1,920 ft³ of air per gallon; burners run 1.2–1.5× that (about 12–15% CO₂).

Target smoke is a No. 0–1 Bacharach smoke spot with 11.5–13.5% CO₂ for retention-head burners; smoke above No. 2 indicates incomplete combustion.

Nozzle ratings are stamped in gph at 100 psi; actual flow varies with the square root of pressure: new gph = rated gph × √(actual psi ÷ 100).

Nozzle spray angle: 70°–80° for most flame-retention burners; 45° for narrow combustion chambers; hollow cone for retention heads, solid cone for older chambers.

Electrode gap is typically 1/8 in (0.125 in) with tips 5/16–7/16 in ahead of the nozzle face; misalignment causes delayed ignition and puffback.

Flame retention head swirls air and recirculates hot gas to stabilize ignition; head position and air shutter setting control CO₂ and smoke.

Delayed ignition and puffback commonly trace to weak ignition transformer (10,000–15,000 V), wet oil, or carbon-bridged electrode insulators.

Draft overfire target is about −0.02 in w.c. for conventional burners; flue draft after the barometric damper is typically −0.02 to −0.06 in w.c.

Combustion air requirement: roughly 1,920 ft³ per gallon at stoichiometric; at 50% excess air, plan about 2,900 ft³ per gallon.

Tank capacity in gallons: rectangular = L × W × H (ft) × 7.48; cylindrical = π × r² × length (ft) × 7.48.

A full 275-gal oval tank holds about 266 gal usable; a 330-gal tank about 300 gal; keep fill level at 80% or less for expansion and gauge accuracy.

Tank slope: pitch toward the drain/outlet end at about 1/4 in per foot so water and sludge collect at the low point for removal.

Fuel line sizing: 3/8 in OD copper is typical for one-pipe runs under about 50 ft; 1/2 in OD for longer runs or two-pipe systems.

Two-pipe system: supply and return from the pump, with a bypass plug removed; one-pipe systems require the bypass plug installed and rely on gravity return.

Lift limit for a single-stage pump is about 8–10 ft; two-stage pumps handle lifts up to about 15–17 ft with adequate vacuum.

Vacuum at the pump should not exceed about 12 in Hg (roughly 6 in Hg for two-stage); higher vacuum signals a restricted line, clogged filter, or air leak.

Tank vent must be at least 1-1/4 in and unobstructed; a plugged vent causes slow fill, tank pressurization, and oil spill during delivery.

Oil filter (10–50 micron) should be installed on the supply line ahead of the pump; a clogged filter causes low pressure, noisy pump, and nozzle starvation.

OSHA/NFPA 31 requires tanks to be on stable supports, with fill and vent outside, and a gauge or whistle to prevent overfill.

Primary control (cad cell) sequence: thermostat calls → burner motor and ignition start → cad cell senses flame → control holds flame; if no flame in 15–45 s, lockout occurs.

Cad cell resistance: below about 1,600 ohms indicates flame present; above about 1,600 ohms (often 10,000+ with no flame) the control drops out and locks out.

Ignition transformer output is typically 10,000–15,000 V at 10–23 mA; a weak transformer causes delayed ignition and puffback.

Limit control (high-limit aquastat) breaks the burner circuit at a set high temperature; set typically 180°F–200°F for hydronic systems.

Circulator control: low-limit aquastat holds the circulator off until boiler water reaches about 120°F–140°F to prevent cold-start condensation.

Thermostat wiring: R to W calls for heat on a 24 V system; C is common; a heat-only system uses R and W only.

Blower motor and burner motor are typically 120 V single-phase; verify supply voltage and amp draw against nameplate before diagnosing.

Lockout reset: press the reset button once after correcting the fault; repeated resets without diagnosis risk flooding the combustion chamber with oil.