How does a Furnace work? Here's how...

How Does a Furnace Work?

Looking for some quick tips on how to keep your gas furnace in tip-top shape this winter?

The Ultimate Gas Furnace Troubleshooting Guide

will answer all of your questions, including how to turn on a furnace, how to change a furnace filter, how to test a furnace's airflow, what to do if a furnace doesn't ignite, and much more.

We hope you find this guide to be useful -- and we hope you stay warm this winter!


A) How to Turn On Your Gas Furnace

It sounds easy, but believe it or not, many people don’t know how to turn their furnace on or off. Here’s a simple, step-by-step breakdown:

* Find the breaker for your furnace. It's part of the electrical panel, which is usually located in your basement, utility room or garage. The breaker should be clearly labeled with a gas furnace sticker. Turn the breaker to the "On" position. 

* Locate and turn on the furnace switch. It is typically somewhere near the bottom of your basement stairs – sometimes in the ceiling, sometimes in the furnace room itself, and usually at eye level or slightly higher. 

* Set your thermostat. Check that the furnace is on and then make sure that the selector switch is set to "Heat". Finally, adjust your set point temperature, and that’s it – your furnace is turned and ready to heat your home!


B) How to Check and Change Your Furnace Filter

Proper maintenance of your furnace filter can help optimize airflow from your furnace, which will keep it running efficiently and economically during the coldest months of the year. To prevent burnout of your furnace parts, we recommend inspecting your filter monthly and changing it every three to six months. Here’s how it’s done:

* Begin by carefully opening the furnace’s external rack or panel door. 

* Remove the furnace filter by sliding it out. Take a look at the filter to see if there’s any darkness or discoloration.

* If you can’t see through the filter, it’s definitely time for a new one. Slide the new filter into the furnace, making sure that it is installed in the direction of the airflow. And that’s it – you’re done!


C) How to Test Your Furnace's Airflow

If a room in your house is too cold during the winer, it might be because your furnace isn't supplying enough warm air through the registers in that particular room. There's a simple way to test the airflow from your furnace registers to make sure they are working properly -- the garbage bag airflow test.

The test is a quick way to estimate airflow by determining how long it takes to fill a common plastic garbage bag. While it is not a precise measurement, it is better than no measurement at all and will give you a good ideas as to whether you need to call a technician to look at your ductwork.

To do the garbage bag airflow test:

* Tape the mouth of the garbage bag to a coat hanger or piece of cardboard to keep it open.
* Crush the bag flat and place it over the register or exhaust hood.
* Count the number of seconds it takes for the bag to fully inflate.

If the inflation time is:

* 2 seconds = 37 L/s (75 cfm)
* 4 seconds = 20 L/s (40 cfm)
* 10 seconds = 10 L/s (20 cfm)

If the measured airflow is less than 10 L/s, the furnace is delivering only a small amount of heat to a room and needs to be further inspected to determine why the heat is not going through.

The garbage bag airflow test is also useful if you have changed your heating or cooling systems or have made major renovations to your house.


D) If Your Gas Furnace Fails to Ignite

If you can hear that your furnace is on and the fan is running, but all you’re getting is cold air, you likely have ignition failure. Try resetting your furnace by turning the switch to off for at least ten seconds, then, turning it on again. If that doesn’t do the trick, give your HVAC contractor a call.


E) Furnace Troubleshooting Checklist

Think your gas furnace has quit? You could save yourself the cost of a no-heat service call by checking the following:

* Is the furnace switch in the on position? It may have been turned off by mistake.

* Is the thermostat properly set to the "heat" position and the temperature set to your normal heat setting?

* Is the furnace venting blocked by snow or ice? If so, try to remove the blockage.

* Are the programmable thermostat batteries fresh?

* Is the circuit breaker in the electric panel in the proper position?

* Is the furnace door properly closed?

* Has the filter been changed recently? 

If you’ve checked everything on the list and your furnace still isn’t working, give your local HVAC contractor a call, and they'll get your furnace back up and running as soon as possible.


F) How to Properly Remove Vent Blockages

If you become aware of a furnace problem that might involve the exterior vent, do a quick inspection and see if any snow, ice or some common household item might be blocking the vent. If so, before removing the obstruction, head inside first and turn the off the power to your furnace. Now, you can safely clear away whatever is blocking your exterior vent. Once you’re done, turn your furnace back on using either the breaker or the switch.

A lack of airflow through the furnace’s interior vents could mean your furnace fan motor has seized. If this has happened, don’t try to fix the problem yourself – contact the expert furnace repair technicians at your local HVAC contractor.


G) How to Stop Air Leaks and Prevent Heat Loss

Air leaks account for a significant amount of a home's heat loss in winter – resulting in increased heating costs as your furnace constantly tries to replace the warm air that has escaped from your home. Fixing these leaks will save you money on your heating bills.

To stop leaks around windows and doors:

* Remove the trims carefully. 
* Fill large cracks or gaps with foam backer rod, oakum, or expanding polyurethane foam.
* Replace the trims and caulk along the edges.

To stop leaks along baseboards:

* Caulk along the seams without removing the baseboard.
* Remove the baseboard and caulk between the wall and the floor.

To stop leaks around electrical outlets on outside walls:

* Turn off power to the outlet and remove the outlet cover. 
* Install a foam insulating pad. 
* Replace the outlet cover turn the power back on to the outlet.

To stop leaks in an unfinished basement:

* Caulk under the basement sill plate and around the joists with a rubber–based caulking or acoustical sealant. 
* Caulk any gaps where ducts enter a wall or ceiling. 
* Insulate ducts with preformed wraps or duct–taped insulation batts.

To stop leaks in your attic:

* Seal any cracks. 
* Weather strip your attic door and close it tightly.


H) If a Gas Smell is Detected

In the unlikely event that you smell something like a rotten egg smell, you could have a gas leak. Don’t turn on any electrical switches and open all of your windows.

​Then, go to a neighbor’s house and contact your HVAC contractor or your local gas provider immediately.

The number one thing you can do to keep your furnace in top working order is to clean it regularly!

If you hire someone to do the job they will replace the air filter and clean the filter system, motor, and blower.

If you’re cleaning the furnace yourself, you may use a toothbrush to clean the fan blades and any other small areas accumulating dust.

Before providing any maintenance on your heater be sure to switch off the power to the appliance by locating a switch.

Also, do not forget to shut off the gas to the furnace by locating the knob near the gas line. The gas line is a black pipe that will be connected to the furnace. Turn the knob so that it sits perpendicular to the line to shut it off.

It is now safe to open the furnace door, and vacuum in all the crevices your vacuum attachment can reach. Remember to vacuum out the base and around the base of the furnace.

Try to remove as much debris and dust as possible. After cleaning the actual unit, make sure the area around the furnace is clean and clear of clutter as well.

Never store any flammable liquids nearby or even in your furnace room.


Once all debris is removed, remove the old filter and replace it with a new one. The filter on your heater should be replaced monthly during the heating season.

Continue to check all fan belts. The fan belts help the furnace blower operate. If you ever find that your furnace is running but heat is not being pushed through the vents, check for broken  or worn fan belts.

Replace any that look frayed, cracked, or worn. Check the tautness of the belts by pushing on the center of the belt. If the belt gives in more an inch, the belt needs to be tightened by adjusting the the attachment bracket. If at any point you smell a gas leak, this is a serious problem.

Do not continue to work on the furnace but leave your home and report the gas leak to your fire department and gas company.

If the unit constantly shuts off whenever the furnace kicks on and trips the circuit, this is due to an electrical problem.

In this case, it is necessary to call an electrician or service professional to take a look at the problem. They will know how to safely fix the issue.


When all inspections and cleaning has been completed. Replace the anything that you removed such as the access panel.

Turn the gas line valve back on and return power to the unit. You may need to relight the pilot light as well. Allow your pilot light to continually run. This will keep condensation at bay even when the furnace is not being used.

Finally, use your furnace only as much as you need. In the colder months, dress a little warmer so you can set your thermostat lower. You’d be surprised how much you could save over the season by dropping your thermostat a couple of degrees!

​Also, try turning down the temperature even more when no one is home to maximize your energy savings.

While the exact specifics might differ, the basics of how a furnace operates remain the same. The specifics vary depending on the type of heat fuel you use or type of furnace, but understanding your furnace helps troubleshoot issues.

Additionally, understanding your furnace makes you a savvy homeowner and helps identify solutions to any problems.

For our discussion below, we are using the example of a gas-powered forced air heating, ventilation, air conditioning (HVAC) system. Without further ado, here’s everything you need to know about how a furnace works.

Furnace Components and How They Work To first understand how a furnace works, it helps to know a little bit about the furnace components.

All furnaces contain a heat exchanger, burner, ductwork, plenum, blower, and flue or vent pipe. Additionally, some furnaces include an air conditioning unit responsible for cooling during summer months.

The heating process begins with the burner. Once the temperature drops low enough to trigger the furnace to run, the burner engages.

This drives the gas to the heat exchanger. For reference, your thermostat controls the burner. Depending on the size of your home, your furnace contains either a single burner or dual burners (which heat larger volumes of air).

Next, the heat exchanger intakes air and turns the gas and air into heat, which ultimately moves through your home’s ductwork. Then, combustion occurs. Combustion occurs when fuel combines with air and ignites, which creates heat. Finally, the heat air transfers into the air distribution system, which includes the blower.

How the Furnace Heats Your Home

As the combustion process happens, outside air enter the system through a separate vent. The outside air mixes with heat.

Next, the air warms to the correct temperature and distributes throughout the house through the ductwork. However, this new and fresh outdoor air requires filtration. As a result, the filtration process removes dust and debris before it is stored in an enclosed space known as the plenum of the furnace.

Next, the he blower in your furnace engages and blows heated air through your ventilation system or ductwork. Ensuring a clean blower that is in good working condition helps the furnace blow hot air when you need it.

Additionally, the air distribution system maintains responsibility for adjusting the temperature of the air to what is set on the thermostat.

The flue and vent pipe of your furnace is responsible for keeping the air in your home safe to breathe.

It does this by exhausting harmful byproducts from the combustion process, such as carbon monoxide and other noxious fumes, outside your home. The flue opens and closes accordingly to ensure that there is no backup of this potentially dangerous exhaust into your home.

Finally, the ductwork of your HVAC system allows heated air to be distributed throughout your home. Therefore, ensure ducts remain clean and free of blockages or debris for optimal air circulation.

Additionally, air circulates through the furnace air filter, which helps clean and purify the air. This is important for maintaining your HVAC system and helps cut down on allergens and odors in the air. Plus, this helps cut down on the amount of dusting and vacuuming required in your home.
Once the temperature in your home reaches its set level, this process stops until the next time heat is required.
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Some furnaces include an attached humidifier, which adds moisture back into the heated air before sending it through the ductwork. This helps create a more comfortable environment.
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Hiring the right Furnace Repair Company to help you with your dream kitchen is important. Choose the right company for your project. 
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Kingdom Heating, Air & Construction are professional AND experts in kitchen remodeling Sacramento and have completed over 100 remodels in Sacramento, Ca. 

Learn more about Kingdom's Furnace Repair & Furnace Replacement on our page.
Call (916) 318-3080 to book your appointment today!
We will come check out your home and give you an estimate for your project. ​