12.2 Atmospheric & Power Burners: Operation, Aeration & Flame Adjustment

Atmospheric & Power Burners: Operation, Aeration & Flame Adjustment

Gas burners are precision devices engineered to mix fuel gas with atmospheric oxygen in exact proportions, deliver the combustible mixture to burner ports, and maintain a stable, clean flame. A Gas Technician 3 (G3) must master burner classifications, aeration fluid dynamics, manifold pressure testing, and flame troubleshooting to ensure high combustion efficiency and zero carbon monoxide ($CO$) production.

---

1. Burner Classifications and Operating Mechanics

Burners are classified according to how air for combustion is supplied and mixed with the fuel gas:

Atmospheric Burners

Atmospheric burners operate without mechanical fans to deliver combustion air. Instead, they harness the static pressure of the fuel gas itself. As fuel gas under pressure ($3.5\text{ in. w.c.}$ for natural gas) is forced through a precision-drilled orifice (spud), it expands rapidly into a high-velocity jet. This jet discharges into the throat of a venturi tube (mixing tube).

                    ATMOSPHERIC BURNER VENTURI DYNAMICS

  Gas Orifice     Air Shutter           Venturi Throat             Burner Ports
  [===] -> -> ->   (Primary Air)       (Narrow Section)          (Flame Emerges)
  High-Pressure      \\     //       Low-Pressure Vacuum            ( ( ( ( (
   Gas Stream         \\===//         (Bernoulli Effect)           ===========
                       |   |            Gas & Air Mix             Burner Head

Common atmospheric burner configurations include:

• Inshot Burners: Feature a straight horizontal venturi tube firing directly into individual tubular or clamshell heat exchanger passages. They are the universal standard in modern multi-port residential furnaces.
• Upshot Burners: Fire vertically upward from a curved elbow throat, commonly found in older conversion burners and cast-iron sectional boilers.
• Drilled Port Burners: Constructed of cast iron or stamped steel with precision circular drillings along the burner head, widely utilized in atmospheric storage water heaters.
• Ribbon Burners: Feature continuous corrugated or crimped metal ribbons inserted into a slot along the burner top, producing a continuous sheet of flame ideal for commercial bake ovens and radiant unit heaters.

Power Burners & Mechanical Draft Systems

Power burners utilize an electric motor-driven fan to supply combustion air under controlled mechanical pressure:

• Forced-Draft Burners: The blower is mounted upstream of the combustion chamber, pushing combustion air through the burner head under positive pressure. Power burners can overcome the resistance of high-efficiency, multi-pass commercial boilers.
• Induced-Draft Burners: The blower is mounted downstream at the heat exchanger exit, creating a negative pressure (suction) that pulls combustion products through the heat exchanger and exhausts them into the vent.
• Mechanical Premix Burners: A variable-speed blower draws in fuel gas and combustion air simultaneously through a modulating venturi, thoroughly homogenizing the mixture before forcing it into a stainless steel or ceramic mesh cylindrical burner head ($100\%$ premix).

---

2. Aeration Dynamics: Primary Air vs. Secondary Air

Complete hydrocarbon combustion requires substantial volumes of oxygen. Air is composed of approximately $20.9\%$ oxygen ($O_2$) and $79.1\%$ nitrogen ($N_2$) by volume. Consequently, supplying $1\text{ cu ft}$ of oxygen requires approximately $4.76\text{ cu ft}$ of atmospheric air.

The Combustion Chemistry Equation

For methane (the primary constituent of natural gas, $>95\%$):

$\text{CH}_4 + 2\text{O}_2 + 7.52\text{N}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + 7.52\text{N}_2 + \text{Heat}$

• Stoichiometric Ratio for Natural Gas: $1\text{ cu ft}$ of natural gas requires $2\text{ cu ft}$ of pure $O_2$, which equates to $9.53\text{ to }10.0\text{ cu ft}$ of air.
• Stoichiometric Ratio for Propane ($C_3H_8$): $1\text{ cu ft}$ of propane requires $5\text{ cu ft}$ of pure $O_2$, which equates to $23.8\text{ to }25.0\text{ cu ft}$ of air.

Primary Air Entrainment & The Venturi Effect

In atmospheric burners, combustion air is delivered in two distinct stages:

1. Primary Air: Ambient air drawn into the burner mixing tube through the adjustable air shutter by the vacuum created by the high-velocity gas jet issuing from the orifice. Under Bernoulli's Principle, as fluid velocity increases through the narrow venturi throat, static pressure drops below atmospheric pressure. This pressure differential entrains primary air. Primary air typically accounts for $30\%\text{ to }50\%$ of total stoichiometric air requirements. It premixes thoroughly with raw gas inside the venturi tube before arriving at the burner ports.
2. Secondary Air: Ambient air drawn into the combustion chamber around the exterior of the burner flame ports by natural thermal buoyancy or draft inducer action. Secondary air supplies the remaining $50\%\text{ to }70\%$ of oxygen required to complete the combustion reaction.

The Necessity of Excess Air

Under real-world field conditions, perfect molecular contact between gas and oxygen is impossible. If exactly $10\text{ cu ft}$ of air is supplied per cubic foot of natural gas, pockets of unburned fuel will decompose into deadly carbon monoxide ($CO$) and soot. To ensure complete combustion, systems are engineered to supply $20\%\text{ to }50\%$ excess air:

$\text{Total Air Required} = \text{Stoichiometric Air} + \text{Excess Air} = 10\text{ cu ft} + (0.30 \times 10) = 13\text{ cu ft air / cu ft gas}$

---

3. Flame Characteristics & Combustion Diagnostics

A properly adjusted burner produces a quiet, stable flame securely seated on the burner ports:

                         PROPER ATMOSPHERIC FLAME ANATOMY

                                  Outer Envelope (Mantle)
                                    - Pale blue / violet
                                    - Secondary air completes combustion
                                    - Up to 3,500°F (1,927°C)
                                     /\
                                    /  \
                                   / /\ \
                                  / /  \ \
                                 | |    | |
                                 | |____| |
                                  \  ||  /
                                   \_||_/
                                 Inner Cone
                                   - Bright blue, sharp outline
                                   - Primary air partial reaction
                                   - ~1,800°F - 2,000°F (982°C - 1,093°C)
                                 Burner Port

Flame Instability Conditions

When air-gas ratios or mixture velocities deviate from design parameters, three distinct malfunctions occur:

---

4. Manifold Pressure Verification & Firing Rate Clocking

Operating appliances at certified gas pressures is essential to maintain rated thermal output and prevent overfiring or underfiring.

Standard Manifold Operating Pressures

Combination gas valves incorporate internal gas pressure regulators that reduce building supply piping pressure to precision burner manifold pressure:

• Natural Gas Standard Manifold Pressure: $3.5\text{ in. w.c.}$ ($0.87\text{ kPa}$)
• Propane Standard Manifold Pressure: $10.0\text{ to }10.5\text{ in. w.c.}$ ($2.49\text{ to }2.61\text{ kPa}$)

Testing Manifold Pressure with a Manometer

To test manifold gas pressure:

1. Shut off electrical power and close the manual gas shutoff valve.
2. Remove the $1/8\text{-in.}$ NPT manifold pressure tap plug downstream of the gas valve seat, or loosen the barbed pressure screw.
3. Attach a calibrated digital manometer or U-tube water manometer hose securely.
4. Restore gas and power, set the thermostat to call for heat, and observe the pressure while all burners operate at steady-state.
5. If adjustment is required, remove the regulator cap screw and rotate the adjustment slot clockwise to increase pressure or counter-clockwise to decrease pressure.

Clocking the Gas Meter to Verify Firing Rate

To verify whether an appliance is consuming fuel at its certified nameplate input rating (BTU/h), the technician must "clock" the utility natural gas meter:

1. Ensure all other gas appliances in the building (water heaters, ranges, fireplaces) are shut off.
2. Fire the furnace at high fire and allow the flame to stabilize for at least 5 minutes.
3. Using a stopwatch, record the exact elapsed time in seconds for the test dial (typically the $1\text{ cu ft}$ or $2\text{ cu ft}$ dial) to complete exactly one full revolution ($360^\circ$).
4. Calculate the actual firing rate using the standard gas clocking formula:

$\text{Input Rating (BTU/h)} = \frac{3{,}600 \times \text{Dial Volume (cu ft)} \times \text{Heating Value (BTU/cu ft)}}{\text{Time in Seconds}}$

Worked Firing Rate Example

A furnace test dial of $2\text{ cu ft}$ completes one revolution in $72\text{ seconds}$. Using a nominal natural gas heating value of $1{,}000\text{ BTU/cu ft}$ (use the utility's actual value when you know it):

$\text{Input} = \frac{3{,}600 \times 2 \times 1{,}000}{72} = \frac{7{,}200{,}000}{72} = 100{,}000\text{ BTU/h}$

If the appliance nameplate specifies an input of $100{,}000\text{ BTU/h}$, the appliance is operating at exactly $100\%$ certified firing rate.