Your Inputs Summary
| Check | Your Answer |
|---|
How to Use the Furnace Not Heating Tool
A surprising number of "furnace not heating" calls trace back to a simple thermostat issue: wrong mode, a setpoint below room temperature, or dead batteries. These take seconds to check and cost nothing, which is why this tool starts here before considering any furnace-side faults.
If you can safely observe through the viewport (never remove burner compartment covers or attempt to manually light a furnace), note exactly where ignition stops or fails. Does the ignitor glow or spark? Does a flame appear briefly then go out? This, combined with control board indicator behaviour, helps categorize the fault. See the fault code lookup tool for blink code categories.
If the furnace ignites and produces heat but temperature rise seems off, or the system short-cycles on a limit switch, the cause is often airflow-related rather than combustion. See the airflow diagnosis calculator for temperature-rise-based analysis.
Diagnosing a Gas Furnace That Won't Produce Heat
A gas furnace that fails to produce heat, whether it doesn't attempt to start at all or goes through part of its startup sequence before stopping, follows a predictable diagnostic pattern that can help homeowners understand what's happening even before a technician arrives, and can sometimes reveal a simple fix that doesn't require a service call at all.
The Normal Ignition Sequence
A modern gas furnace follows a specific sequence when the thermostat calls for heat: the control board first checks safety switches (pressure switch, limit switch, rollout switch) are in their normal position, then energizes the inducer motor to establish draft, confirms draft through the pressure switch, energizes the ignitor (hot surface or spark type) and waits for it to reach ignition temperature, opens the gas valve, and confirms flame through the flame sensor within a few seconds. If flame isn't confirmed, most systems attempt this sequence 3-4 times before locking out and requiring a manual reset. Understanding where in this sequence the furnace stops is the single most useful piece of diagnostic information available without opening the unit.
No Ignition Attempt: Power or Safety Circuit Issue
If the blower runs (or the furnace makes no attempt to start heating at all) but there's no sign of ignition attempt whatsoever -- no ignitor glow, no spark sound, no gas odor -- this points toward either a power/control issue preventing the sequence from starting, or an open safety switch preventing the control board from proceeding past its initial safety checks. Common causes include a tripped limit switch (often from a recent overheating event that hasn't been addressed), an open door safety switch (many furnaces require the blower access panel to be properly seated to complete a safety circuit), or a control board fault.
Ignition Attempted But Flame Doesn't Establish or Hold
When the ignitor visibly glows or sparks but a flame either never establishes or lights briefly before extinguishing, the fault typically lies in gas delivery, the ignitor's ability to actually ignite the gas-air mixture, or the flame sensor's ability to detect the flame that is present. A weak or aging ignitor may glow but not reach sufficient temperature to ignite gas reliably. A fouled or misaligned flame sensor can fail to detect a flame that's actually burning normally, causing the control board to shut gas flow believing no flame is present -- a very common cause of "lights then immediately shuts off" symptoms. Low gas supply pressure or a nearly empty propane tank can also prevent reliable ignition. See the combustion analysis calculator for combustion quality assessment once ignition is achieved.
Short Cycling and Limit Switch Trips
A furnace that heats briefly, then shuts down, potentially restarting after a cooldown period, is exhibiting classic short-cycling behaviour usually caused by a high-temperature limit switch tripping. This safety device opens when the heat exchanger temperature exceeds a safe threshold, most commonly due to restricted airflow preventing adequate heat removal from the heat exchanger by the circulating air. Checking and replacing a dirty filter is the first and most common fix for this pattern -- see the airflow diagnosis calculator for a full airflow assessment if filter replacement alone doesn't resolve it.
Flame Rollout: A Serious Safety Condition
If flame is visible extending outside the normal burner combustion area -- rolling out toward the front of the furnace rather than staying contained within the heat exchanger -- this indicates a serious combustion or venting problem requiring immediate shutdown and professional inspection. Flame rollout is typically caused by restricted venting preventing combustion gases from exiting normally, a blocked or damaged heat exchanger disrupting normal combustion airflow, or in worst cases a cracked heat exchanger. A furnace that has tripped its rollout safety switch should not be operated again until a licensed technician has identified and corrected the underlying cause. See the heat exchanger crack test and flue gas analyzer for further investigation of this condition.
Frequently Asked Questions
Usually the burners never ignited, or ignited briefly then shut down on a safety lockout. Common causes: a failed ignitor unable to reach ignition temperature, a dirty or misaligned flame sensor causing shutdown after briefly detecting no flame, a closed gas valve or empty propane tank, or a tripped pressure switch preventing the sequence from starting. Observing exactly where the startup sequence stops helps distinguish between these causes.
Often the thermostat fan setting is on "On" rather than "Auto," combined with a separate ignition or gas supply fault preventing the actual heating cycle from completing. It can also occur after a limit switch trip, since some control boards keep the blower running afterward to dissipate residual heat even though no new heat is being added.
Flame rollout occurs when burner flames extend outside their normal combustion path, typically toward the furnace front. It's usually caused by a blocked heat exchanger, restricted venting, or a cracked heat exchanger disrupting normal combustion airflow. A tripped flame rollout switch requires professional inspection before further operation, since rollout indicates combustion gases aren't flowing through the intended path and may pose fire or CO risk.
Yes -- worth ruling out first. Common causes: dead thermostat batteries causing display/program loss, a setpoint accidentally below room temperature, system mode switched to "Cool" or "Off" instead of "Heat," or a smart thermostat schedule conflict overriding the expected call for heat. These checks take a minute and can save an unnecessary service call.