When a furnace runs but the air from the vents feels cool, the blower is working but the burners aren’t heating, or aren’t heating for long. In Chicago homes, the cause is usually one of these seven, roughly in order of how often they turn up.
1. The thermostat fan is set to On
If the fan switch is set to On instead of Auto, the blower runs all the time, including between heating cycles, so you feel room-temperature air for long stretches. Switch it to Auto and see whether the air turns warm when the furnace fires.
2. A clogged air filter
A packed filter restricts airflow until the furnace overheats. A safety switch called the high limit then shuts the burners off while the blower keeps running to cool things down, so you get lukewarm or cold air. Pull the filter; if it’s gray and matted, replace it.
3. A dirty flame sensor
The flame sensor proves the burners are lit. When it’s coated with residue, the furnace lights, can’t confirm the flame, and shuts the gas off after a few seconds. It may retry several times and then lock out. This is one of the most common furnace repairs and usually a quick fix for a technician.
4. A failed igniter
Most furnaces built in the last few decades use a hot-surface igniter that glows before the gas opens. Igniters crack with age. If yours doesn’t glow, the burners never light.
5. Gas supply problems
If other gas appliances also aren’t working, the issue may be the gas supply. If you smell gas, leave the house and call Peoples Gas or 911 from outside.
6. A clogged condensate drain (high-efficiency furnaces)
High-efficiency furnaces produce water that drains away through a small tube. If the drain or trap clogs, a float switch stops the furnace from heating. Look for water around the base of the furnace or a full condensate pump.
7. Ductwork leaks
If the furnace is heating but a few rooms still get cool air, leaky or disconnected ducts in the basement or attic can be losing the heat before it arrives. This is common in older Chicago homes converted to forced air. See duct sealing and repair.
What to check before you call
- Thermostat set to Heat, fan on Auto, temperature a few degrees above the room.
- Filter replaced if dirty.
- Furnace switch on and the front panel seated properly.
- Any error light code on the furnace’s control board, which a technician can read.
If the air still isn’t warm, call (312) 924-7223 for furnace repair in Chicago. A technician can test the flame sensor, igniter, limit switch and gas valve and find the cause.