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The Home Depot

How to Repair a Furnace

Pro Referral > Home Guides > Heating & Cooling > How to Repair a Furnace
How to Repair a Furnace

So your heater isn’t heating or your furnace just isn’t keeping you warm. Maybe your vents simply aren’t blowing enough warm air, or instead they just keep blowing ... and won't stop. Sounds like a problem with your furnace, right?


Well Wait! Like your car’s proverbial “engine knock,” the real challenge with identifying a heating problem is first identifying exactly where the problem lies. While some issues with heating can, of course, be traced to your furnace, many more can be found in the system that surrounds it.


Some repairs are surprisingly easy and can be easily accomplished by you; others, however, will require the help of a professional. Knowing how to address these heating issues hinge on the understanding that your furnace is only part of a larger home heating system. This article will focus as much on how to repair a furnace as it will on how to troubleshoot and solve heating problems.


It will focus on conventional (vs. high-efficiency), combustion, forced-air systems. These are by far the most common systems in the US. High-efficiency set-ups, those installed in greater numbers in the past few years, use more complex science and require slightly different steps when troubleshooting.

Tools Materials Skill Level Estimated Time
• 4-in-1 Multi-screwdriver • Furnace filter Intermediate 1 to 6 hours
• Adjustable wrench • Foil duct tape
• Channel locks • Electrical tape
• Flashlight • Steel Wool or Sand Cloth
• Duct cleaning brush
  1. 1.Know Your Heating System

    All Conventional Gas Furnaces Perform a Few Basic Functions: These types of furnaces:

    1. 1) Take in cold air;
    2. 2) Clean that air with an air filter;
    3. 3) Heat up that air with a burner;
    4. 4) Incorporate a heat exchanger to help manage that heating;
    5. 5) Vent exhaust; and
    6. 6) Employ a blower that distributes the then warmed air throughout your home's ductwork.

    The Anatomy of a Heating System

    The furnace is the heart of your home heating system. The thermostat is the brain, the exhaust venting is the waste system, and the ductwork is the circulatory system. Often powered by natural gas, oil, or propane, the furnace is a complex appliance at the center of a forced-air heating system.


    In combustion systems, those driven by the “fuels” mentioned above, you'll find a combustion chamber, a blower, and some form of attached ductwork. The ductwork itself has outlets, both supply-side registers and return registers. The supply registers deliver heated air into the individual areas of the house. Return registers, also strategically placed throughout the home, make up the points at which air is pulled into the system. These "returns" may or may not be fitted with air filters.


    Within the combustion chamber, you'll find a set of burners, which may or may not have an integrated pilot and/or other related modules, such as a flame sensor or a thermocouple. Yes, every time your thermostat calls for heat, there is an actual fire that ignites within the furnace. Stoked and managed in part by the heat exchanger, your furnace, in turn, will also be fitted with some means of exhausting any by-product of this said combustion.


    Work to Diagnose Your Problem

    A Cheat Sheet to Causes of and Approaches to Three Common Problems.


    When looking for the source of a heating problem, a few common causes jump right to mind. Start your checks with this graph in mind.


    Problem: The furnace isn’t heating.


    Common Possible Causes:

    1. 1) Thermostat (or other power-related issue)
    2. 2) Gas, aka fuel, flow-related
    3. 3) Flame-related (pilot-, igniter-, burner-, heat exchanger-related)

    Start Troubleshooting by:  Inspecting the thermostat, checking the power and the gas supply.


    Problem: There is not enough heat.


    Common Possible Causes:

    1. 1) Dirty air filter
    2. 2) Obstructed or imbalanced airflow (possibly register-related)
    3. 3) Leaky ducts
    4. 4) Burner(s) dirty or in need of other servicing

    Start Troubleshooting by: Replacing the air filter, checking the registers and the ductwork.


    Problem: The gas furnace goes on and off too frequently. It turns off when it shouldn’t and turns on when it shouldn’t.


    Common Possible Causes:

    1. 1) Dirty air filter
    2. 2) Blower motor problem
    3. 3) Thermostat problem
    4. 4) Venting/flue issue

    Start Troubleshooting by: Replacing the air filter and inspecting the thermostat, then consider calling for service.

  3. 2.Rule out the Obvious: Four Simple Checks

    As with any other troubleshooting, it is always smart to quickly eliminate the obvious. Perform these checks:


    Thermostat: Is your thermostat set to the proper setting, usually labeled something like “Heat”, "On," or "Auto"? Is the thermostat then set at a temperature that will call for heat? In other words, is your thermostat set at a temperature higher than the temperature registered at the thermostat? Some thermostats are battery-powered, others are tied into the electrical system, and still others, like programmable thermostats, may be powered by both. If yours has batteries, replace the batteries.


    Electrical: Has the furnace’s breaker tripped? Because a furnace is an appliance, it requires a small amount of electricity to function. This power, like other power in the home, is tied into the home's main breaker panel. For additional safety, and for ease of servicing, forced-air furnace systems also have a "kill switch" installed. Take a quick trip to your mechanical room and check that both the furnace switch (sometimes located in a basement stairwell or on a wall near the furnace), as well as the breaker are the not source of the issue.


    Fuel: Gas systems have a shut-off located on the gas piping near the furnace. Is the gas supply to the furnace open?


    Registers: No (or not enough) warm air is circulating? Check your supply registers, often located on the floor or in the ceiling. Are they obstructed in any way? Is a piece of furniture covering them perhaps? (Yes, it matters!) Are they open? Most registers have built-in toggles that will allow you, by design, to limit airflow. Are they currently limiting it?

  4. 3.Making the Smaller Repairs

    Check and Change the Furnace Filter(s): Filters clean the air that is headed back to the furnace. Dirty or clogged filters limit airflow. In turn, the heat exchanger may overheat, perhaps causing the furnace to shut off too quickly. Newer furnaces meanwhile may cut off completely if they sense a clogged filter, while older units will continue to function, but with decreased heat output. While a dirty furnace filter may seem like a relatively small item, it in fact can be the cause of (or at least a contributing factor to) a significant number of problems relating to the furnace.


    If the blower is running, your registers are open, but you still don't have heat coming out, replace the filter. Filter locations may vary based on how your system is set up. In most cases, a furnace filter can be found on the side of the blower, at the base of the furnace. Often a dirty filter can be identified by simply inspecting it. Unless the manufacturer of your filter advises specifically, do not attempt to clean a filter. (They are relatively inexpensive items anyway and should be replaced when dirty.)


    If you have found the correct matching size, replacing the filter is as easy as sliding out the old and slipping in a new. Arrows on the frame of the filter typically indicate the correct orientation when you reinstall.


    Filters come in a variety of sizes and grades. Manufacturers generally say pleated filters are good for three months, but change them more frequently if you have pets, kids, or have a busy household generating lots of dust. Refer to your owner's manual or locate your currently installed filter to find the size you'll need.


    Check for Blocked or Leaky Ducts: Blocked or leaky ducts can affect airflow. Examine any ductwork you can get access to and look for gaps between sections. Seal gaps with foil duct tape.


    Important! Don't use standard duct tape. In fact, look for its presence as you inspect. It will likely already be creating problems.


    Check Your Flue: If you find that your furnace is constantly cycling on and off, the issue may lie in the furnace’s exhaust venting. Drawn by the warmth, critters sometimes enter the exhaust vent. While in some cases a visual inspection may do, you may have to perform some disassembly. Turn the furnace off at the furnace switch, then dismantle the duct to check for debris. Be sure to reassemble the sections in the same order and direction.


    Tip: Photos taken on your smartphone can be handy when you go to reassemble.


    Check the Pilot: If you furnace is older than 25 years, you may have a pilot that may need to be lit. Lighting instructions are often found on the inside of cover(s) found on the front of your furnace. These covers often require you to lift up and pull out.


    Note: Pilots can become an issue for any number of reasons including a faulty thermocouple, dirty gas tube, or even because of a draft.


    Check, Clean, and Replace the Thermocouple or Flame Sensor: If you can get the pilot lit but it will not stay lit, this is likely due to a dirty or faulty thermocouple (on traditional piloted systems) or a dirty or faulty flame sensor (on furnaces fitted with other ignition control modules). These are typically a simple replacement requiring only a turn of a screw or a nut or two. While there are universal options in many cases, the hardest part with this task is finding the exact right new part that will work with your furnace.

  5. 4.Making the Bigger Repairs

    Other Ignition Control Systems and Issues: Newer systems will not have a traditional pilot, instead they will be fitted with an ignition control device. Malfunctions here can cause intermittent or no heat. There are typically two types of ignition systems that you'll find on furnaces installed in the last 25 years: hot surface ignition and/or intermittent pilot.


    These ignition devices are similar in that they both employ electronics to perform their required functions. Because of this, though, they are both particularly prone to failure. Each, however, will require different steps in troubleshooting.


    Hot Surface Ignition System: Say your furnace burner will not ignite or ignites but won't stay ignited. In furnaces fitted with this module, first try a reset of the system (toggling the furnace switch off and on). Typically you can use your senses (i.e. listening or smelling) to determine if your furnace is supplying gas.


    If you have reset, and still have no ignition, turn off the power and gas to the furnace and proceed.

    1. 1) Remove and check hot surface igniter being careful not to touch the heat element.
    2. 2) If igniter appears cracked or damaged, replace the igniter.
    3. 3) If it is a matter that the burner(s) light but will not stay on, remove the flame sensor from its bracket. Flame sensors are similar to a thermocouple (described above).
    4. 4) Lightly clean the surface of the flame sensor with fine emery cloth and replace.
    5. 5) If this does not work, replace the flame sensor.
    6. 6) If this does not address your problem, it may be wise to call in a service technician, who in turn will likely start by inspecting the gas controls.

    Intermittent Pilot Ignition System: Paired with a pilot, this setup replaces manual lighting and an always-on, “standing” pilot with an electronic igniter similar to the one you might find on a gas grill. Instead of lighting the burners directly, as you’d see with that grill, instead, the igniter generates a spark, lighting that paired pilot, but only when needed.


    Say you can identify a pilot igniter, but no spark appears, you’re possibly looking at a damaged spark electrode, basically a faulty component. You could continue troubleshooting by replacing the igniter.


    If a spark is present, but the pilot still does not light, you are more than likely looking at a dirty or damaged gas tube or a faulty gas valve, possibly a more significant repair.


    If you can get the pilot to light, but the main burner still does not come on, this could indicate any number of possible problems, some of which that could be related to the electronic and gas control components found within the furnace. Check for the presence of a flame sensing rod and go from there, but it may be best at this point to call in a pro.


    Gas Pressure Regulator: Located adjacent to your furnace’s combustion chamber, you'll also find a gas control value. If you have ruled out other issues related to the ignition system, this is likely the culprit. Basically, if the gas pressure regulator malfunctions, it closes off the gas feed. With no gas, there is no fuel to fire the furnace’s burners, leading, of course, to no heat.


    Error Codes: If your furnace is newer than 25 years, it is likely fitted with a small window containing a tiny light bulb. If there is an issue with the furnace, this light can flash rhythmically, presenting a code that can help you figure out what’s going on. Refer to your furnace’s owner’s manual or call the manufacturer directly for information about any code that may be flashing. It will help guide any further troubleshooting.

  6. 5.Know When to Call for Help

    Your furnace contains many mechanical parts essential to the proper operation of your furnace. Wear and tear on any of these parts may cause issues. To some degree, wear and tear is expected, especially if you are not providing the unit with regular maintenance.


    The First Things to Fail: If there is a real failure, some of the first things to fail are parts like the flame sensor or the gas control value (listed above). While individuals with a good bit of plumbing skill may want to tackle this repair, a novice may need to call in a pro. It's important to note too that finding the right part is not always the easiest, and having a pro’s help for this alone can be beneficial.


    Remember that while you may know a thing or two, the pros know much, much more. Licensed HVAC pros are schooled (often passing state-administered tests) and are equipped to perform troubleshooting on complex systems like your furnace. Never be afraid to enlist their help, especially when safety or long term performance might become a concern.


    Rule of Thumb: It is usually best to let professionals handle problems related to mechanical wear and tear. While not only an opportunity to invite larger issues, in some cases, you may actually void the warranty on the appliance. And yes, it is always wise to remember that your appliance may still be within its warrantied service window.


    Maintenance: Your furnace would benefit greatly with regular service. When? Every year, at the end of the summer or beginning of fall, hire a furnace repair specialist to inspect and maintain the furnace. The professional will get the unit ready for the upcoming cool weather and fix problems that could cause breakdowns or expensive repairs during busy winter months.

Level of Difficulty

Repairing some minor furnace problems can be a relatively easy proposition. On the other side of the same token, some problems can be relatively complex. In cases where problems can be linked to items involving electricity or gas, it may make sense to call a pro.

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