Heating & AC Repair

Furnace Heating Troubleshooting Guide

The information contained in this article is for reference purposes only.

Never service any equipment where the power hasn’t first been disconnected. Some electrical parts such as capacitors can still hold dangerous electric current even after the power has been disconnected. 

If you suspect that gas may be leaking from your system, get out of your home, shut off the main source of gas, and call your local gas company immediately.

If you live in the our service area (including North Raleigh, Raleigh, Cary, Durham, Chapel Hill, or Wake Forest), call Comfort Monster or a licensed technician to professionally service and repair your HVAC system. 

When your furnace isn’t keeping you warm, start here before doing anything else:

  1. Filters: Make sure all filters have been replaced recently and are clean and free-flowing.
  2. Thermostat: Make sure the thermostat is set to heat and the temperature is set a few degrees above the current indoor temperature. Note that if you recently had your thermostat in cool mode, you might have to wait up to 5-10 minutes to allow the system to switch over. Many thermostats have built-in delays.
  3. Gas/Propane:  You may laugh, but we’ve been to calls where the only problem is the propane has run out or the gas has been shut off for another reason. While it might seem obvious, you might save some money by checking this first.
  4. Condensate Pan/Drain: If you have a “split system” high efficiency (90% furnace) or and you know where your indoor unit is located, check the drain pan and drain line to be sure the condensate is draining properly, and remove any clogs. 90% furnaces create condensation and a clogged drain will cause your system’s safety mechanism to trip and stop operation of system in order to prevent water damage to your home.

If doing one of these things fixed your problem, continue to monitor your system and make sure it’s operating properly. This might be a good time to make a non-emergency call to get a tune-up or inspection on your system if it has been more than six months to a year since you had professional maintenance on it.

Still No Heat? Try this.

If you have your furnace’s homeowner’s manual, it will often have troubleshooting tips that are specific to your furnace. Follow those instructions first.  If that doesn’t work, continue troubleshooting below.

Find your furnace. It is usually located in the attic for upstairs systems, or under the house for the 1st floor. If you have a big box out in the yard (read this if you need help identifying your system), called a package unit, there probably isn’t much you can do but call a professional to take a look. Package systems have doors that are screwed on and even locating the right panel to take off would take some explanation. 

  1. Look for Blinking Lights.  There is often an LED light that can be seen from outside the furnace. This light will usually blink in a sequence to provide status or fault codes for your furnace. Long and short blinks can be combined to match up to a code. For instance, 2 long blinks followed by 3 short ones might indicate a “24” error. A constant blinking light with no longs or shorts will often indicate the furnace is in “standby mode” or it’s receiving a call for heat from the thermostat. You will have to consult your manual or the inside cover of the furnace for details on what the blinking light means. If you aren’t able to learn anything from the blinking status light(s), try step 2 below.
  2. Reset the Power. By code, there should be a power disconnect within a few feet of your furnace and it is often labeled. It is common for this to be a light switch. Flip this off and wait about 20 seconds and then turn it back on. You should hear a few clicks and the blower starting up. Don’t get too excited yet. The blower comes on to “purge” any heat that might still be inside your furnace. After that it will go through the startup sequence and attempt to fire the furnace. If you wait a minute or so and you don’t hear anything happening, proceed to step 2. If the furnace starts normally, keep an eye on it for a few minutes. It might shut back off. No matter what happens, you probably need service but you might be able to get a little heat first. 
  3. Remove the cover. Once you get to this step, it is highly unlikely you are going to fix your own furnace, but if you feel so compelled and have the skill to do it (and have the power disconnected) you can take the cover off. The cover for the furnace usually comes off without removing any screws. There are several different styles from a single door that just pops off to a 2 piece design that you slide and pull, to a 2 piece design with 2 knobs that you turn horizontal to remove. Once you have the cover off, you can make sure there aren’t any loose wires. You can also check to see if the fuse on the control board (3-5 amps) is blown. At this point, you’re advised to call for service, even if you find the issue, you are going to need parts and the sooner a professional diagnoses the problem, the quicker they can get your furnace back up and running.

That Wasn’t the Problem Either? More Furnace Troubleshooting Is Below.

To troubleshoot other specific symptoms affecting your HVAC furnace system, find your symptom in the chart, then click on each of the possible causes you’re interested in learning more about:

SymptomPossible Cause
Thermostat Blank
  • Circuit breaker tripped
  • Condensate drain clogged
  • Thermostat battery dead
  • Wiring issue
Blowing Cool Air
  • Furnace not starting
No Air Moving
  • Worn or thrown belt (older systems)
  • Bad blower motor
Water Leaking from AC
  • Cracked pipe
  • Bad condensate drain pump
  • Condensate drain clogged
Furnace Making Noise
  • Bad fan blower motor
  • Bad fan bearings
  • Loose or slipping fan belt (older systems)
  • Broken fan shroud
  • Broken or unbalanced fan blades
  • Bad inducer motor or bearings
Furnace Won’t Turn Off
  • Bad thermostat
  • Wiring problem or short
Electrical or Rubber Smell in Vents
  • Bad blower motor
  • Bad blower capacitor
  • Bad blower bearings
  • Wiring problem or short


  • Condensate drain pan and drain line troubleshooting:
    • If you have a split system (if you’re not sure what kind you have, go read this article and come back), your indoor unit produces condensation when the hot air from your 90% furnace cools. This condensation is designed to drain outside your house through a plastic PVC drain line. If this line becomes clogged, it will overflow from the unit and into the drain pan. Once the drain pan fills up, a safety cutoff switch will kill the power to the unit to prevent the condensation from causing water damage in your home. 
    • Your indoor unit may be located in the basement, attic, or a closet. Once you find the unit, check the drain pan for water. If it’s full, you’ve found your problem. 
    • The drain line(s) will need to be cleaned out. Professionals may do this with a combination of compressed air and/or a pipe snake or other specialized tools. If you aren’t able to get the water drained out by yourself, call us at 919-MONSTER. 
    • The drain line is one of the many air conditioner parts cleaned and serviced as part of our twice annual MonsterCare maintenance program, which usually prevents issues like this from occurring.
  • Circuit breaker tripped: 
    • Circuit breakers are designed to protect your HVAC equipment from damage and your house from a fire if an electrical problem develops. That being said, it is possible that resetting a circuit breaker will get your system working again, at least for a little while. 
    • How to do it: Locate your electrical panel. It is usually found in an accessible but unobtrusive place such as a laundry room, garage or hallway. Sometimes either your main panel or a sub-panel will be located near your outdoor unit. 
    • Open the panel and look for breakers that are tripped. Some circuit breakers show a red indicator if they are tripped, others might just be flipped to off or partially off. Your breakers should be labeled, so look for ones related to the HVAC system. 
    • To reset a breaker, turn it all the way off and then back to on. If it trips back right away, leave it off. That means it is doing its job of protecting your home, because there is an electrical short somewhere that otherwise might have caused a fire. 
    • Either way, if the breaker being tripped was the issue, you should call Comfort Monster– There is a good chance that whatever made the system trip will happen again. 
  • Thermostat battery dead: 
    • If the problem is a dead battery in your thermostat, the fix can be quick and cheap – But don’t get your hopes up too high quite yet, because these days most thermostats don’t have batteries. 
    • They are most often powered by the main equipment. If you have a modern WiFi type thermostat it most likely doesn’t have batteries. You can check, however. There are too many types to provide instructions, but you can either get your make and model number and Google whether it has batteries and how to replace them or look for yourself. If you want to check for yourself, look carefully at your thermostat. Normally you can open up the lower panel or unclip it from the wall. Once you get it open or unclipped from the wall look for a batter door. Open the battery door and replace the batteries. Hopefully you’re back in business! If not, call 919-MONSTER and we’ll be happy to help.
  • Loose or Slipping/Thrown Fan Belt: 
    • This problem is only possible if you have a split system that has an older-style indoor unit, as newer models are generally “direct drive” rather than using fan belts to power the indoor blower. 
    • Locate your indoor unit (if you don’t have one, read more here). If there is a fan belt, it’s normally pretty easy to see the pulleys on the motor and fan. If the belt is not present, look around for it. It should be near the unit. 
    • If the belt is undamaged, you may be able to put it back on. Make 100% sure the power is off to the indoor unit first! Then place the belt around the fan pulley and motor pulley as far as it will go. Rotate the pulley in the direction that will cause the belt to tighten and pop onto the pulley. Once it’s on, check the tension of the belt. It should be firm, but not so tight that it binds. If you need to, adjust the motor to tension the belt properly. 
    • If the belt is damaged, we recommend that you stop and call an HVAC company to make sure it gets replaced properly and that there’s not another underlying problem. But if you feel confident that you can get the proper replacement belt for your system, follow the directions above to install it.
    • Call Comfort Monster if you need any help!
  • Cracked Drain Pipe: 
    • You may certainly be able to repair a cracked drain pipe yourself, or you can hire a plumber or HVAC technician to do it. We would recommend that since you are having problems with your HVAC system, it’s probably best to have a licensed technician check out the system and make sure there’s no other underlying problems before fixing the drain pipe.
  • Bad Condensate Drain Pump: 
    • Locate your indoor unit and look in the drain pan (that’s the thing under the unit that catches water) for a condensate drain pump. When the system can’t drain directly by gravity due to installation constraints, the installer or contractor may have used a pump to remove the condensate from your system. These are self-contained pumps that come on automatically, as a sump pump does, to get the water out. 
    • Condensate drain pumps are often plugged into an outlet rather than being hard-wired into the electrical system. Test your outlet to make sure there is power. If the outlet is bad, the pump of course won’t work, and you’ll need to get a licensed electrician (or an HVAC tech) to replace that outlet. 
    • If the outlet isn’t the problem (you can also use an extension cord to test the pump with a different power source), then your pump could be bad. If you want to replace it yourself, look at  your local home improvement store for a new pump with a similar size and capacity – It doesn’t need to be something specially tailored to your system. The PVC or tubing is probably not glued to the pump. Gently remove the pipe and then remove and replace the pump. Call 919-MONSTER or another licensed HVAC company if you need help.
  • Bad Fan Blower Motor: 
    • If this is the problem, we recommend you call 919-MONSTER or another licensed HVAC company to check your system and repair or replace the fan blower motor. 
  • Bad Fan Motor Bearings: 
    • If this is the problem, you might hear a screeching or grinding sound like metal on metal, perhaps in a rhythmic, oscillating way. Call 919-MONSTER or another licensed HVAC company to check your system and repair or replace the fan blower motor. 
  • Broken or Loose Blower Wheel or Blower Housing: 
    • On a typical split system, the blower will be located in the inside portion of the split system. These are usually found in the attic, crawlspace or a closet in your house.
    • When you find your furnace/air-handler if you hear grinding, banging, humming, or smell any kind of burning or electrical issue you probably have something wrong. 
    • If you want to proceed further, with the power disconnected. With the unit off, locate the blower (you will need to remove the cover from your air-handler) and make sure it is properly secured and that there are no cracks or breaks. If you find any issues, note the issues. If the blower is just loose, tighten the blots/screws and adjust as necessary. It is common in units 15 or more years old to have issues with the blower where the blower and wheel need to be replaced. 
    • If you need any help, call 919-MONSTER or another licensed HVAC company. 
  • Bad Thermostat: 
    • Diagnosing whether the thermostat or wiring is the issue can be tricky. If you are sure you have a bad thermostat, you can attempt to replace it yourself. Be advised that different equipment requires different setups, especially with smart thermostats. It’s often not as simple as just connecting the right wires and mounting the thermostat on the wall. Read the documentation that came with the thermostat, and watch available online tutorials. 
    • Note: You should ALWAYS turn off power to the indoor unit before removing or installing thermostat. Otherwise, you may damage thermostat or indoor transformer. If you have any problems call 919-MONSTER or another licensed HVAC company to help you. 
  • Wiring Problem or Short:  
    • Diagnosing wiring problems can be one of the most difficult issues to troubleshoot. Intermittent faults such as shorts or poor continuity can appear to work properly when testing and then exhibit strange issues. The new modern systems and thermostats are more sensitive to wiring issues. 
    • For either thermostat wires or other electrical issues affecting your comfort systems, we recommend calling 919-MONSTER or another licensed HVAC company.


When your furnace or heating goes out, there are a few things you might be able to try before calling in the professionals. Some of these are relatively simple, such as cleaning your filters, but others could get you hurt or make the problem worse. Either way, educating yourself about this important part of your home is a good idea — Even if you do have to call someone from Comfort Monster or another HVAC company, you’ll be ahead of the game when it comes to understanding what is going on and needs to be done.