Well, THIS isn’t how you wanted to start the business day. Your building’s heater comes on, and instead of nice, toasty warm air, you get cool air—or worse, an Arctic blast! What’s going on?

Here are some of the most common reasons your building’s furnace could be blowing cold air:

  • A problem with the thermostat
  • The furnace is overheating
  • The ignitor or flame sensor is failed, or the pilot light is out (for gas furnaces)
  • The furnace isn’t getting enough fuel (for gas or oil furnaces)
  • A problem with your ductwork

We’ll go through each of these in this article. One thing to keep in mind before you start troubleshooting: furnaces can take awhile to warm up. If the heater blows cold air right after you turn it on but warms up in a few minutes, it could be as simple as that. If your furnace takes longer to warm up than you remember, it could be slowing down with age.

1. Check Your Thermostat

Your thermostat is the best place to start for most HVAC issues. Here are a few things to check:

  • The fan switch. Many thermostats have a fan setting that can be switched to ON, OFF or AUTO. If it’s ON, the furnace fan may blow continuously even when the furnace isn’t heating the air up to the temperature you set. Try switching the setting to AUTO to see if this resolves your issue. Keep in mind that a fair amount of commercial spaces and associated equipment are “constant volume.” This means airflow is continuous and either heating or cooling is applied as the controls specify. In these cases, the above won’t apply to your system.
  • The batteries. Many thermostats run off central power with a battery backup, but some run on batteries exclusively. Dying batteries can cause your thermostat to malfunction and send incorrect temperature information to your heater. If you haven’t done so in awhile, replace the batteries and see if this clears up your cold air problem.
  • The air temperature near the thermostat. A thermostat reads the temperature of the air close around it. If the thermostat is in a very warm spot, it will apply that temperature to the rest of the building—which may be significantly cooler. That can lead to cooler air than you expected as the thermostat tries to regulate the temperature.

If you’ve checked these things and still suspect the thermostat is the problem, it may need recalibration or replacement.

2. Your Furnace May Be Overheating

Does your furnace blow hot air, then cold or cool air, then nothing? Your furnace may be overheating.

As part of their internal safety controls, commercial furnaces have several limit switches and component operations that have to be “proved” in the sequence of operation before the unit’s control board logic will allow firing. If the unit gets too hot, the safety controls will shut down the burners. The cool air then continues awhile to cool the unit down to a safe level.

Dirty air filters are the most common cause of furnace overheating. Dirty coils are another culprit. Try changing the air filter and see if this corrects the problem. If not, call in a professional HVAC technician without delay. Repeated overheating will damage the heat exchanger.

3. For Gas Furnaces: Check the Ignitor and Flame Sensor or Pilot Light

Most modern gas-fired furnaces use an electronic igniter coupled with a flame sensor. These can fail, so they’re a good place to check if you’re having issues. A commercial HVAC technician can repair or replace these components if needed.

If you have an older gas furnace with a physical pilot light, the pilot light going out can cause the furnace to blow cold air. This usually happens when the gas company shuts down lines for maintenance or an emergency. Once the gas line is safely back in operation, try relighting the pilot light and see if this resolves your cold air problem. If the pilot light won’t stay lit, call in a professional to take a look.

4. For Oil Furnaces: Check the Filter and Flame Sensor

If your oil heater is blowing cold air, it may not be getting enough fuel. A common culprit is a clogged filter. Check the filter and have it cleaned or replaced if necessary—this needs to be done regularly for your heater to perform properly.

The flame sensor is another possible trouble spot. It may be faulty or, more commonly, dirty. An HVAC technician can clean or replace the sensor if needed.

5. You May Have a Problem With Your Ductwork

Ductwork tends to be out of sight, out of mind. Your heater blowing cold air or not enough air can be a warning sign that there’s a problem with your ducting.

When you turn on the heat, the heated air must travel from the furnace and through the ductwork before coming out through your vents. If the duct is broken, collapsed or obstructed, you’ll notice a reduction in airflow. In turn, this can cause the furnace to go out on a high limit due to not enough air being allowed across the heat exchanger. The burners shut down as described above in the section about overheating.

How do ducts break? Over time, the various temperatures surrounding and going through the ductwork cause stress and strain. Good insulation goes a long way in preventing problems, but even when used normally, ducts will degrade over the years. Internally insulated ducting can be an issue as well. Over time, the insulation can disadhere and fall into the duct space causing a clog and reduced airflow.

If you suspect a leak or other problems with your ductwork, call a professional who specializes in that area.

Still not sure why your building’s heater is blowing cold air? Call in a professional.

A cold air problem with your heater isn’t going to resolve itself. Waiting to address it could lead to a more costly repair down the road or even a safety issue. If these troubleshooting tips didn’t lead to a simple fix, or if you aren’t sure how to proceed, call in a professional without delay.

If you’re in the Wichita area, Commercial Trade Services is nearby to help you with your commercial heating needs. We can also help you prevent unexpected heater problems through proactive maintenance. Contact us today.