Most homeowners take for granted that their furnace will keep them comfortable during the winter. Unless there is a problem, people don’t usually stop to think about how their heating system works. Still, smart property owners take time to learn the fundamentals of the furnace that works to make a warm and welcoming home during the coldest months of the year. With that in mind, here’s a closer look at some of the essential components of the average furnace.

Anatomy of a furnace

    • Air filter: Helps remove dust, pet dander, pollen and other pollutants from indoor air.
    • Cold air return: Brings circulated air from your home back into the furnace, where it gets reheated and vented back into the house.
    • Compartment blower: Circulates air throughout your home.
    • Electric blower motor: Powers the blower/fan.
    • Flue: A vent that discharges potentially dangerous byproducts of your furnace, such as carbon monoxide, outside your home.
    • Pilot light: A small gas flame that lights a burner that heats the circulated air through the furnace.
    • Gas burner: The heat source of the furnace.
    • Gas valve: Allows the burner flame to be on both high and low settings and shuts off the furnace’s gas flow if the pilot light goes out.
    • Heat exchanger: Reheats filtered air.
    • Heat recovery ventilator (optional): Transfers humidity between indoor and outdoor air streams to maintain balance; recovers about 85% of heat loss.

Tools and Materials Required

Furnace repair requires the use of specific tools — some of which you may already have in your toolbox, and others that are more specialized.

    • Screwdrivers: You’ll need at least one flathead and one Phillips-head screwdriver. However, a full range of screwdriver sizes comes in handy.
    • Pliers: A full set of pliers will allow you to tackle most jobs, including cutting through wires and bolts.
    • Wrenches: HVAC repair often requires different types of wrenches, including crescent and pipe wrenches.
    • Flashlight: A flashlight will allow you to see into the nooks and crannies of your furnace.
    • Drill: Drills are must-have tools for a range of HVAC repair scenarios.
    • Metal cutters: You might need to cut through metal if you are repairing duct work.
    • Multimeter: Safety is essential when working with electricity. Keep a multimeter in your toolkit to test for the presence of electricity and the voltage.
    • Extension cord: You’ll likely find it helpful to have a sturdy extension cord when you need access to electricity for your tools.
    • Personal protective equipment: PPE such as gloves, goggles, masks, earplugs and steel-toed work boots can keep you from experiencing injuries if accidents occur.

Common Problems

    • The furnace isn’t producing any heat: If the furnace isn’t blowing warm air, there are a couple of quick things you can check. Are all the registers open? Is your thermostat set to heat, not cool? Is the pilot light still lit? Is the circuit breaker on? Have you changed your furnace filters recently?
    • The furnace isn’t producing enough heat: Your furnace may be working too hard to keep up with your demands because there is a buildup of dirt and debris. Remove the access panels to your furnace and inspect the components. Use a vacuum with an extension hose to clean up the area.
    • The furnace isn’t blowing air: If your furnace isn’t blowing at all, check your thermostat to make sure it’s on and set to the correct temperature. If that doesn’t solve the problem, ensure your circuit breaker is on and nothing is blocking your furnace air filter.
    • The blower is continuously running: When you have your thermostat set to auto, your furnace fan should blow only when your furnace is in the middle of a heating cycle. But if your fan runs constantly, regardless of whether your furnace is heating, that’s not normal. The first thing to check is your thermostat settings, then your fan limit switch. If you have any problems with your wiring, you may have a problem for a professional to assess.
    • The furnace is noisy: Banging, whistling, rattling and vibration may indicate gaps in the duct work, improperly sized ducts or a clogged air filter blocking the airflow.

Areas You Should Inspect First

Furnace door safety switch

If your furnace has stopped working correctly, one of the first things you can check is the safety switch on the furnace door. This device prevents the fan and burner from coming on while the door is open or missing. The door must be in place to activate the switch and allow the furnace to operate. If you discover you need to install a new safety switch, shut off the power, prepare the door, wire the new switch, then double-check the installation.

Burner flames

Contamination on the burners can cause them to stop working. Dust can accumulate over time, which is why you should inspect your furnace’s burners before fall and winter arrive. When the burners are clean, the flames will be blue and even. If they’re yellow, you likely have dirty burners. You can clean them yourself by shutting off the power and the gas, then using the hose attachment on your vacuum cleaner to clean the burners and the area around the blower. Do this check and cleaning annually.

Limit switch

A bad limit switch could cause the blower to run continuously, significantly shortening the blower’s lifespan as a result. If the high limit switch fails, the furnace should not fire. When the high limit switch trips, it will shut off the heating element and the blower will continue to run until either you reset the system, or the limit switch closes. Replacing the switch will solve the problem.

Electric ignition/pilot control

Gas furnaces can have a pilot light or electronic control, which eliminates the need for a constant flame. A faulty ignition system can result in your furnace being unable to properly heat your home. Your pilot light might go out if you have drafts or clogs in your heating system.

Action items you can handle yourself include the following:

Step 1: Turn the furnace power switch off and back on (you may need to reset the ignition).

Step 2: Turn off power and gas to the furnace, then visually inspect the heating element without touching it. If it looks cracked or damaged, you’ll need to replace the igniter.

Step 3: Consult your furnace’s owner’s manual for the manufacturer-recommended troubleshooting steps. You can harm the system, your property and yourself if you try to cut corners or disregard safety protocol during this task. When in doubt, call for professional service.


Furniture and home decor may block air vents, impeding airflow throughout your home. Make sure all registers are open and have unobstructed access. While closing vents in rarely used rooms may seem like a sensible idea, over time, you’ll do your system more harm than good. Closed air vents increase the amount of conditioned air that gets wasted through leaky duct work, leading to higher energy bills and potentially damaging your HVAC system. Make sure all vents in your home are open year ‘round.

Flame sensor

The flame sensor is an essential safety feature on your furnace. A buildup of rust or soot on your flame sensor may cause your furnace to function improperly. Typically, the flame sensor is a rod-shaped component located near the back of the furnace, right in the path of the burner. You can clean it by turning off power to the furnace at the breaker box; removing the sensor from the furnace; scrubbing it gently with steel wool to remove debris buildup; reattaching the sensor; and powering the furnace back on.

How to Repair Your Gas Furnace

The furnace isn’t producing any heat

If your furnace is on, but isn’t heating your home, the issue may not lie with the unit itself. Instead, you may have a problem with your thermostat. If you have a battery-powered thermostat, check to determine whether it’s time to replace those. Make sure the thermostat is on the heat setting, and that you have it set at least five degrees above room temperature. If you have a programmable thermostat, ensure it has the correct date and time.

Once you’ve ruled out thermostat-related issues as the cause of your furnace woes, check to ensure you haven’t blown a fuse or tripped the circuit breaker on your furnace. On the breaker panel in your home, identify the circuit breaker for your furnace, then manually flip it on and off. Make sure it stays in the on position. If this doesn’t solve the problem, there could be something wrong with the electrical wiring. You can check this yourself if you own a multimeter, or you can call a local service provider to safely troubleshoot electrical issues for you.

The furnace isn’t producing enough heat

What should you do if your furnace is running, but is still failing to adequately heat your house? The first suspect should be a clogged air filter. Find your filter, remove it and hold it up to the light. If nothing shines through, it’s time for a replacement. Most air filters need changing every three months, but you may wish to replace yours more often if you have pets, if anyone in your household smokes indoors or if you have family members with respiratory issues. Put a note on your calendar or set a reminder on your phone so you don’t forget to change your filter.

When your air filter gets clogged with dust and debris, it forces your air handler to work harder to compensate for the blockage of airflow. In addition to increasing your utility bills, the reduced airflow through your heating and cooling system can cause your heat exchanger to overheat and shut off before it has a chance to produce enough heat for your comfort needs. Frequent filter changes will resolve this problem quickly and inexpensively.

The furnace isn’t blowing air

Clogged filters, tripped circuit breakers and improperly set thermostats can also cause the furnace to stop blowing air. If you have checked for all these problems and can rule them out, you might have a problem with your furnace fan. With the furnace off, turn the thermostat to fan-only mode. If the fan turns on, it is working properly. If the fan does not turn on, check the circuit breaker to see if it’s tripped. If so, you likely have a problem with your unit’s electrical wiring, and you should call for professional service if you aren’t familiar with the ins and outs of working with electricity. If the circuit breaker didn’t trip, it’s a sign that the thermostat or the blower motor is failing, and an HVAC technician needs to come inspect the unit.

The blower is continuously running

When you have your thermostat set to auto, your furnace fan should blow only when your furnace is in the middle of a heating cycle. But if your fan runs constantly, regardless of whether your furnace is heating, that’s not normal. First, check your thermostat settings and make sure it isn’t calling for continuous heat or forcing the fan to blow nonstop. 

Set your thermostat to a temperature that’s a few degrees lower than the current room temperature. Doing so will shut the furnace down and prevent your thermostat from calling for more heat. If the furnace was in the middle of a heating cycle, wait three to four minutes for the blower fan to stop running.

Did your furnace fan stop blowing? If so, it’s likely that you’ve set your target temperature too high, causing your furnace to heat continuously as it tries to reach that temperature. Another issue that could cause your furnace to heat constantly is a house with poor insulation and weatherization, allowing cold air to enter the home as hot air escapes. If your furnace continues to run without meeting your desired temperature, you may also have a problem with the fan limit switch or your electrical wiring. In this case, you may wish to have a professional technician diagnose the problem and recommend specific repairs.

Rapid cycling problems

Rapid cycling, also known as short cycling, may indicate an incorrectly sized furnace for your home. A furnace that’s too large will use a lot more energy to heat the space, leading to uneven temperatures that force the unit to power on more frequently to compensate. 

Another common cause of a short-cycling furnace is that the furnace is overheating and shutting itself down to prevent damage. Overheating is often a result of restricted airflow caused by things such as cracked heat exchangers, clogged filters and blocked air vents. 

Finally, another common cause of a short-cycling furnace isn’t the furnace itself, but a malfunctioning thermostat. A faulty thermostat can tell your furnace that it has already met the set temperature, causing the furnace to prematurely turn off.

A short-cycling furnace could cause your home heating system to break down prematurely, which can be a headache during the fall and winter. If your furnace is cycling rapidly and you can’t seem to pinpoint the reason, contact the heating contractors at a professional HVAC repair service today.

The furnace is noisy

Does your furnace make loud banging, whistling and rattling noises while it’s running? There are several potential causes of these sounds, including wear and tear inside the unit.

Banging: If you hear banging noises when your unit cycles on, it could be because temperature and pressure changes cause your duct work to flex. Listen carefully to determine if you can pinpoint the source of the noise. There may be a weak spot in your ducts that you can shore up by installing a thicker piece of metal like a patch. 

Whistling: Whistling noises indicate a variety of airflow problems. One reason is a clogged filter that is having trouble sucking air. Other possible causes are holes or gaps in the duct work, or ducts that are too small.  

Rattling: If there’s rattling or vibrations in your duct work, try tightening loose ducts with screws or duct tape. If the furnace itself is rattling, you may be able to lever the unit up just enough to slide a thin rubber or cork pad underneath it.

You can smell gas

The smell of sulfur or rotten eggs coming through your air vents is a sure sign that your furnace is leaking natural gas. You may also notice a hissing sound. Follow the sound and smell to pinpoint the source.

NOTE: If you suspect a gas leak, immediately shut off power to your furnace and open all the windows in your home, then call a professional HVAC repair company, your utility provider or your local fire department. Do not try to troubleshoot the issue on your own. Saving a few dollars on DIY diagnostics and furnace repair is not worth the potential danger to your home and family involved in a gas leak. Carbon monoxide poisoning from natural gas is a life-threatening medical emergency, especially for young children and senior citizens.

Maintaining Your Furnace

Clean/replace the filters

Filters play a crucial role in your furnace’s function. Clogged or dirty air filters will impede airflow and will also shorten the lifespan of your furnace by forcing it to work harder to keep up with your demands. The indoor air quality in your home may also suffer if you fail to replace or clean your filters, which should be a particular concern to you if anyone in your household lives with respiratory problems such as allergies or asthma. Most air filters need cleaning or replacement every 60 to 90 days. Consult with your owner’s manual to ensure you know what kind of filters you have, their location and how often you need to clean or replace them.

Make sure the surrounding area is clear

Most furnaces reside in the garage or basement — areas of our home where we tend to accumulate extra clutter. It’s crucial to keep three to five feet of clearance around your furnace. Doing so helps the furnace run better because it allows free circulation of air around and through the unit. It’s also a safety issue, since surrounding your furnace with various items can become a fire hazard.

Oil the furnace blower annually

Your furnace has a large barrel-shaped component called a blower motor that forces the warmed air from the furnace to all the rooms of your home. The motor rapidly turns a fan to push the air out of the furnace. To prevent this fan from seizing up, you should carefully remove it and lubricate it to help prolong the lifespan of your furnace. 

With all power to your furnace shut off, locate the furnace blower and detach the bolts from its housing with a wrench. Gently remove the blower, then locate the oil ports on its motor and shaft. Consult your furnace’s owner’s manual for the specific oil port locations and how much oil to add to each port. Reattach the blower to the furnace, then power the unit back on. The newly oiled blower motor should run smoothly.

Clean all debris from heat pumps, as well as intake and exhaust vents

Even if your furnace seems to be working normally, it’s essential to keep its various components clean. Make sure there is a minimum clearance of five feet around your intake and exhaust pipes. If you have any shrubs or other vegetation growing nearby, trim them back and clear the area of all clutter. While you may feel tempted to hide or cover your intake and exhaust vents, it can be dangerous to do so. With a flashlight, look inside the pipes to make sure there is nothing blocking airflow, such as small animal nests, ice, dead leaves or any dirt or debris.

Not Sure How to Fix the Problem? Call in the Pros!

At Above & Beyond HVAC Services, we have been providing HVAC repair, installation and maintenance to our neighbors in the Delmarva area for more than 20 years. We provide both residential and commercial service, and provide 24-hour emergency response for your peace of mind. Whether you need a standard HVAC maintenance plan, a free estimate on a new HVAC installation or a free second opinion on a repair estimate from a different company, we invite you to give us a call. We pride ourselves on our ability to provide fast, friendly and honest service, no matter how large or small your needs may be.