You don’t have to be an expert to understand how your furnace works or to tell when something’s off. Most people just want their home to stay warm when it’s cold outside. But when things start taking longer to heat up or a room never quite feels right, knowing the basics can help you figure out what’s worth checking and what can wait.
If you’ve just moved into a home with forced air, or you’re trying to get a handle on why your system sounds different than it used to, this guide will walk you through what a furnace actually does, how it works with the rest of your home, and what warning signs are worth a second look.
The Job It’s Built to Do
A gas furnace does one thing: It takes air from inside your home, heats it, and sends it back out through the ducts. That cycle continues until the thermostat says you’ve hit the temperature you asked for.
What happens in between is pretty straightforward. When the thermostat kicks on, it tells the furnace to open the gas valve. A small flame ignites the gas, which heats up a part called the heat exchanger. A blower then pushes air across that heated surface. That air moves through the ducts, comes out of the vents, and starts warming the space.
You might hear the system start with a soft click or a quiet hum. A few seconds later, warm air should start coming from the vents.
The Role of the Thermostat
The thermostat monitors indoor air temperature so that when it drops below the number you’ve set, it can signal the furnace to start heating. That signal triggers the gas valve to open, the burners to ignite, and the fan to start pushing air through the system.
Once the furnace gets going, it keeps running until the thermostat senses that the room’s warmed up. Then it tells the system to shut off. The burners stop first, and the fan usually keeps running for a bit to clear out any leftover heat.
If the thermostat isn’t reading the room correctly or isn’t sending that signal like it should, the furnace short cycles or it might not start at all.
Don’t Ignore the Filter
This is the part most people forget. And it’s usually the thing causing the problem when airflow suddenly drops or the system shuts itself off. Your furnace pulls air through a filter before it heats anything. If that filter’s clogged, the furnace has to work harder to move the same amount of air. That can cause it to overheat or shut down early.
Some homes need a new filter every month. Others can go longer. Monthly checks are smart if you have pets or a lot of foot traffic. Pull it out and take a look. If it’s gray and dusty, or you can’t see through it, it’s time for a fresh one.
How Long Should Heating Take?
That depends on a few things—how cold it is outside, how well your house holds heat, and how big the system is. On a typical Minnesota winter day, the furnace might cycle on every 15 to 30 minutes. If it’s below zero, you’ll hear it more often. That’s normal.
What you want to watch for is a change in behavior. If the system starts running non-stop but the house still feels cold, or if the furnace keeps shutting off before the rooms are warm, something’s likely interfering with how the system is running. It could be something simple like a dirty filter or a stuck damper. It could also be something electrical or mechanical inside the unit.
Keeping the System Safe
Modern furnaces have multiple sensors built in to keep everything running safely. If something starts behaving outside normal limits, the system shuts down to prevent damage or a safety issue.
It may try to restart on its own, but if the problem sticks around, the furnace locks out and stops running until someone resets it or clears the fault.
Flame sensor confirms that the flame is steady and burning clean. If it doesn’t detect a flame within a few seconds, it shuts the gas off to keep it from filling the combustion chamber.
High-limit switch monitors how hot the heat exchanger gets. If the furnace is overheating, which is usually due to poor airflow or a dirty filter, the switch will shut everything down to prevent heat damage or a cracked exchanger.
Pressure switch checks proper airflow through the combustion chamber and vent pipe. If the vent is blocked or the inducer fan isn’t moving air the way it should, the pressure switch won’t allow ignition to start.
Rollout switch trips if flames start spilling outside the burner area, which can happen if the heat exchanger is cracked or blocked. It shuts the whole system off immediately and usually needs to be manually reset after inspection.
Sensors can get dirty, wires can corrode, and switches can stick. A yearly check gives the Schwantes team the chance to clean, test, and replace what needs attention before it shuts down on a subzero morning.
