2.5 Oil-Fired Furnaces, Burners, Fuel-Oil Storage & Piping

Oil-Fired Furnaces, Burners, Fuel-Oil Storage & Piping

It is tempting to treat Michigan as a natural-gas-only state and skip oil heat. Do not. PSI's published content outline for the HVAC Equipment examination lists "gas piping systems utilizing natural, propane and fuel oil" as part of the scope, and LARA's own classification definitions use fuel oil twice: the limited heating service definition sets a firing-rate limit in gallons per hour on No. 1 and No. 2 fuel oil, and the unlimited heating service definition is written as servicing "without restrictions concerning thermal capacity or grade of fuel oil or type of fuel." Rural northern Michigan, older Upper Peninsula housing stock, and properties beyond the gas main still run oil, and the exam reflects that.

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Fuel Oil Properties You Must Know Cold

The Btu-per-gallon figure is the one that turns nozzle sizes into input ratings:

$\text{Input (Btu/h)} = \text{Nozzle rating (gph)} \times 140{,}000$

A 0.75 gph nozzle fires 105,000 Btu/h. A 1.25 gph nozzle fires 175,000 Btu/h. Output is input times the AFUE, so an 85% AFUE furnace with a 1.00 gph nozzle delivers roughly 119,000 Btu/h.

Notice how this interacts with licensing. LARA caps limited heating service on oil-fired equipment at a maximum firing rate of less than five gallons per hour per unit — about 700,000 Btu/h of input. Above that, the servicing work requires unlimited heating service.

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The High-Pressure (Pressure-Atomizing) Gun Burner

Essentially every modern residential oil burner is a pressure-atomizing gun burner. Trace its components in order, because sequence-of-operation questions follow this order:

1. Fuel unit (oil pump). A gear pump driven off the burner motor shaft. It draws oil from the tank and raises it to atomizing pressure — classically about 100 psig, and 140 to 150 psig on many modern high-static burners. The pump contains a built-in pressure-regulating valve and a cutoff valve that snaps the oil flow on and off cleanly so the burner does not dribble at start and stop.
2. Nozzle. A brass-bodied orifice with a swirl chamber. It is specified by three characteristics that must all match the manufacturer's rating plate:
   • Flow rate in gph at 100 psig,
   • Spray angle (30°, 45°, 60°, 70°, 80°, 90°),
   • Spray pattern — hollow (A/H), solid (B/S), or semi-solid.
3. Electrodes and ignition transformer / solid-state igniter. The transformer produces roughly 10,000 volts across the electrode tips, which sit above and ahead of the nozzle centerline and never in the oil spray itself. Electrode gap and position are set with a gauge, not by eye.
4. Blast tube, air tube, and end cone. Combustion air is metered by an air band and air shutter on the housing.
5. Cad cell. A cadmium sulfide photocell mounted to see the flame. Its resistance falls with light: dark it reads on the order of hundreds of thousands of ohms; sighted on a bright flame it should read roughly 300 to 1,500 ohms.
6. Primary control. The safety brain. On a call for heat it energizes motor, igniter, and valve and starts a trial for ignition (the "safety timing," commonly 15, 30, or 45 seconds). If the cad cell does not prove flame inside that window, the control locks out and requires a manual reset.

Exam trap. A burner that runs, ignites, then drops out and locks out is behaving exactly like a gas furnace with a dirty flame sensor. The oil equivalent is a sooted or loose cad cell or a cracked cad-cell holder. Check cad-cell resistance with the flame established before condemning the primary control.

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Combustion Setup and Testing

Oil is tuned with instruments, never by flame color alone:

Set the smoke number first, then trim the air band to the highest CO₂ that holds a zero-to-trace smoke, then back off slightly for a safety margin. Adjusting for CO₂ before smoke produces a burner that looks efficient on a meter while packing the heat exchanger with soot — and 0.03 inch of soot on a heat exchanger surface costs roughly 2% to 3% in efficiency.

Draft is regulated by a barometric draft regulator in the vent connector. Unlike a gas draft hood, it is adjustable by moving the counterweight, and on oil it must be set to the manufacturer's over-fire draft.

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Fuel-Oil Storage and Piping (Michigan Mechanical Code Chapter 13)

Fuel-oil piping and storage live in Chapter 13 of the Michigan Mechanical Code, which is a separate chapter from the gas piping rules in the IFGC. The essentials:

Tanks

• Indoor tanks are typically 275-gallon steel tanks supported on legs and set level on a firm, noncombustible base.
• Tank vent piping must be sized and run so the tank cannot be pressurized during filling, and the vent terminates outdoors with a weatherproof cap. Vent whistles let the delivery driver hear the tank filling and stop before overfill.
• Fill piping must be accessible from outside, capped, and clearly identified.
• Tanks must be protected against vehicle impact where exposed, and underground tanks bring additional Michigan environmental requirements outside the mechanical permit.

Supply piping

• One-pipe system: a single supply line, gravity feed, tank outlet above the burner. Simple, but any air that gets in must be manually bled.
• Two-pipe system: supply plus return, required when the tank is below the burner so the pump can lift oil (the pump's bypass plug must be installed — a classic no-fire callback is a two-pipe system with the bypass plug left out, or a one-pipe system with the bypass plug left in).
• Install a filter or strainer at the tank and, on many jobs, a secondary filter at the burner. Use flare fittings, not compression fittings, on oil lines; soft copper tubing routed and supported so it cannot be damaged.
• Anti-siphon protection. Where the tank is located so that its oil level can be above the burner, the code requires protection — an anti-siphon valve or an approved arrangement — so a broken line cannot siphon the entire tank onto the floor. This is a frequent inspection item and a frequent exam question.
• A readily accessible manual shutoff valve must be provided at the tank and at the burner.

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Diagnostic Quick Table