Oil Heating: Nozzles, Pressure and Combustion Testing
Oil burners: nozzles, pressure and combustion testing
Oil heat questions revolve around the burner's hydraulic and electrical parts. Know the sequence: thermostat calls, the primary control starts the burner motor and ignition transformer, the fuel unit builds pressure (typically 100 psi, sometimes 140 psi), the nozzle atomizes oil, the electrodes ignite the spray, and the cad cell proves flame to hold the control in. Without flame proof, the primary locks out on its safety timing.
Know that nozzle ratings are stated in gph at 100 psi, and that flow changes with the square root of pressure, so raising pressure from 100 to 140 psi increases flow about 18%. Know the 140,000 Btu per gallon rule, electrode settings per the manufacturer, and combustion targets: CO2 roughly 11-13%, smoke 0-2 (ideally 0), plus stack temperature and draft per the data plate, often about -0.02 in. wc. overfire.
Traps: sizing a nozzle by eye, overfiring a derated boiler, ignoring a delayed-ignition pattern that soots the chamber, and setting electrodes by guesswork. One-pipe systems with excessive lift, or air in the lines, starve the pump and cause lockouts that look electrical.
Worked example: a 0.85 gph nozzle at 100 psi delivers 0.85 x 140,000 ≈ 119,000 Btu/hr input. If the nameplate calls for 100,000 Btu/hr, the correct nozzle is about 0.70 gph, not a larger nozzle turned down with pump pressure.