Furnace Running but Not Heating? A Peru & Bunker Hill Homeowner’s Guide

Nov 4, 2026

You can hear the furnace. Air is coming out of the vents. But the house is not getting any warmer, and the air feels lukewarm at best. That is one of the more frustrating furnace problems because it looks like everything is working.

The good news is that “running but not heating” usually narrows the problem down to a few likely causes. Here is how we think through it on calls around Peru, Bunker Hill, Mexico and Denver, and what you can safely check yourself first.

If at any point you smell gas or the carbon monoxide alarm goes off, stop. Get everyone outside, then call your gas utility and 911.

Start with the fan setting

Look at your thermostat. Most have a fan setting of AUTO or ON.

AUTO runs the blower only when the furnace is heating.
ON runs the blower all the time, even between heating cycles.

If the fan is set to ON, you will feel room-temperature air blowing between cycles, and it can feel cold. Switch it to AUTO and give it a few minutes to see if the air warms up. This one setting accounts for more “it’s blowing cold air” calls than you would think.

While you are there, make sure the thermostat is set to HEAT and a few degrees above the current room temperature.

Is the furnace actually heating, or just moving air?

Next, figure out whether the burners are lighting. On many furnaces you can see the flame through a small window in the front panel. You might also hear a whoosh when the burners light a minute or so after the furnace starts.

If the burners never light

The blower might be running on its own while the furnace tries and fails to ignite. That points to the ignitor, the flame sensor, the gas supply, or a safety switch. A blinking light on the control board usually gives a code. Write down the pattern before you call.

If the burners light and then go out

A dirty flame sensor is the most common reason. The burners light, the sensor does not confirm the flame, and the furnace shuts off the gas. The blower keeps running to cool things down, so you get cool air.

Check the filter and airflow

A clogged filter does two things. It cuts airflow, so less warm air reaches the rooms. And it can make the furnace overheat, which trips a safety switch that shuts off the burners while the blower keeps going. Either way, the result feels like cold air.

Pull the filter and look at it. If it is gray and packed with dust, replace it. Then make sure supply vents are open and the return grilles are not blocked by furniture.

High-efficiency furnace? Look outside

If your furnace vents through white plastic pipes, check where they leave the house. Snow, ice or a bird nest blocking the intake or exhaust will stop the furnace from firing. Clear anything you can reach safely. Also look for water around the base of the furnace, which can mean a clogged condensate drain has shut it down.

Older homes and ductwork

Many homes in Peru and Bunker Hill have been around a while, and some have had additions, finished basements or ductwork changes over the years. Leaky or disconnected ducts in a crawlspace or attic can dump warm air where nobody feels it. If one part of the house is always colder, or a room got colder after a remodel, ductwork may be part of the story.

A simple test: on a cold day, feel the air at the register closest to the furnace and the one farthest away. If the near one is warm and the far one is barely warm, the furnace is probably fine and the heat is getting lost on the way.

That is not usually a quick fix, but a tech can look at the system as a whole and tell you if the ducts are the problem or the furnace is.

When to book furnace repair

Call us for furnace repair if:

The fan is on AUTO, the filter is clean, and you still get cool air
The burners will not light, or light and quickly go out
The furnace starts and stops every few minutes
You see a blinking error code that will not clear
You smell burning or hear grinding or banging

If the furnace was not serviced this fall, a furnace tune-up covers the flame sensor, ignition and airflow checks that cause most of these problems.

We have been serving Peru since 1969, including families out in Bunker Hill and across Miami County.

Schedule furnace service or call us at 765-460-2660.

Frequently asked questions

Why is my furnace blowing cold air?

Common causes are the thermostat fan set to ON, a dirty flame sensor, a clogged filter, or the furnace overheating and shutting off the burners while the blower keeps running.