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Unit 5 — Pressure Testing, Tubing, and Piping
Section 2 — Air-Fuel

1.2 — Air-Fuel Assembly & Lighting

Assembly procedures establish a safe, ready-to-use outfit; correct lighting technique produces a stable flame and prevents gas accumulation before ignition. This lesson covers air-fuel outfit assembly sequence, pre-use inspection, and torch lighting procedure.

Air-Fuel Torch Assembly Flame Types 313A / 313D

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2.1.1 — Assembly Procedures for an Air-Fuel Outfit

Assembly means selecting the correct torch body and tip, confirming hose condition, connecting components correctly, and ensuring regulators and valves operate smoothly before introducing flame. A systematic pre-use inspection and assembly sequence eliminates the most common causes of torch malfunction and hazardous gas leaks.

Air-Fuel Outfit Components

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Fuel Cylinder

Propane or MAPP gas in a portable cylinder. Propane is the most common air-fuel source for HVAC/R soldering and brazing. MAPP burns hotter than propane and is suitable for brazing. Cylinder pressure varies with temperature — check the regulator gauge before each use.

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Regulator

Reduces cylinder pressure to a safe, consistent working pressure. For air-fuel work, a single-stage regulator is typical. Confirm the regulator is rated for the fuel gas being used (propane regulators are not interchangeable with acetylene regulators).

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Hose

Red hose carries fuel gas. Inspect the full length for cuts, abrasions, kinking, or deterioration before each use. Hose fittings on fuel lines use left-hand threads (identified by a notch on the nut) to prevent accidental cross-connection with oxygen equipment.

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Torch Body and Tip

The torch body mixes air and fuel at the tip orifice. Tip size determines heat output and flame spread: smaller tips for tight copper fittings and lighter-gauge tubing; larger tips for bigger diameter pipe. Always match the tip to the joint size and wall thickness.

Assembly Sequence

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Step 1 — Inspect All Components Before Connecting

  • Examine the hose for cuts, cracks, soft spots, or oil contamination — remove from service if damaged
  • Check hose fittings for stripped or crossed threads
  • Inspect the torch body for cracks, blocked air vents, or valve leaks
  • Inspect the tip orifice: it must be round, clean, and undamaged; replace if deformed or enlarged
  • Verify the regulator diaphragm and gauges are not physically damaged
  • Confirm the cylinder valve and protective cap are intact; check cylinder for dents, corrosion, or heat marks
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Step 2 — Connect the Outfit

  • Secure the cylinder upright in a cart, stand, or against a stable surface before opening any valve
  • Remove the cylinder valve cap and inspect the valve outlet for debris or damage; blow out briefly with the valve cracked open (away from ignition sources)
  • Thread the regulator onto the cylinder valve hand-tight, then snug with a wrench — never use a pipe wrench that could damage the regulator body
  • Confirm the regulator adjusting screw is backed out (counterclockwise, pressure off) before opening the cylinder valve
  • Connect the hose from the regulator outlet to the torch body inlet — fuel hose uses left-hand (reverse) threads; tighten firmly but do not over-torque
  • Thread the selected tip onto the torch body and tighten finger-tight plus a quarter turn; a snug tip prevents air leaks at the seat
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Step 3 — Pressurize and Verify

  • Open the cylinder valve slowly, one full turn; watch the high-pressure gauge rise to cylinder pressure
  • Turn the regulator adjusting screw clockwise until the working-pressure gauge reaches the manufacturer’s recommended pressure for the tip size selected
  • With the torch valve closed and the system at working pressure, perform a leak check on all connections before lighting (see Lesson 1.4)
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No Oil or Grease on Any Fitting

Never apply oil, grease, or pipe thread compound to fuel gas fittings. Contamination can damage regulator seats, cause valve sticking, and create fire risk. Fuel gas connections are metal-to-metal or use purpose-made tape only where a manufacturer specifies it.

2.1.2 — Air-Fuel Torch Lighting Procedures

Lighting procedures should be performed in a consistent sequence so the flame stabilizes quickly and can be adjusted to heat the joint evenly without overheating flux or base metal. A repeatable lighting procedure also ensures fuel does not accumulate before ignition, which is the primary cause of torch pop and minor flashback events.

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Always Use a Friction (Flint) Lighter

Never light a torch with a match, cigarette lighter, or any open-flame source held in the hand. Use an approved friction (flint) striker every time. An open-flame lighter in the hand puts the ignition source in the path of escaping gas and greatly increases burn risk.

Lighting Sequence

  1. Confirm the work area is safe: check that the joint area is clear of combustibles, heat shields are in place, fire-watch materials are accessible, and ventilation is adequate.
  2. Confirm cylinder valve is open and regulator is set to working pressure (completed during assembly). The torch valve should still be closed.
  3. Hold the friction lighter at the tip, cupping it slightly in front of the tip orifice at a 45° angle. Do not block the air intake holes on the torch body.
  4. Open the torch valve approximately one-quarter turn — just enough to allow fuel flow. Do not open more than one-half turn before igniting.
  5. Strike the lighter immediately after opening the valve; the flame should catch within one to two strikes. If it does not light within two attempts, close the torch valve, allow any accumulated gas to dissipate for a few seconds, then retry.
  6. Adjust the torch valve until the desired flame type is achieved. For most HVAC/R soldering and brazing, a soft, slightly reducing flame (small inner cone, no harsh roaring) heats the joint evenly and does not oxidize the copper surface.
  7. Allow the tip to warm briefly (10–15 seconds) before applying heat to the joint. A warm tip stabilizes the flame and reduces the likelihood of the flame lifting off the tip during work.

Flame Types for Air-Fuel Torches

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Soft / Gentle Flame

Low gas flow, small inner cone, quiet burn. Used for light-gauge soft solder joints (e.g., drain lines, small-bore water lines) where excess heat would damage flux or melt tubing wall before the joint fills.

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Full / Working Flame

Larger inner cone with a strong, defined tip. Standard setting for most copper refrigerant brazing with silver-alloy filler. Provides sufficient heat to bring the joint to brazing temperature without excessive dwell time that could overheat flux.

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Excessive / Roaring Flame

Gas flow too high: harsh sound, flame lifts from tip, inner cone poorly defined. Overheats and oxidizes the joint. Back off the torch valve until the flame is stable and the inner cone is well-defined with no lifting.

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