- A gas furnace fires in a strict 6-step sequence every time — if any step fails, the furnace shuts down before gas ever flows, which is a built-in safety feature, not a malfunction.
- The inducer motor runs first, before ignition, to clear the heat exchanger of any residual gases — skipping this step is why older standing-pilot furnaces were more dangerous.
- Hot-surface igniters made of silicon carbide or silicon nitride are fragile enough to crack from a fingerprint’s skin oil — understanding this helps you avoid an expensive service call.
- In the Ohio River valley’s freeze-thaw winters, pressure switch failures and cracked heat exchangers are more common than in climates that stay consistently cold — knowing the sequence tells you exactly where to start troubleshooting.
- The blower motor is always the last component to start, and that delay is intentional — keep reading to understand why turning it on too early would actually damage your system.
Your gas furnace runs through the same precise sequence every single time it fires — and when something goes wrong, it’s almost always because one step in that chain broke down.
Louisville HVAC Pro’s serves Louisville and the surrounding Jefferson and Bullitt County communities, and the ignition sequence is one of the most common things our technicians walk homeowners through when a furnace isn’t behaving. Understanding it yourself means you can describe the problem accurately, narrow down the cause, and avoid paying for a diagnostic call on something you could have caught yourself.
Your Furnace Follows a Strict Order Every Time It Fires Up
Every component in your furnace has a job, and each one waits for the previous component to finish its job before it starts. The control board manages this timing. If anything falls out of sequence — a pressure switch that won’t close, an igniter that doesn’t reach temperature, a flame sensor that doesn’t detect a flame — the board shuts everything down and logs a fault code.
Why the Sequence Matters for Homeowners
When your furnace short-cycles, locks out, or blows cold air, the sequence is your diagnostic roadmap. Instead of guessing, you can work through each step in order and find exactly where the chain broke. That’s true whether you’re doing a visual inspection yourself or explaining symptoms to a technician over the phone.
The Mnemonic That Makes It Easy to Remember: “Take It Slow, It’s Gonna Blow”
HVAC instructor Jason Obrzut uses this phrase to teach the sequence: Thermostat, Inducer motor, Safety switch, Igniter, Gas valve, Blower motor. Each word maps to a real component, in order. Run through it whenever your furnace isn’t starting and you’ll know immediately which part of the system to look at first.
Step 1: The Thermostat Sends the Call for Heat
Nothing happens until the thermostat asks for it. When your indoor temperature drops below the setpoint, the thermostat closes a low-voltage circuit — typically 24 volts — and sends that signal to the furnace’s control board.
How the Thermostat Signals the Circuit Board
That 24-volt signal travels from your thermostat through the R and W terminals on your wiring. The R terminal carries the 24V power, and W is the heat call wire. When W closes to R, the board sees the call for heat and begins executing the sequence. A bad thermostat wire, a loose terminal, or a tripped breaker on the air handler can break this signal before the board ever sees it.
What Happens When the Board Receives the Signal
The control board doesn’t immediately open the gas valve. Instead, it starts a pre-ignition checklist. It checks for any existing fault conditions, verifies the pressure switch is in the open position (which confirms the inducer isn’t already running incorrectly), and then sends 120 volts out to the inducer motor. The gas valve stays closed until much later in the sequence.
Step 2: The Inducer Motor Kicks On
The inducer motor is a small but critical fan mounted at the top of the furnace near the flue outlet. It runs before any gas flows, and that timing is intentional.
What the Inducer Motor Actually Does
The inducer pulls air through the burner compartment and pushes combustion gases out through the flue. Before ignition, it runs a pre-purge cycle — typically 15 to 30 seconds — to clear any residual gas or combustion byproducts out of the heat exchanger. This is a core safety function. If the inducer motor fails to start, the board will not proceed to ignition.
How Combustion Air Moves Through the Heat Exchanger
Once the burners light, the inducer maintains negative pressure inside the heat exchanger. This draws fresh combustion air in through the burner orifices and pushes exhaust gases — carbon monoxide, water vapor, nitrogen oxides — out through the secondary heat exchanger (on a 90%+ efficiency furnace) or directly out the flue (on an 80% furnace). The direction of airflow is always from burner to flue, never the reverse.
Why the Inducer Is Critical to Safe Furnace Operation
If the inducer can’t create enough negative pressure — due to a failed motor, a blocked flue, or a cracked inducer housing — the pressure switch won’t close, and the furnace won’t light. In Louisville’s climate, flue blockages from ice at the exhaust termination are a real problem in late January and February, especially on PVC-vented 90% furnaces where the exhaust pipe exits near ground level. That’s one of the first things to check on a no-heat call after a hard freeze.
Step 3: The Safety Switch Closes
The pressure switch — sometimes called a negative pressure switch or draft inducer pressure switch — is a small diaphragm-operated switch connected to the inducer housing by a rubber hose. Its only job is to confirm that the inducer is actually moving air.
What a Negative Pressure Switch Is
The pressure switch is a small, round disc — usually about the size of a half-dollar — mounted on the side of the inducer housing or the furnace cabinet. It has two electrical terminals and a small rubber hose port. When the inducer creates enough suction, that negative pressure pulls the diaphragm inside the switch, which closes the electrical contacts and sends a signal back to the control board.
A pressure switch stuck in the open position is one of the most common no-heat calls in the Louisville area. The switch itself is inexpensive, but before replacing it, a technician will always check the rubber hose for cracks, water in the line, or a blocked port — because those cause identical symptoms to a failed switch.
How Negative Pressure From the Inducer Closes the Switch
Pressure Switch Diagnostic at a Glance
- Symptom: Furnace starts inducer but won’t ignite
- First check: Is the rubber hose connected and crack-free?
- Second check: Is there water sitting in the hose or switch port? (Common on 90%+ furnaces in humid Ohio River valley winters)
- Third check: Use a manometer or magnehelic gauge to verify the inducer is actually producing the rated negative pressure
- Fourth check: If pressure is correct and hose is clear, the switch diaphragm has likely failed — replace the switch
The inducer motor needs to produce a specific level of negative pressure — typically between -0.5 and -1.8 inches of water column, depending on the furnace model — before the pressure switch will close. That spec is printed on the switch itself. If the inducer is weak, the flue is partially blocked, or the heat exchanger has a crack that’s allowing pressure to bleed off, the switch won’t see enough suction to close.
On 90% AFUE furnaces, condensate can collect inside the pressure switch hose or even inside the switch body itself. This is especially common here in the Ohio River valley, where high humidity means the secondary heat exchanger is pulling a lot of moisture out of the flue gases. A waterlogged pressure switch hose will hold the diaphragm in the wrong position and keep the switch from closing — even when the inducer is working perfectly.
If you have a 90% furnace and it’s locking out on pressure switch errors repeatedly through the winter, check the condensate drain line first. A slow or blocked drain backs condensate up into the system, and the pressure switch hose is often the first place it shows up.
The 24V Signal That Travels Back to the Board
Once the pressure switch closes, it completes a 24-volt circuit back to the control board. The board reads this as confirmation that the inducer is running and airflow is established. Only after receiving this signal does the board move to the next step: energizing the igniter.
Some furnaces have two pressure switches — a low-pressure switch and a high-pressure switch — each monitoring a different stage of the venting system. Both have to close before ignition proceeds. If either one stays open, the board stops and flashes a fault code, usually two or three blinks depending on the manufacturer’s code chart.
Step 4: The Igniter Heats Up
With the pressure switch confirmed closed, the board sends 120 volts to the igniter. This is where you start to see heat delays — typically 15 to 34 seconds — while the igniter reaches the temperature needed to light the gas.
Silicon Carbide vs. Silicon Nitride Igniters
Most furnaces built in the last two decades use one of two hot-surface igniter materials. Silicon carbide igniters glow orange-hot and are the older of the two technologies — they work reliably but are brittle and prone to cracking with age. Silicon nitride igniters are newer, more durable, and more resistant to thermal shock. Silicon nitride igniters also tend to have a longer service life, though both types will eventually fail. A healthy silicon carbide igniter draws around 3.2 to 3.5 amps at 120V; a significant drop in amperage draw usually means the element is starting to fail.
Why Igniters Are Fragile and How to Handle Them
The oils from your skin can create a hot spot on a silicon carbide igniter that causes it to crack the first time it heats up. Always handle igniters by the ceramic base or the wiring harness — never touch the glowing element itself. The same rule applies during installation of a replacement. A new igniter that fails on the first heating cycle is almost always a handling issue, not a defective part.
Hot-Surface vs. Intermittent-Spark vs. Direct-Spark Ignition
Hot-surface ignition is the most common type in residential furnaces today. Intermittent-spark ignition uses an electronic spark — similar to a gas range igniter — to light a small pilot flame, which then lights the main burners. Direct-spark ignition skips the pilot entirely and sparks directly at the main burner. You’ll still find intermittent-spark systems in older equipment, and some commercial applications still use direct-spark, but if your home furnace was installed after the mid-1990s, it almost certainly uses hot-surface ignition.
Step 5: The Gas Valve Opens
Once the igniter has reached operating temperature — hot enough to reliably ignite natural gas, which requires approximately 1,100°F for silicon carbide — the board sends 24 volts to the gas valve. This is the moment gas actually enters the system for the first time in the sequence.
How the Board Sends 24V to the Gas Valve
The gas valve is an electromechanical device with two internal operators: a redundant safety valve and a main valve. Both have to open for gas to flow. The 24-volt signal from the board energizes a solenoid coil inside the valve, which lifts the valve seat and allows gas to pass through to the burner orifices. Most residential gas valves also have a manual shutoff knob — the one labeled On/Off/Pilot — that must be in the On position for any of this to work.
The board starts a trial-for-ignition timer the moment it energizes the gas valve — typically 7 to 10 seconds. If the flame sensor doesn’t detect a flame within that window, the board closes the gas valve, waits, and tries again. Most boards allow two or three ignition attempts before locking out completely and flashing a fault code.
How Flame Sensors Prove the Burner Is Lit
The flame sensor is a thin metal rod — usually stainless steel or Inconel — that extends into the burner flame. It works through a process called flame rectification: a live flame conducts a tiny AC current (typically less than 5 microamps) and converts it to DC, which the board reads as proof of ignition. If the board doesn’t see that DC signal within the trial-for-ignition window, it assumes the burner failed to light and shuts the gas valve.
A dirty flame sensor is one of the most common causes of a furnace that lights briefly and then shuts off after a few seconds. The ceramic insulator or the rod itself gets coated with oxidation over time, which increases resistance and reduces the microamp signal below the board’s threshold. Cleaning the rod with fine steel wool or light emery cloth — carefully, without bending the rod — usually restores full function. It’s a legitimate DIY maintenance item for homeowners who are comfortable working around furnace components with the power off.
Step 6: The Blower Motor Comes On Last
The burners are lit, the heat exchanger is warming up — and the blower still isn’t running yet. That’s normal, and it’s by design. The control board holds the blower off for 30 to 90 seconds after ignition, depending on how the furnace is configured.
Why There Is a Delay Before the Blower Starts
If the blower started the instant the burners lit, it would push cold air through the ducts for the first minute or two of every heating cycle. That cold blast isn’t just uncomfortable — it also chills the heat exchanger surface right when it’s trying to reach operating temperature, which reduces efficiency and adds thermal stress to the metal over thousands of cycles. The delay lets the heat exchanger reach a stable temperature first, so the air coming out of your vents is warm from the very first second the blower kicks on. On most boards, this delay is adjustable via DIP switches — typically set to 30, 45, 60, or 90 seconds.
What Happens If the Blower Fails to Start
If the blower motor doesn’t start after the heat delay, the heat exchanger will overheat. Every furnace has a high-limit switch — a thermal safety device mounted directly on the heat exchanger — that trips when the metal reaches a temperature threshold, typically between 150°F and 200°F depending on the model. When the limit trips, it cuts power to the gas valve and shuts the burners off. The furnace will then try to restart, overheat again, and cycle on the limit repeatedly. This is called short-cycling on the high limit, and it’s hard on the heat exchanger. A cracked heat exchanger from repeated thermal stress is an expensive repair — often more than half the cost of a new furnace.
If your furnace is blowing warm air for a few minutes and then stopping, check the filter first. A clogged filter restricts airflow enough to cause the same high-limit trip as a failed blower motor. A 1-inch filter should be replaced every 30 to 60 days in a Louisville home that runs heat regularly through the winter. If the filter is clean and the blower still won’t run, you’re likely looking at a failed capacitor, a worn blower motor, or a board that’s not sending the run signal — all of which require a technician.
Common Ignition Problems and What They Mean
Most furnace faults trace back to one of three places: the venting and pressure system, the igniter or flame sensor, or the gas valve circuit. Knowing which step in the sequence failed tells you which category to focus on.
Safeties That Won’t Close
A pressure switch that won’t close is the single most common ignition-related service call. Before assuming the switch itself is bad, check the rubber hose for cracks or water, verify the inducer is spinning freely, and make sure the flue isn’t blocked. In Louisville, ice blockages at the PVC exhaust termination on 90% furnaces are a regular January problem. The fix is sometimes as simple as clearing a 2-inch pipe opening on the side of the house. Other safeties — like a rollout switch or a secondary limit — trip for different reasons, but they all stop the sequence cold and produce a specific flash code on the control board. Count the blinks, look up the code on the inside of the furnace door panel, and you’ll know exactly which safety opened.
Cracked Heat Exchangers and Flue Pressure Issues
A cracked heat exchanger doesn’t always produce obvious symptoms, but it will affect the ignition sequence in a specific way. A crack allows supply air from the blower side to bleed into the combustion chamber, which changes the pressure dynamics the inducer is working against. This can cause intermittent pressure switch failures — the switch closes sometimes and not others, or the furnace lights but then trips a limit. In the Ohio River valley, the freeze-thaw pattern through a typical Louisville winter puts more thermal stress on heat exchangers than a climate that drops to 10°F and stays there. A heat exchanger that sees 40 heating cycles in a day during a week of temperature swings between 20°F and 50°F is working hard. If you’re seeing repeated pressure switch faults on a furnace that’s more than 15 years old, a heat exchanger inspection is worth having before replacing parts.
Now You Know Exactly How Your Furnace Fires Up
The gas furnace ignition sequence — thermostat, inducer motor, pressure switch, igniter, gas valve, blower motor — runs in that order every single time, and every step depends on the one before it. When your furnace isn’t heating, you now have a roadmap. Start at the thermostat and work forward. Listen for the inducer to start. Watch for the igniter to glow. Check whether the burners actually light. Note how long before the blower comes on. Each observation tells you something specific, and that information makes every service call faster and more accurate — whether you’re troubleshooting yourself or describing the problem to a technician.
If your furnace isn’t starting and you can hear the inducer running but nothing else is happening, the pressure switch circuit is the next place to look. If you can’t hear the inducer at all, that’s your starting point — check for power to the motor, listen for any hum that might indicate a seized bearing, and inspect the capacitor if it’s a PSC-type motor.
For homeowners in Louisville, Jeffersontown, Shepherdsville, or across the river in Southern Indiana, Louisville HVAC Pro’s is available to diagnose furnace ignition problems accurately and explain exactly what failed and why.
Frequently Asked Questions
What is the first thing that happens when a gas furnace starts?
The first thing that happens is the thermostat sends a 24-volt signal to the furnace control board through the W terminal. The board receives that call for heat and immediately energizes the inducer motor — not the gas valve, not the igniter. Gas doesn’t enter the system until step five of the six-step sequence.
Why does my furnace blower turn on after the heat has already started?
The blower delay is intentional. The control board holds the blower off for 30 to 90 seconds after the burners ignite, allowing the heat exchanger to warm up first. This prevents cold air from blowing through your vents at the start of each heating cycle and reduces thermal stress on the heat exchanger over time. The delay timing is often adjustable via DIP switches on the control board.
What is the difference between hot-surface ignition and intermittent-spark ignition?
Hot-surface ignition uses a resistive element — either silicon carbide or silicon nitride — that heats to over 1,100°F when 120 volts is applied, igniting the gas on contact. Intermittent-spark ignition uses a high-voltage spark to light a small pilot flame first, and that pilot flame then lights the main burners. Hot-surface ignition is the standard in residential furnaces installed after the mid-1990s. Intermittent-spark systems are more commonly found in older equipment or certain commercial applications.
What does a flame sensor do in a gas furnace?
The flame sensor confirms that the burners actually lit after the gas valve opened. It does this through flame rectification — a live burner flame conducts a small AC current through the sensor rod and converts it to DC, which the control board reads as proof of ignition. If the board doesn’t detect that signal within 7 to 10 seconds of opening the gas valve, it closes the valve and either retries ignition or locks out completely. A coated or oxidized flame sensor rod is one of the most common causes of a furnace that lights and then shuts off within seconds.
Why is the inducer motor so important in the furnace ignition sequence?
The inducer motor serves two functions: it purges residual gases from the heat exchanger before ignition, and it maintains the negative pressure that combustion depends on throughout the entire heating cycle. Without it, the pressure switch won’t close, the igniter won’t energize, and the gas valve won’t open. It’s the gatekeeper of the entire sequence. Beyond safety, the inducer also protects air quality — if combustion gases can’t exit through the flue reliably, they have nowhere to go except into your living space.