Gas furnace operation and troubleshooting field guide

What a gas furnace is and what it does

A gas furnace burns natural gas or propane to heat air, and a blower moves that heated air through the building's duct system. The burners fire inside a metal heat exchanger, the house air passes over the outside of that exchanger and picks up the heat, and the products of combustion stay sealed inside and go up the flue. That separation is the whole safety premise of the machine. Burned gas on one side, the air you breathe on the other, and a wall of steel between them that has to stay intact.

Everything a furnace does happens in a fixed order, started by the thermostat and run by the control board. Knowing that order is how you troubleshoot one, because a furnace that fails stops at a specific step and that step points at the part. This guide is built around the sequence and the components that drive it. For the same gas heat section riding inside a packaged rooftop unit, see the rooftop unit installation and startup guide. For where the blower sits and how the warm air gets distributed through a central system, see the air handler guide. This guide stays on the furnace itself, the combustion side, and the no-heat call.

The trap is treating a furnace as a box that either runs or it doesn't. It is a chain of safeties, each one proving the last step before the next is allowed, and gas does not flow until draft is proven and the igniter is hot. Learn the chain and most no-heat calls walk you straight to the fault.

The sequence of operation, step by step

The sequence of operation is the fixed order a gas furnace follows from the thermostat call to shutdown, with each step proving itself before the next is allowed to run. On a modern induced-draft furnace it goes like this. The thermostat closes on a call for heat and sends 24 V to the control board. The board starts the inducer motor, which pulls combustion air through the burners and pushes the flue gas out the vent. With the inducer running, the pressure switch senses the negative pressure the inducer creates and closes, proving draft. No proof, no gas.

Once draft is proven, the board energizes the igniter. On the common hot-surface system the glow bar heats for a warm-up period, usually somewhere around 15 to 45 seconds depending on the design, until it is hot enough to light gas. Then the board opens the gas valve, gas flows to the burners, and the hot igniter lights them. The flame has to be sensed within a trial-for-ignition window, commonly a few seconds, or the board shuts the valve and tries again.

With flame proven, the burners heat the exchanger. After a fixed blower-on delay, often in the 30 to 60 second range, the board starts the supply blower so it does not blow cold air into the house before the exchanger is warm. The furnace runs until the thermostat is satisfied, then the board closes the gas valve, the burners go out, the inducer runs a post-purge to clear the flue, and the blower runs an off-delay to pull the last of the heat out of the exchanger before it stops. Memorize this order. The fault is almost always the step that won't complete.

The exact timings, the trial-for-ignition window, and the number of retries before a hard lockout are set by the manufacturer and live in the installation and service literature for that model. Treat the numbers here as the shape of the sequence, not as the spec for the furnace in front of you.

The burners, the manifold, and the gas valve

The gas valve, the manifold, and the burners are the fuel side of the furnace. The gas valve is the control point. It is a 24 V solenoid valve with a built-in pressure regulator that opens when the board calls and meters the gas to a regulated outlet pressure. From the valve, gas flows into the manifold, a pipe with a threaded orifice, called a spud, screwed in ahead of each burner. The orifice is a precise hole that sets how much gas enters each burner.

Each burner is a tube with ports along it. Gas leaves the orifice, draws in primary air on the way into the burner, mixes, and burns at the ports as a row of tight blue flames carried into the heat exchanger. A clean, properly set burner makes crisp blue flames that stay put. Lazy yellow flames, flames that lift off the ports, or flames that roll out the front are a problem with the gas, the air, the orifice, or a fouled burner, and they are a combustion and carbon monoxide warning, not a cosmetic one.

The pressure the valve delivers to the manifold is the manifold pressure, and it sets the firing rate together with the orifice size. That pressure gets set with a manometer to the value on the data plate, covered in its own section below. Get the orifice or the pressure wrong and the furnace fires too high or too low, with the high side overheating the exchanger and making carbon monoxide.

How does a furnace light the burners?

A furnace lights its burners with one of four ignition systems, and which one you have changes the troubleshooting. The oldest is the standing pilot: a small flame that burns continuously and lights the burners whenever the gas valve opens, with a thermocouple proving the pilot is lit before the main valve will open. It is reliable and simple, and it wastes gas keeping a flame lit around the clock. You still find them on older equipment.

Intermittent pilot lights a pilot only on a call for heat, usually with a spark, proves that pilot with flame rectification, then opens the main valve. It keeps the pilot idea but drops the constant burn. The two systems on most furnaces built in the last few decades are hot surface ignition and direct spark.

Hot surface ignition is the most common, and the igniter is the part techs replace most. A silicon carbide or silicon nitride element, the glow bar, heats to incandescence when the board sends 120 V to it, and the burner gas lights off the glowing element. The bar is brittle and degrades with every heat cycle, so a cracked or open igniter is a frequent no-heat cause. Handle a new one by the ceramic base only, because skin oil on the element shortens its life. Direct spark skips the pilot and the glow bar entirely and lights the main burners straight off a high-voltage spark, then proves the flame with the same rectification a flame sensor uses. Know which system is in front of you before you start chasing a no-light.

The igniter, the warm-up time, and the trial-for-ignition window belong to the manufacturer's design, so match the replacement igniter to the unit and confirm the timing against the service literature rather than a generic part.

What is a flame sensor?

A flame sensor is a single metal rod that sits in the burner flame and proves to the control board that the burners actually lit, so the board knows it is safe to keep the gas valve open. It is the most common no-heat call there is, and the fix is usually a two-minute cleaning, not a part. The classic symptom is a furnace that lights, burns for a few seconds, then drops the flame and shuts off, often retrying two or three times before it locks out.

It works on flame rectification. The board puts an AC voltage on the rod, and a lit flame conducts a tiny direct current from the rod through the flame to the grounded burner, because the ionized gas in the flame passes current in one direction far better than the other. The board reads that rectified current in microamps, commonly in the low single digits on a healthy system, often somewhere around 1 to 6 microamps with the exact minimum set by the design. See a steady signal above the threshold and the board holds the valve open. See nothing, and it assumes the burners did not light and shuts the gas for safety.

Because the signal is only a few millionths of an amp, a thin film of oxidation, soot, or the white silica scale that builds up on the rod is enough to choke it below the threshold. The furnace really did light, but the dirty rod cannot prove it. Cleaning is gentle: pull the sensor, lightly scuff the rod with fine abrasive pad or emery cloth, wipe it, and reinstall. Do not sand it to bare bright metal aggressively or you remove material. The other half of the call is the ground. Flame rectification needs a solid burner ground, so a corroded burner, a bad sensor wire, or a poor chassis ground produces the same low-signal symptom as a dirty rod. Read the microamps with a meter in DC microamps in series with the sensor wire, and you know whether you are chasing a dirty rod, a bad ground, or a genuinely weak flame.

The heat exchanger and the cracked-exchanger hazard

The heat exchanger is the sealed metal chamber the burners fire into, and it is the one part of the furnace that has to stay perfect, because it is the only thing separating the combustion gas from the air your customer breathes. House air blows over the outside of it and picks up the heat. The flue gas, including carbon monoxide, stays inside it and vents outside. When that steel is intact, the two streams never touch. When it cracks, they do.

A cracked heat exchanger lets combustion gas bleed into the airstream the blower is pushing through the house, and that gas carries carbon monoxide. This is the deadly furnace failure. Carbon monoxide is colorless, odorless, and tasteless, low exposure brings headaches, nausea, and confusion that get mistaken for the flu, and high exposure kills. A crack is most likely on an old exchanger, one that overheated for years on low airflow, or one that ran with a high temperature rise. The tells are a flame that changes shape or rolls when the blower starts, soot, rust flakes in the burner area, repeated CO alarms, and visible cracking on inspection with a mirror, a light, and often a camera.

Be blunt with the customer and with yourself here. A cracked heat exchanger is not a repair. The standard response across the trade is to shut the furnace down and red-tag it so it cannot run until the exchanger or the furnace is replaced, because there is no reliable field patch and the failure mode is fatal. Confirm the inspection method and the condemnation criteria against the manufacturer's instructions and the adopted fuel-gas code, and when in doubt about a suspected crack, shut it off. A cold house is a callback. A house full of carbon monoxide is a funeral.

Why won't my furnace ignite?

When a furnace will not ignite, the most common stopping point after the flame sensor is draft proving, where the inducer and the pressure switch have to agree before any gas flows. The inducer is a small blower that runs first on every call. It pulls combustion air through the burners and pushes the flue gas out the vent, and on a furnace it creates the draft instead of relying on hot-gas stack effect alone. No inducer, no draft, no gas.

The pressure switch is the safety that proves the inducer is actually moving air before the board will open the valve. It is a pressure-sensing switch connected by a small hose to the inducer housing, and the negative pressure the inducer pulls closes it. The board watches that switch. If it does not close, the board never energizes the igniter or the valve, because firing the burners with no draft would dump combustion gas into the house. This is one of the most common lockouts in the field, and it is usually not the switch itself.

The thing that trips it is a blocked or restricted vent. Snow or ice over a sidewall termination, a bird nest, a crushed flue, water sitting in the vent or in a condensing unit's blocked condensate drain, or a cracked or plugged pressure hose all keep the switch from making. A weak or failing inducer motor that cannot pull its rated draft does the same. The honest order is to confirm the vent is clear, the hose is intact and routed right, and the inducer spins and pulls draft, before you condemn the switch. A pressure switch that gets replaced when the real fault was a snow-covered vent is a part that did not need changing and a callback waiting on the next storm.

The blower and the high-limit switch

The blower is what actually delivers the heat, moving house air across the hot heat exchanger and out to the rooms. It runs after a timed delay on a call so it does not blow cold air, and it keeps running on an off-delay after the burners shut so it strips the residual heat out of the exchanger. The blower on a furnace is the same air mover that feeds the duct system, and how it is set drives the temperature rise covered in the next section.

The high-limit switch is the safety that watches the temperature near the heat exchanger and shuts the burners off if it climbs too high. It exists for one reason: to protect the heat exchanger from overheating when airflow is inadequate. If the blower fails, the filter is plugged, or a belt-drive blower slips, the exchanger gets too hot, the limit opens, and the gas valve closes while the blower keeps trying to cool things down. That is the system protecting itself.

Here is the rookie trap. The limit is a backstop, not a thermostat. A furnace that only stays in its temperature range because the high limit keeps cutting the burners is a furnace running too hot for its airflow, and the cycling is a symptom to fix, not a control to lean on. When a furnace short-cycles on the limit, you have an airflow problem until proven otherwise, and the first suspect is the filter.

What is the temperature rise on a furnace?

Temperature rise is the supply air temperature leaving the furnace minus the return air temperature entering it, and it has to land inside the range stamped on the furnace data plate, commonly shown as something like 40 to 70°F or 35 to 65°F. The plate gives a low number and a high number. The measured rise has to fall between them, and where it lands tells you whether the airflow and the firing rate are matched to the equipment.

Measure it with the furnace at full fire, a clean filter in place, and a thermometer in the return and in the supply. Put the supply probe far enough downstream, past the first elbow if you can, that you are reading air temperature and not radiant heat straight off the heat exchanger, which reads falsely high. Subtract the return from the supply. That is your rise.

A rise above the data-plate range means too little airflow or too much fire, and low airflow is by far the more common cause. A dirty or undersized filter, closed registers, undersized or restricted duct, or a blower speed set too low all starve the air and drive the rise up, which overheats the exchanger and trips the limit. A rise below the range means too much airflow or too little fire. The two levers are blower speed and manifold pressure, and the order matters: fix airflow first, then trim the manifold pressure to the plate. Do not measure rise on a dirty filter, because the false high reading sends you chasing a problem that a filter change would have solved. The same temperature-rise check applies to the gas heat section inside a rooftop unit, covered in the rooftop unit guide.

The acceptable range is the manufacturer's number on that furnace, not a memorized figure. Set the rise to the data plate, and let the plate govern.

What is the difference between a condensing and a non-condensing furnace?

A condensing furnace adds a second heat exchanger that pulls so much heat out of the flue gas that the water vapor in it condenses to liquid, which is where the extra efficiency comes from, and it runs at 90 percent AFUE or higher. A non-condensing furnace vents hotter flue gas straight up the flue without that second pass and runs around 80 percent AFUE. That single difference, whether the flue gas is cooled enough to condense, drives the venting, the materials, and the install.

Because a non-condensing 80 percent furnace vents hot gas, it uses metal venting, typically Type B vent or a lined chimney, and it falls under venting Category I as a natural-draft or fan-assisted appliance with the vent under negative pressure. A condensing 90-plus furnace vents cool, wet gas under positive pressure, so it falls under Category IV and vents through plastic pipe, commonly PVC or CPVC, often with a second pipe bringing combustion air in directly from outside as a sealed-combustion unit.

The condensate is the part that bites on a 90-plus furnace. The water that condenses out of the flue gas is acidic, so it has to run through plastic or stainless components and drain through a trap to a proper disposal point, sometimes through a neutralizer where the code or the local sewer authority requires it. A plugged condensate trap or a frozen condensate line on a high-efficiency furnace backs water up, trips the pressure switch, and shuts the furnace down with a no-heat call that looks like a draft fault and is really a drain. The venting materials, lengths, and combustion-air rules belong to the manufacturer's instructions and the adopted fuel-gas code, so confirm them against both before you cut pipe.

AFUE, staging, and the blower motor

AFUE, annual fuel utilization efficiency, is the percentage of the fuel's energy the furnace turns into useful heat over a heating season. An 80 percent furnace sends roughly a fifth of the fuel energy up the flue; a 95 percent condensing furnace sends about a twentieth. AFUE is a rated lab number from the AHRI test procedure, and the furnace only delivers near it when it is set up and maintained right, the same way a cooling unit only hits its SEER with the charge and airflow correct.

Firing stages change how the furnace runs against the load. A single-stage furnace has one firing rate, full on or off, and it cycles more. A two-stage gas valve has a low and a high fire, running on low most of the time and stepping up to high only on the cold days, which means longer, quieter, more even cycles. A modulating furnace varies the firing rate across a range to match the load closely and holds the tightest comfort.

The blower motor matters as much as the gas valve. A standard PSC motor runs at fixed speeds. An ECM, an electronically commutated variable-speed motor, ramps its speed and can hold airflow steady as the filter loads or static changes, which is why ECM blowers are paired with two-stage and modulating furnaces and why they hold the temperature rise better across conditions. The efficiency on the rating plate is a promise the install and the maintenance have to keep, not a number the box delivers on its own.

Venting and combustion air

Venting carries the products of combustion safely outside, and combustion air supplies the oxygen the burners need, and both are governed by the fuel-gas code and the manufacturer's instructions rather than by eye. The venting splits by category. A Category I furnace, the 80 percent non-condensing type, vents hot flue gas under negative pressure through metal, typically Type B vent or a properly lined masonry chimney, sloped up and sized so the gas rises and clears. A Category IV furnace, the 90-plus condensing type, vents cool, wet gas under positive pressure through plastic pipe, with the joints sealed because the vent is pushing gas out, not relying on it to rise.

Combustion air is the other half. An open-combustion furnace pulls its combustion air from the space it sits in, so that space has to supply enough air, through openings to outside or to a large enough volume, or the burners starve and combustion goes bad. A sealed-combustion furnace pipes its combustion air directly from outdoors, which sidesteps the room-air question and the backdraft risk that comes with it.

Backdrafting is the hazard that hides behind venting. In a tight house, exhaust fans, a clothes dryer, or a powerful range hood can pull the building negative enough that a natural-draft appliance spills its flue gas back into the room instead of up the vent, and that spilled gas carries carbon monoxide. This is why combustion air and venting are code-governed, not best-effort. Confirm the vent category, the materials, the sizing, the slope or the sealed joints, the termination clearances, and the combustion-air provisions against the National Fuel Gas Code, NFPA 54, or the International Fuel Gas Code as adopted, plus the manufacturer's vent tables, and verify the appliance actually drafts and does not spill once it is running.

Combustion safety and carbon monoxide

Combustion safety is the part of furnace work that can kill someone, so it gets treated as the priority on every gas job, not as a final box to check. The furnace burns a fuel inside a house and is supposed to keep every product of that combustion sealed and vented. Carbon monoxide is the danger. It is a colorless, odorless gas produced by incomplete combustion, it is fatal at high levels and damaging at low ones over time, and the symptoms read like the flu, so the people being poisoned often do not know it.

Three failures put carbon monoxide where it does not belong: incomplete or improper combustion at the burners making more CO than a clean burn, a cracked heat exchanger leaking flue gas into the airstream, and a venting failure or backdraft spilling flue gas back into the house. A clean blue flame, an exchanger with no cracks, and a vent that actually drafts are the three things that keep CO out of the air, and you confirm all three.

Prove it with instruments, not assumptions. A combustion analyzer reads the carbon monoxide and the oxygen or carbon dioxide in the flue and tells you whether the burn is clean and whether the CO is in an acceptable range for the appliance. Run a spillage test at the draft hood or check that a sealed unit is venting, look the heat exchanger over, and confirm the manifold pressure and the flame are right. Set the manifold to the data plate, because overfiring drives CO up. And the carbon monoxide alarm in the house is the last line of defense, not the first. It is there to wake people up when everything upstream has already failed, so it never substitutes for safe equipment, a clean burn, a sound exchanger, and a vent that works. Every CO figure, test method, and acceptance limit is governed by the manufacturer's instructions and the adopted fuel-gas code, so verify against both.

Manifold and gas pressure

Manifold pressure is the gas pressure the valve delivers to the burners, and it sets the firing rate together with the orifice size, so it gets set with a manometer to the value on the data plate, not by feel. For natural gas the manifold pressure is commonly set near 3.5 in. w.c. and for propane near 10 to 11 in. w.c., but those are typical figures and the data plate value governs the furnace in front of you. Connect the manometer to the manifold pressure tap on the outlet side of the valve, fire the furnace, and read it under fire.

Two pressures are in play and they are different. The inlet, or supply, pressure is what the gas line delivers to the valve, and the manufacturer states an acceptable inlet range and a maximum the valve will take. The manifold, or outlet, pressure is what the valve sends to the burners. Read the inlet at the inlet tap and the manifold at the outlet tap, and confirm both are in range, because a low inlet starves the furnace at full fire even when the manifold setting looks right.

Clocking the meter is the cross-check on firing rate. With every other gas appliance off, time how long the gas meter's test dial takes to make one revolution, convert to cubic feet per hour, multiply by the heating value of the gas, and you have the actual input the furnace is burning. Compare that to the input on the data plate. Do not overfire. Setting the manifold pressure high to push more heat overheats the heat exchanger, raises the temperature rise, shortens the exchanger's life, and makes carbon monoxide. The data plate input and manifold pressure are the limits, and the fuel-gas code and the manufacturer's instructions govern how you set and confirm them.

The control board and flash codes

The control board, often called the integrated furnace control, runs the sequence, watches the safeties, and reports faults through a diagnostic LED, and reading that LED is the fastest way into a no-heat call. The board is what energizes the inducer, watches the pressure switch, powers the igniter, opens the gas valve, watches the flame sensor, and starts the blower, all on the timings the manufacturer built in. When something in that chain does not happen, the board records it and flashes a code.

The code shows on an LED visible through a sight glass or on the board behind the access panel, and you read the blink pattern against the legend printed on the furnace, usually inside the blower-door panel. A code that means pressure switch did not close points at the vent, the inducer, or the hose. A code for failed ignition or flame not sensed points at the igniter, the gas, the flame sensor, or the ground. A limit-open code points at airflow. The legend on that specific furnace is the authority, because the patterns differ by manufacturer and model.

Lockouts come in two flavors and the difference matters. A soft lockout pauses and retries on its own after a delay. A hard lockout shuts the furnace down until someone cuts the power and resets it. A furnace that drops into a hard lockout repeatedly is not asking to be reset again, it is telling you there is a real fault that keeps stopping the sequence at the same step. Read the code, find the step, fix the cause, then reset. Resetting a hard lockout without finding the fault just postpones the same failure, sometimes onto a colder night.

Why is my furnace not heating?

When a furnace is not heating, you run the sequence of operation in order and find the step where it stops, because the step that won't complete is the fault. Start at the thermostat: confirm it is calling and the board is getting 24 V, and check the low-voltage fuse on the board, which is a common and quick find. Confirm the line voltage and the furnace switch and door switch, because a furnace with the blower door off or the service switch flipped will not run and looks dead.

Then follow the chain. Does the inducer start on a call? If not, suspect the inducer motor or its power. Does the pressure switch close? If the inducer runs but the switch does not make, check the vent for blockage, the hose for cracks or water, and the inducer's draft before condemning the switch. Does the igniter glow? An igniter that never lights up is often cracked or open, or has lost its 120 V supply. Does the gas valve open and the burner light? If the igniter glows and the burner never lights, suspect gas supply, the valve, or the manifold pressure. Does the flame stay lit, or does it light and drop after a few seconds? That last symptom is the flame sensor almost every time, dirty or poorly grounded.

Let the flash code shortcut the walk. Read the LED first, match it to the legend on the furnace, and it usually names the step that failed, so you start at the right place instead of at the top. The discipline is the same either way: the furnace stops somewhere specific, the sequence tells you where, and you fix the cause of that stop rather than swapping parts down the line until something works.

Why does my furnace short-cycle?

A furnace short-cycles, firing and shutting off in short bursts before the house warms, most often because restricted airflow overheats the heat exchanger and trips the high-limit switch. The burners fire, the air cannot carry the heat away fast enough, the temperature near the exchanger climbs past the limit setpoint, and the limit cuts the gas while the blower keeps running. The space cools, the thermostat calls again, and the cycle repeats. The number-one cause is the simplest one: a dirty filter.

Work the airflow causes first because they are the common ones. A clogged filter, closed or blocked registers, a collapsed or undersized return, dirty blower wheel, or a blower speed set too low all starve the airflow and trip the limit. Confirm the filter is clean, the registers are open, and the blower is moving air before you look anywhere else, and check the temperature rise, because a high rise confirms the airflow problem the limit is reacting to.

Not every short cycle is airflow. An oversized furnace satisfies a small zone fast and cycles a lot by nature. A flame-sensor or ignition fault makes a furnace fire and drop out in seconds, which reads as short-cycling but is really a flame-proving failure, so check whether it is the limit cutting it or the flame never being proven. And a failing limit switch itself can open early and cut a furnace that is not actually overheating. The fix follows the cause, but the airflow check comes first, every time, because it is right most of the time.

The filter and airflow

The filter is the cheapest part on the furnace and the one that causes the most trouble when it is ignored, because a plugged filter starves the airflow the whole heating side depends on. Low airflow drives the temperature rise up, overheats the heat exchanger, and trips the high limit, which is why a dirty filter is the leading cause of short-cycling and a real contributor to the kind of long-term overheating that cracks an exchanger. Change the filter and a surprising number of no-heat and short-cycle calls disappear.

Airflow is more than the filter, though the filter is where it starts. The return has to be big enough and open, the supply registers have to be open, the duct has to be sized for the air, and the blower has to be set to move the design airflow against the static the duct presents. A furnace cannot heat right on starved air no matter how the gas side is set, and pushing the manifold pressure up to compensate just overheats things faster.

The same airflow that the furnace heats, the cooling coil and the rest of the system also depend on, and selecting filter media is a trade between cleaner air and the static the blower can afford. The air handler guide covers the filter section and the static-pressure picture for central systems by topic. For the furnace, the field rule is short: keep the filter clean, keep the airflow up, and measure the temperature rise to prove the air is moving before you touch the gas.

Natural gas versus propane

Natural gas and propane are not interchangeable in a furnace, and running propane through a furnace set up for natural gas without converting it is dangerous. Propane carries more energy per cubic foot and runs at a higher manifold pressure, so a furnace built for natural gas has larger burner orifices and a regulator spring sized for low-pressure natural gas. Feed propane through those natural-gas orifices and you grossly overfire the furnace, overheat the exchanger, and make carbon monoxide.

Converting between the two fuels is a real procedure with manufacturer parts, not a setting you dial in. It means changing the burner orifices to the size for the new fuel and changing the pressure regulator spring or conversion components in the gas valve, then setting the manifold pressure to the new fuel's value on the conversion data, clocking the input, and running a combustion analysis to confirm a clean burn. The conversion kit and the instructions come from the furnace manufacturer for that model, and they are what governs the parts and the pressures.

Read the rating plate before you fire any furnace, and confirm the fuel it is set for matches the fuel at the building. A propane furnace fed natural gas underfires and runs poorly; a natural-gas furnace fed propane overfires and becomes a carbon monoxide source. The conversion has to be done with the right parts to the manufacturer's instructions and the fuel-gas code, and on a propane install the heavier-than-air fuel and leak detection bring their own code requirements. Do not improvise the fuel side.

Annual service and what it covers

Annual furnace service exists to keep the combustion clean, the exchanger sound, and the airflow up, which is to say it keeps the carbon monoxide where it belongs and the furnace doing what it was rated to do. The single most reliable maintenance win is cleaning the flame sensor, because the slow buildup on that rod is what causes the most common no-heat call, and cleaning it on a fall service heads off the January lockout before it happens.

The rest of the visit follows the components. Inspect the burners for soot, rust, or debris and confirm the flames are clean and blue. Inspect the heat exchanger for cracks, scaling, and corrosion with a light, a mirror, and a camera. Check and replace the filter, and verify the blower wheel is clean and the airflow is right. Confirm the inducer runs and the pressure switch makes, and on a condensing furnace clear and flush the condensate trap and line, because a plugged trap is a no-heat call waiting on the coldest week. Confirm the venting is clear and intact and the termination is not obstructed.

Then prove the combustion. Set or confirm the manifold pressure against the data plate, measure the temperature rise and confirm it lands in the plate range, and run a combustion analysis for carbon monoxide and a clean burn. Confirm the carbon monoxide alarms in the house are present and working. A service visit that produced no combustion numbers and no exchanger inspection on a gas appliance is not a furnace service, it is a filter change with a clipboard.

Commercial and rooftop gas heat by topic

The gas heat in a light-commercial rooftop unit is the same furnace section, the same sequence, and the same safeties, packaged into the cabinet on the roof instead of standing in a basement. The burners, the gas valve, the manifold, the heat exchanger, the flame proving, the inducer and pressure switch, and the temperature rise all carry over, and so does the combustion-safety discipline. The differences are the install context: gas piped to the roof and sized for the input over the developed length, the manifold and inlet pressures set with a manometer at startup, and the combustion and CO check as part of commissioning rather than a standalone service.

On the rooftop side the gas heat shares the cabinet with the cooling, the blower, and often an economizer, so the airflow that sets the temperature rise is the same air the cooling coil and the duct system see. Set the airflow first, then the gas, the same order as a residential furnace. The deep treatment of the rooftop install, the gas piping to the roof, the startup pressures, and the commissioning record lives in the rooftop unit installation and startup guide. The point here is that the combustion knowledge transfers intact: a cracked exchanger on a rooftop unit is the same life-safety problem it is in a house, and the manifold pressure and temperature rise are set to the same data-plate logic.

What to document

A furnace service or startup that left no numbers is one nobody can stand behind later, and on a gas appliance the numbers are also the safety record. Capture what you set, what you measured, and what you found, so the next tech has a baseline and the customer has proof the combustion was checked. The table below is the core, organized as the sequence runs: the step, what that component does, and the fault that shows up there.

Record the manifold pressure set against the data plate, the inlet gas pressure, the clocked input if you clocked it, the measured temperature rise against the plate range, the combustion analysis results including carbon monoxide, the flame-sensor microamp reading, the condition of the heat exchanger and burners, and the venting and condensate condition. Note the fuel type the furnace is set for and confirm it matches the building. The tradeos tool keeps the readings, the data-plate photo, and the heat-exchanger inspection images together so the record is one package instead of a sheet that gets lost between the roof and the truck.

Common mistakes

• Replacing a flame sensor when a two-minute cleaning of the rod, or fixing the burner ground, was the fix.
• Suspecting a cracked heat exchanger and skipping the CO check, or finding a crack and not red-tagging the furnace.
• Setting the manifold pressure high to push more heat, overfiring the furnace and making carbon monoxide.
• Measuring temperature rise on a dirty filter, then chasing a problem a filter change would have solved.
• Condemning the pressure switch when the real fault was a blocked vent, a cracked hose, or a plugged condensate drain.
• Running propane through a furnace still set up for natural gas, with no orifice and regulator conversion.
• Resetting a hard lockout repeatedly without reading the flash code or finding the step that failed.
• Closing a gas job with no combustion analysis, no CO reading, and no heat-exchanger inspection.

Field checklist

Standards and references

The manufacturer's installation and service instructions govern the furnace itself. They set the orifice size, the manifold pressure for each fuel, the temperature-rise range, the igniter and the sequence timings, the trial-for-ignition window and the number of retries, the venting category and materials, and the inspection and conversion procedures. When any other document and the manufacturer disagree on a setting, the manufacturer's instructions and the appliance listing control, and going against them voids the listing and the warranty.

The fuel-gas code carries the gas piping, the venting, the combustion air, and the appliance install. In the United States that is the National Fuel Gas Code, NFPA 54, also published as ANSI Z223.1, or the International Fuel Gas Code, depending on what the jurisdiction adopts, and the two are closely aligned but not identical. Combustion testing and carbon monoxide acceptance follow the appliance listing and the analyzer manufacturer's procedure, and AFUE ratings come from the AHRI test procedure behind the catalog efficiency. Where a propane install applies, the propane code and leak-detection requirements add to the fuel-gas code.

Cite the document that owns the point, set every furnace value to the manufacturer's instructions, and confirm the code requirements against the edition the jurisdiction has actually adopted and any local amendments, because these codes revise on their own cycles. The pressures, rise ranges, and efficiencies in this guide are typical figures for orientation, not a substitute for the data plate and the adopted code. On combustion and carbon monoxide, when the reading or the inspection leaves any doubt, the safe call is to shut the appliance down.

Units, terms, and definitions

Furnace work carries its own vocabulary and a couple of unit systems, so the same value reads differently across a data plate, a manometer, and a spec.

Gas pressure is in inches of water column, written in. w.c. or in. wg, where 1 in. w.c. is about 249 pascals and roughly 0.036 psi. Heating input and capacity are in Btu/h, or in kilowatts in metric work. Temperature rise is in degrees Fahrenheit on most US data plates and Celsius in metric. The flame-sensor signal is in microamps, millionths of an amp, read in DC. Efficiency is AFUE as a percentage. Venting is sorted into Category I through IV by flue temperature and pressure, with Category I and IV the two a residential furnace uses.

Sequence of operation

The fixed order a furnace fires in, each step proving itself before the next runs

Inducer

The small blower that pulls combustion air and pushes flue gas out the vent

Pressure switch

The safety that proves the inducer is moving air before the gas valve can open

Hot surface igniter

The glow bar that heats to incandescence and lights the burners directly

Flame rectification

How a flame sensor proves flame, by the tiny DC current a lit flame passes to ground

Heat exchanger

The sealed metal chamber that transfers heat while keeping flue gas out of the air

Temperature rise

Supply minus return air temperature, held inside the data-plate range

AFUE

Annual fuel utilization efficiency, the seasonal percentage of fuel energy turned into heat

Manifold pressure

The gas pressure the valve delivers to the burners, set to the data plate

Related tools

Calculators and readiness checks for this work

Compare your options

FAQ

People also ask

• How do you start up an RTU?
• What is an air handling unit?
• What is building pressurization?
• How does a cooling tower work?
• How do you use a multimeter?