Oil Heating Fundamentals
This domain tests how No. 2 heating oil behaves as a fuel and how a technician judges whether a system converts it efficiently. Expect questions on fuel properties, the combustion triangle, draft, stack temperature, CO2 and O2 readings, smoke number, and steady-state efficiency.
Fuel and combustion basics
- No. 2 oil is a distillate with roughly 138,000–140,000 Btu per gallon; its flash point is typically above 100°F, so a room-temperature puddle will not flash.
- Viscosity rises as oil cools; thick oil atomizes poorly and can overload the pump or make a smoky flame.
- Complete combustion needs fuel, air, and ignition in proportion. Oil burners normally run 20–50% excess air, about 10–13% CO2 and 4–9% O2.
- Draft is measured in inches of water column. Overfire draft is normally slightly negative; a blocked flue or weak chimney changes it at once.
Efficiency
Steady-state efficiency comes from stack temperature, net stack temperature (stack minus room), CO2 or O2, and smoke. A clean heat exchanger and correct excess air raise efficiency; high net stack temperature and high excess air send heat up the chimney.
Worked example
A nozzle rated 1.00 gph at 100 psi fires 1.00 × 140,000 = 140,000 Btu/hr. With 11% CO2, zero smoke, and 430°F net stack, the efficiency chart reads about 80%. Output = 140,000 × 0.80 = 112,000 Btu/hr. If net stack rose to 550°F while input stayed the same, efficiency would drop several points and output would fall - the extra heat is leaving through the flue.
Traps
- Nozzle flow is rated at 100 psi; raising pump pressure raises firing rate.
- Zero smoke does not mean high efficiency; a hot stack can still waste heat.
- Read net stack temperature, not gross, on an efficiency chart.