When the furnace kicks on and only blows weak or lukewarm air, a dirty filter is one of the most common weekend surprises. A clogged filter can shut your system down, trigger safety switches, and leave you cold right when you need heat the most. A regular furnace tune up plus simple filter checks can prevent late night calls for local emergency furnace repair and keep your energy bills under control.
If your home in Boulder or nearby towns feels chilly even though the furnace is running, the filter is one of the first things to check before you call for same day furnace repair.
How to check air filter in furnace
Start at the thermostat and make sure the furnace is actually set to Heat and calling for warmth. If it is, you can safely check the filter in a couple of minutes.
Most furnace filters sit in one of three spots:
- In a slot beside or under the furnace cabinet
- Behind the lower blower door
- Inside a filter rack on the return air duct
Look for the removable cover, then slide the filter out and hold it up to the light. If you can barely see light through it or it looks gray and fuzzy, it is overdue for a change. During pre winter prep, many Boulder homes do this at the same time as a furnace tune up so the system starts the season with clean airflow.
If the filter is hard to reach, stuck, or you are not sure what size to buy, call us and we can take care of it as part of a quick maintenance visit.
Can you change furnace filter while its running
Most furnaces will still run while you swap the filter, but it is not a good idea. For safety and comfort, turn the system off first.
Use the thermostat or the power switch near the furnace to shut it down. That way the blower is not pulling dust into the equipment while the filter is out, and you avoid any chance of the blower door tripping a safety switch. Once the new filter is in place and the panel is secure, turn the furnace back on and let it run a full cycle.
If you are nervous about turning off or restarting the system, a short furnace tune up visit can include filter replacement, basic checks, and a test run so you know everything is working correctly.
Can a dirty filter cause furnace not to work
Yes. A dirty filter is one of the top reasons a furnace will not stay on or refuses to start a full heat cycle.
A dust packed filter forces the blower to work harder, which can lead to:
- Overheat and shut down on a safety limit
- Short cycle and only blow warm air for a few minutes
- Trip protection switches that prevent the burners from lighting
For you it just feels like a weak airflow. But inside the furnace, it is a safety shutdown designed to protect the heat exchanger and blower motor from damage.
If you have already replaced the filter and the furnace still will not run, you may need a local emergency furnace repair to reset safety limits, clean internal components, or diagnose a failing part.
Need a hand today?
Call (720) 680-2303 or schedule at www.mesa-plumbing.com. Full furnace tune-up + fresh filters, all to get your home winter-ready.
How do you reset a furnace after a dirty filter?
First replace the dirty filter with the correct size and type. Then turn the thermostat to Off and wait a few minutes. If your furnace has a power switch nearby, switch it off and back on to give the control board a fresh start. After a short pause, set the thermostat back to Heat and raise the temperature a few degrees above room level.
Signs of a clogged furnace filter?
Weak airflow, longer run times, uneven room temperatures, frequent cycling, extra dust, or a warm furnace cabinet are common warning signs. If you notice any of these during pre-winter prep, replace the filter and consider a furnace tune-up before the colder weather hits.
What does a dirty furnace filter smell like?
A dirty furnace filter usually smells dusty, stale, or earthy, similar to an attic or a closed-up room. If pets, smoke, or cooking grease are trapped in the filter, the odor can be stronger and linger longer in the air.