Why Does My Furnace Turn on but No Heat Comes Out? A New Jersey Tech Answers | Air Experts

Cold air coming from your furnace is the last thing you want going into a New Jersey winter.

Your furnace could be producing cool air for a lot of different reasons, but the most common causes are:

  • Thermostat is set to “ON”
  • Bad/stuck fan limit switch

To help you solve your “no heat” problem, we’ll walk you through how to troubleshoot the 2 problems below.

Troubleshooting tip #1: Check the thermostat

If your furnace blows hot air sometimes and cool air at other times, you probably have your thermostat set to “ON”. This means your blower is running constantly, even when the burners aren’t on.

What to do: Set your thermostat to “AUTO” (see picture below). This will set the blower to run only when the burners run, so you shouldn’t get cool air coming from your vents anymore.

furnace blowing cold air

Troubleshooting tip #2: Check the fan limit switch

Some furnaces have a combination fan limit switch that:

  1. Tells the blower when to turn on (by measuring the temperature of the heat exchanger)
  2. Acts as a safety device that shuts off your burners if the furnace gets too hot

A fan limit switch has 3 primary jobs:

  • Turn the blower motor ON (when the heat exchanger rises to a certain temperature at the start of a heating cycle)
  • Turn the blower motor OFF (when the heat exchanger drops in temperature after a heating cycle)
  • Turn the furnace blowers off (when the heat exchanger hits dangerously high temperatures)

That said, if your furnace fan limit switch is damaged or has been tampered with, it might allow the fan to run even after the heat exchanger has cooled off. This will cause the fan to blow chilly air into your home.

Also, if the fan limit switch override (a small knob on the switch) somehow got set to MANUAL instead of AUTO, the blower will run constantly—whether or not your furnace is going through a heating cycle.

Note: If you or a tech has recently tampered with your furnace, it’s possible that the fan limit switch was set incorrectly by accident.

What to do: To change your fan limit switch controls, follow the steps below:

  1. Remove your furnace’s access panel. You should be able to simply slide off or unhook the panel.
  2. Locate the fan limit switch. It will likely be located in a small box towards the top of your furnace.
  3. Open the switch cover. Inside you’ll see a metal dial labeled “FAN LIMIT” that will look something like this.
  4. Slide the controls to the right temperature. We recommend checking the owner’s manual to find the appropriate temperature settings for your furnace.
  5. Set the override to AUTO. On most furnace models, you push in the white knob for automatic blower motor operation and pull it out for continuous. When the override is set to AUTO, the blower motor will only run when your furnace needs to heat your home, so you shouldn’t get any more cool air.

Note: Only follow these DIY steps if you’re familiar with how a furnace operates. Otherwise, you could risk damaging your unit or even injuring yourself.

Still getting cool air? You might have leaky ducts.

If you tried troubleshooting the 2 tips above and you’re still getting cool air coming from your vents, it could be caused by leaks in your supply ducts.

When your supply ductwork has holes or connection gaps, the air your furnace heats escapes into your attic or crawlspace. Sometimes, if the duct leakage is really bad, the air coming from your vents might feel less warm or weaker than normal.

furnace duct leaks
Leaks in your supply ducts push heated air into your attic or crawlspace.

Plus, leaky ducts reduce the efficiency of your furnace because it has to run longer and work harder than normal.

What to do: Call a professional to repair your ducts. Trying to repair the leak yourself could do more harm than good (ducts can be fragile), so we recommend leaving duct repairs to a professional.

Need a pro to look at your furnace? Call a New Jersey tech.

We’ll send one of our pros to figure out why your furnace isn’t blowing hot air and fix the problem.

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