7.1 Gas-Fired Heating: Natural Gas, LP, Burners, Heat Exchangers, and Gas Piping

Combustion Fundamentals and Gas Fuel Chemistry

Gas-fired heating systems generate thermal energy through the chemical reaction of hydrocarbon fuels with atmospheric oxygen. To design, install, and service gas furnaces safely in accordance with Texas mechanical codes and International Fuel Gas Code (IFGC) standards, HVAC contractors must master the physical and thermodynamic differences between Natural Gas and Liquid Propane (LP).

Physical Properties of Fuel Gases

Hydrocarbon fuel gases differ significantly in specific gravity, energy density, air-to-fuel combustion ratios, and operating pressures. Specific gravity is the ratio of the weight of a gas volume to the weight of an equal volume of dry air (where air equals 1.00).

Safety Mandate: Because Liquid Propane has a specific gravity of 1.52, unburned LP gas settles in basements, crawlspaces, floor drains, and low-lying structural areas. Leaking LP gas poses a severe explosion hazard because it does not dissipate upward into the atmosphere like natural gas.

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Liquid Propane (LP) Gas Conversion Field Procedures

Most modern gas furnaces leave the factory configured for Natural Gas operating at 3.5 in. w.g. manifold pressure. When installing an appliance on a Liquid Propane supply system, the HVAC technician must install a manufacturer-approved LP conversion kit prior to firing the unit.

Required LP Conversion Steps

1. Burner Orifice Replacement: LP gas has 2.5 times the heat content per cubic foot compared to natural gas ($2,500 \text{ BTU}$ vs $1,000 \text{ BTU}$) and operates at higher manifold pressure. Therefore, natural gas orifices must be removed and replaced with physically smaller orifice openings (higher drill bit number size). If natural gas orifices are left in place on an LP system, the furnace will be severely over-fired, producing dangerous levels of soot and Carbon Monoxide (CO).
2. Gas Valve Pressure Regulator Modification: The internal pressure regulator spring of the combination gas valve must be replaced with a stiffer LP regulator spring (or flipped, depending on valve design) to increase regulated manifold output from 3.5 in. w.g. up to 10.0 - 11.0 in. w.g.
3. Pilot Orifice Replacement: On standing or intermittent pilot units, the pilot burner orifice must be replaced with a smaller LP pilot orifice to match the higher LP supply pressure.
4. Pressure Switch & Burner Adjustments: Verify low-pressure cut-out switches (required on LP installations by IFGC) and check primary air shutter settings if adjustable.
5. Conversion Rating Label: Affix the completed conversion rating plate and label to the furnace casing stating the date of conversion, technician license number, and new orifice/pressure parameters.
6. Manometer Verification: Connect a digital or U-tube manometer to the gas valve outlet pressure tap to verify manifold static pressure under full burner load.

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Burner Designs and Aerodynamics

Furnace burners mix fuel gas with primary air and deliver the mixture to the ignition point inside the heat exchanger. Texas contractors encounter two primary burner types:

Inshot Burners

Modern induced-draft and forced-draft furnaces utilize inshot burners. An inshot burner features a venturi-shaped steel body mounted on a distribution manifold alignment rail.

• Operation: High-velocity gas issuing from the orifice creates a low-pressure area inside the burner throat (Bernoulli's principle), drawing in primary air around the orifice. The gas-air mixture shoots horizontally into the entry tube of an individual heat exchanger cell.
• Advantages: Highly stable flame, compact footprint, resistance to draft disturbances, and optimized mixing for low-emission combustion.

Ribbon and Ported Slotted Burners

Older natural-draft, atmospheric furnaces utilized multi-port ribbon burners or cast-iron slotted burners located underneath vertical clamshell heat exchangers.

• Operation: Gas flows from multiple orifices along a horizontal manifold pipe into long slotted ports. Primary air enters through adjustable air shutters at the burner entry.
• Maintenance: Prone to rust scale accumulation from overhead heat exchangers, which clogs burner ports and causes yellow, soot-producing flames due to lack of primary air.

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Heat Exchangers: Primary vs. Secondary and Condensate Chemistry

The heat exchanger transfers thermal energy from hot combustion gases to the circulating indoor air stream while keeping toxic flue gases completely isolated from the occupied building envelope.

Non-Condensing Furnace (80% AFUE):
Combustion Gas -> Primary Heat Exchanger (Steel) -> Vent Stack (300°F - 450°F Flue Gas)

Condensing Furnace (90%+ AFUE):
Combustion Gas -> Primary Heat Exchanger -> Secondary Heat Exchanger (Stainless) -> PVC Vent (100°F - 130°F Flue Gas + Acidic Condensate)

Primary Heat Exchangers

• Construction: Manufactured from heavy-gauge aluminized steel or stainless steel stamped into clamshell halves or bent into serpentine tubular configurations.
• Operating Temperature: Used in 80% AFUE non-condensing Category I furnaces. Flue gas exit temperatures remain high ($300^\circ\text{F} - 450^\circ\text{F}$), well above the flue gas dew point ($\approx 130^\circ\text{F}$).
• Failure Modes: Continuous thermal expansion and contraction can cause metal fatigue, leading to cracks around stress points, welds, and turn bends. A cracked heat exchanger allows carbon monoxide to enter the supply air stream, requiring immediate unit shutdown under Texas safety standards.

Secondary Heat Exchangers (Condensing Furnaces)

• Construction: Condensing furnaces (90%+ AFUE) add a secondary heat exchanger constructed of corrosion-resistant AL29-4C stainless steel or 316L stainless steel tubing equipped with high-density exterior cooling fins.
• Latent Heat Extraction: Flue gases exiting the primary heat exchanger enter the secondary heat exchanger, where cooler return air ($65^\circ\text{F} - 70^\circ\text{F}$) cools the combustion gases below their dew point ($\approx 130^\circ\text{F}$). Water vapor ($\text{H}_2\text{O}$) produced during combustion condenses into liquid water, releasing its latent heat of vaporization ($970 \text{ BTU per pound of water}$). This latent heat extraction boosts furnace thermal efficiency above 90-98%.
• Acidic Condensate Chemistry: Combustion condensate contains dissolved carbon dioxide, nitrogen oxides, and sulfur traces, forming a weak solution of carbonic and nitric acids with a pH of 3.0 to 5.0.

$\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \quad \text{(Carbonic Acid, pH 3-5)}$

• Drainage Requirements: Acidic condensate will rapidly corrode standard copper, iron, or steel piping. Condensing furnaces mandate Schedule 40 PVC, CPVC, or ABS plastic drain lines leading to a dedicated condensate trap. Where discharged into public sewers, local plumbing codes often mandate passing condensate through a limestone condensate neutralizing kit to elevate pH toward neutral (7.0).

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Gas Ignition Systems

Furnace controls have evolved through four distinct ignition methods:

1. Standing Pilot: A continuous gas pilot burns 24/7. A bimetallic thermocouple positioned in the pilot flame generates $30 \text{ mV DC}$ through the Seebeck effect, powering an electromagnet inside the main gas valve to hold the safety pilot valve open.
2. Intermittent Pilot (IP): Energy-saving system that ignites the pilot flame only upon a call for heat using a high-voltage spark electrode. Once a flame rod proves pilot flame, the electronic module opens the main gas valve. The pilot turns off when the thermostat call ends.
3. Direct Spark Ignition (DSI): Eliminates the pilot entirely. Upon call for heat, an electronic control board opens the main gas valve and generates a high-voltage ($10,000\text{V}$) repetitive spark directly across the main burner ports to ignite gas immediately.
4. Hot Surface Ignition (HSI): Utilizes a solid-state resistance element composed of silicon carbide or silicon nitride. Upon a call for heat, the board applies 120V AC to the HSI element, heating it to $1,800^\circ\text{F} - 2,500^\circ\text{F}$ (glowing bright yellow/white) before opening the main gas valve.

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Flame Rectification Principle and Microamp Diagnostics

Modern DSI, IP, and HSI gas furnaces rely on flame rectification to verify burner flame presence and prevent explosive gas accumulation.

How Flame Rectification Works

1. Ionization: A gas flame contains free electrons and positively charged ions created by high-temperature chemical reactions, making the flame an electrical conductor.
2. AC Voltage Application: The furnace control module applies a $120\text{V AC}$ potential to a stainless steel flame sensor rod inserted directly into the burner flame.
3. Asymmetric Surface Area: Current flows from the small tip of the flame rod through the flame to the grounded metal burner assembly. Because the surface area of the grounded burner is vastly larger than the flame rod tip (often a 10:1 or 20:1 ratio), electrons flow far more easily in one direction.
4. Rectification to DC: This physical asymmetry converts (rectifies) the alternating current (AC) signal into a pulsating direct current (DC microampere signal, $\mu\text{A}$).

$\text{Healthy Flame Signal} = 1.0\, \mu\text{A DC} \text{ to } 5.0\, \mu\text{A DC} \quad (\text{Microamp Drop-out Threshold} \approx 0.5 - 1.0\, \mu\text{A DC})$

Control Board (120V AC Output) ---> Flame Rod ---> Ionized Flame ---> Grounded Burner (Large Area) ---> Control Board (DC µA Sense Input)

Microamp Diagnostic Troubleshooting

• Low Microamp Signal (< 0.5 µA): Causes furnace nuisance lockout after 3 to 5 seconds of main burner ignition.
• Common Causes: Oxide/silicon coating on the flame rod (clean using fine steel wool or abrasive Scotch-Brite pad; never use sandpaper, as quartz/silicate dust bakes into a non-conductive glass insulator on the rod), loose furnace chassis ground wire, or flame lifting off burners due to excessive manifold gas pressure or over-firing.