If your furnace is blowing cold air, it can be frustrating—especially when you’re counting on it to keep your home warm. This issue is fairly common and can point to several underlying problems, ranging from simple fixes to more serious mechanical failures
Symptom: Furnace runs, but the air feels cool or room temp.
Cause: Thermostat is set to “On” instead of “Auto”, which makes the fan run constantly—even when the furnace isn’t heating.
Fix: Set the fan to “Auto” so it only runs during a heating cycle.
Symptom: Cold air blows briefly when the system first turns on.
Cause: The air handler kicks on before the heat exchanger has warmed up.
Fix: Give it a few minutes—this is normal at startup.
Symptom: Weak airflow and cold air.
Cause: A clogged filter restricts airflow, causing the furnace to overheat and shut off the burners for safety.
Fix: Replace the air filter and reset the system.
Symptom: The blower runs, but there’s no flame or heat.
Cause: The pilot light is out or the electronic ignition has failed.
Fix: Relight the pilot (if manual), or call a technician to inspect the igniter or flame sensor.
Symptom: Furnace lights briefly but shuts down after a few seconds, and the fan continues blowing cold air.
Cause: A dirty or malfunctioning flame sensor prevents the system from confirming ignition and turns off the burners.
Fix: Clean or replace the flame sensor (best handled by a pro).
Symptom: Furnace cycles on/off quickly, blowing cold air.
Cause: Furnace overheats (due to dirty filters, blocked vents, etc.), triggering the limit switch to shut off the burner.
Fix: Change filters, check airflow, and call a technician if it persists.
Symptom: Furnace runs but no heat is produced.
Cause: A closed gas valve, empty propane tank, or supply issue prevents ignition.
Fix: Check the gas supply, valve position, or contact your utility provider.
Symptom: Inconsistent operation, cold air only, strange noises.
Cause: Electrical failure in the furnace’s control system or blower.
Fix: Requires professional diagnosis and repair.
Symptom: Furnace won’t ignite, fan runs only.
Cause: A backed-up condensate drain line can trigger a safety shutdown.
Fix: Clear the line or call for service.
Check thermostat settings.
Change the air filter.
The pilot light is on (if applicable).
Error codes or flashing lights on your furnace panel.
Make sure all vents are open and unblocked.
If none of the simple fixes work, it’s time to call Fast HVAC to safely inspect and diagnose the problem.
We are authorized dealers and installers of the most common and trusted furnace manufacturers.
A faint gas smell when first starting your furnace can be normal, especially at the beginning of the heating season. However, a strong, persistent gas odor could mean there’s a serious gas leak. Leave your home immediately and call 9-1-1 from a safe location. Once you’re in a safe place, contact an HVAC professional to inspect your system.
Furnaces burn fuel, which produces carbon monoxide—a deadly, odorless, and colorless gas. A well-maintained furnace safely vents this gas, but a malfunctioning unit can allow it to accumulate to dangerous levels. If your carbon monoxide detector sounds an alarm, evacuate your home immediately and call 9-1-1.
Loud hissing, banging, or other unusual sounds could point to an electrical issue or another serious internal problem. If your furnace is making strange noises, stop using it and call a licensed HVAC technician to investigate and make necessary repairs.
If you’ve tried basic troubleshooting or don’t feel comfortable doing it yourself—and your home is still cold—it’s time to call in the experts. Your furnace may need professional repairs or replacement beyond DIY fixes.
Let’s bring your vision to life with expert craftsmanship and thoughtful design.
16120 Woodinville Redmond Rd NE STE. 15
Woodinville, WA 98072
(425) 200-4874
hello@fastwaterheater.com
Monday – Friday 7:30am-5pm
24/7 Emergency Service Available
HVAC License
NORDSHA772L2
Electrical License
NORDSHA774MR
