12.3 Gas Appliance Troubleshooting, Combustion Analysis & CO Safety

12.3 Gas Appliance Troubleshooting, Combustion Analysis & CO Safety

Quick Answer: The safety-inspection sequence in IFGC 2018 Appendix D requires you to confirm that the main burner is burning properly with no floating, lifting or flashback, adjust the primary air shutter as required, and then test for spillage at the draft hood relief opening after 5 minutes of main burner operation using smoke (D.5.2). The match-or-candle wording belongs to the 2000-2015 editions; the 2018 text says smoke.

Everything in this section is diagnostic: the appliance is already installed, and your job is to prove it burns cleanly, drafts properly and cannot poison the occupants. Two numbers anchor the topic -- 400 ppm air-free carbon monoxide (CO), the ANSI Z21 ceiling for a central furnace's flue gas, and 50 ppm, the OSHA permissible exposure limit for the air a worker breathes over 8 hours. They are not the same measurement.

Reading the Flame

Primary air is the air pre-mixed with gas inside the burner venturi, metered by the air shutter. Secondary air is the air drawn in around the flame at the burner ports and inside the combustion chamber. A correctly adjusted natural gas burner shows a soft blue flame with a well-defined inner cone; a propane burner shows the same blue flame, on which slight yellow tips are normal for many appliances. Set the air shutter to the smallest opening that produces a stable inner cone with no yellow tipping.

A floating flame is the most dangerous of the four, because the cause is starved secondary air or a plugged vent -- exactly the conditions that push combustion products back into the building.

Orifices, Conversions and Manifold Pressure

Natural gas carries about 1,000 Btu per cubic foot; propane carries about 2,500 Btu per cubic foot. To deliver the same input, a propane appliance passes less than half the gas volume, which is why a propane orifice is drilled substantially smaller and why a conversion always means changing orifices and the appliance regulator spring or setting. IFGC 301.7.1 allows a fuel conversion only where complete instructions come from the installation instructions, the serving gas supplier or the manufacturer.

Manifold pressure is the pressure the burners see downstream of the appliance regulator inside the combination gas valve. It is read with a manometer at the gas valve outlet tap while the appliance fires. Typical settings are about 3.5 inches water column (in. w.c.) for natural gas and 10 to 11 in. w.c. for propane, but the rating plate is the authority -- two-stage equipment carries separate low-fire and high-fire figures. Because 1 psi equals 27.7 in. w.c., 3.5 in. w.c. is only 3.5 / 27.7 = 0.126 psi; a psi gauge will never resolve it.

Worked Example -- Clocking the Meter to Confirm Input

Shut off every other gas appliance including standing pilots, fire the unit at high fire until steady, and time one revolution of a test dial:

cfh = (3,600 x dial size) / seconds per revolution

A furnace nameplated at 100,000 Btu/h turns the 1-cubic-foot hand once in 32 seconds on natural gas at 1,000 Btu/ft3:

• 3,600 / 32 = 112.5 cfh
• 112.5 x 1,000 = 112,500 Btu/h
• 112,500 / 100,000 = 1.125, so the unit is 12.5 percent overfired -- reduce manifold pressure to the plate value and re-clock.

The same 100,000 Btu/h appliance on propane should draw only 100,000 / 2,500 = 40 cfh.

Draft, Spillage and Depressurization

Natural draft appliances (Category I with a draft hood) rely on the buoyancy of hot flue gas and on the draft hood to admit dilution air and decouple the burner from vent conditions. Induced-draft appliances use a fan to pull products through the heat exchanger; forced-draft appliances push them. Only natural-draft equipment can spill at a draft hood, which is why the spillage test is the core field check.

Following IFGC Appendix D, you visually confirm the burner flame, adjust primary air, then check for spillage using smoke at the draft hood relief opening after 5 minutes of main burner operation. If flue gas rolls out instead of room air being drawn in, the appliance is spilling. The procedure then requires you to turn on every other fuel-gas-burning appliance in the same room at full input and repeat the flame check and the spillage test, because a common vent can be overwhelmed.

Real installations fail under depressurization rather than at rest. Run the worst case: clothes dryer on, kitchen and bath exhaust fans on, whole-house fan on, fireplace damper open, and all interior doors in their normal position. The US Department of Energy field protocol treats a gas furnace or water heater on an atmospheric chimney as acceptable at roughly -0.020 in. w.c. (-5 Pa) below 20 degrees F outdoors, easing to about -0.004 in. w.c. (-1 Pa) above 80 degrees F; less negative than that and the appliance is prone to backdrafting. That same protocol tells the technician to stop the draft test and ventilate whenever ambient CO in the appliance zone exceeds 20 ppm.

Combustion Analysis

A combustion analyzer samples the flue and reports CO in ppm, oxygen (O2) percent, carbon dioxide (CO2) percent, stack temperature and draft. O2 and CO2 move opposite each other: high O2 means high excess air, a cool inefficient fire and diluted readings; O2 near zero means the burner is starving and CO will spike.

Because excess air dilutes the sample, CO is normalized to an air-free basis:

CO air-free = CO measured x 20.9 / (20.9 - O2 percent)

Worked example. An 80 percent induced-draft furnace reads 100 ppm CO at 9 percent O2: 20.9 / (20.9 - 9) = 20.9 / 11.9 = 1.756, so 100 x 1.756 = 176 ppm air-free. Air-free CO is always the higher number, which is why a power-vented water heater can look clean on a raw CO reading and still be producing a hazardous amount of CO at the burner.

An elevated CO reading with normal O2 points at the flame itself: flame impingement on a dirty or misaligned burner, a cracked or sooted heat exchanger, or an overfired gas valve. A cracked heat exchanger classically shows up as a flame that changes shape, rolls or blows sideways the instant the circulating blower starts, together with a CO reading that climbs when the blower energizes. Confirm with a borescope inspection of the exchanger, a soap or tracer-gas test, or a manufacturer-specified pressure check -- never on flame movement alone.

Carbon Monoxide: Limits, Symptoms and Alarms

CO is a colorless, odorless, tasteless flammable gas produced by incomplete combustion of any carbon fuel -- a starved burner, a blocked flue, a backdrafting water heater, an unvented heater, a running engine in an attached garage or a charcoal grill brought indoors. It binds to hemoglobin far more readily than oxygen does, so the blood stops delivering oxygen even while breathing continues.

Symptoms track dose. Low-level exposure produces headache, fatigue, shortness of breath on exertion, nausea and dizziness and is routinely misdiagnosed as flu -- the giveaway is that symptoms improve when occupants leave the building. Rising concentrations bring confusion, impaired coordination, vomiting, chest pain and collapse; at several hundred ppm, life-threatening symptoms appear within a couple of hours, and above roughly 800 ppm unconsciousness can occur within minutes.

Alarms. The 2018 IRC requires CO alarms outside each separate sleeping area in the immediate vicinity of the bedrooms, and inside a bedroom where a fuel-burning appliance is located in that bedroom or its attached bathroom (R315.3); alarms are listed to UL 2034 (R315.1.1). Maryland layers its own statute on top: Annotated Code of Maryland, Public Safety Article, Title 12, Subtitle 11 (sections 12-1101 to 12-1106) requires alarms in specified dwellings that rely on fossil fuel or have an attached garage, and permits sealed, tamper-resistant units with a 10-year battery where battery power is allowed. Some jurisdictions go further -- Montgomery County has required alarms outside sleeping areas and on every level of covered pre-2008 dwellings since July 1, 2019.

When to Shut It Down

IFGC Appendix D states the professional standard plainly: if the inspection finds a condition that could result in unsafe operation, the appliance should be shut off and the owner advised of the unsafe condition. Red-tag conditions include a cracked or leaking heat exchanger, persistent spillage or backdrafting under normal operating conditions, flue gas CO above the appliance threshold, a disconnected or badly deteriorated vent, a defeated safety control, and any gas leak.

Exam Trap: The most common miss on this topic is treating 400 ppm as the CO level at which you evacuate a house. It is not an ambient air limit at all -- it is the air-free flue gas ceiling for a central furnace under ANSI Z21. The limit for the air people actually breathe is far lower: 50 ppm as an 8-hour OSHA PEL, with 1,200 ppm as the NIOSH IDLH. Read the question carefully to see whether it is describing a flue sample or room air.