Common Questions about Gas Furnaces

This page of the Gas Furnace Guide contains some of the more common and frequently asked questions we answer. If you have a question that’s not listed, please use the form at the bottom of this page and ask! Names or email addressed are never shared, and your question might just get added to the page!

Click on the question to jump to the answer!

(Last updated:10/2016)

Gas Furnace FAQ’s

Do you sell Gas Furnaces?

No, at the Gas Furnace Guide, we only help you with the buying and/or repair process.

What is a gas furnace?

A Gas furnace is a piece of HVAC equipment in both homes and business that converts natural gas or propane into heat for the space.

What is AFUE?

AFUE is an acronym for Annual Fuel Utilization Efficiency and is a measure of how well a gas furnace or heating system converts fuel into heat. To get a basic understanding of the gas furnace rating, you could use a money analogy.

  • An 80AFUE furnace converts .80 cents of every dollar spent on fuel (Gas) to heat.
  • A 93AFUE furnace converts .93 cents of every dollar spent on fuel (Natural Gas or Liquid Propane) into heat.

How does a gas furnace work?

This is a very complicated answer, since different furnaces work in different ways. Basically however…a gas furnace converts fuel like natural gas or liquid propane into a flame that heats up a heat exchanger. When it reaches a manufacturer setpoint, a blower built into the furnace starts up and blows air across the outside of the heat exchanger. This process heats up the air before it is distributed through the ducting system.

Can I install my own gas furnace?

Installing a furnace is a detailed and technical process that requires several specialized tools for working with sheet metal. That said… If you can swap a car transmission, or replace the engine on your lawn mower, you can probably install your own furnace.

Disclaimer: Improper installation of a gas furnace CAN KILL YOU! Working with natural gas usually requires a license in most states. In addition, most states or municipalities require permits and inspections to replace or install a gas furnace. It is our recommendation that you always use a trained and licensed professional to install your furnace!

Can I buy a gas furnace online?

In some cases you can find a furnace and buy it online at places like eBay or Amazon. Due diligence should be exhausted to the extreme! Buying a gas furnace online and installing it yourself or having a friend help, comes with great risk.

Before you do buy it online, make sure you speak directly to the furnace manufacturer about their warranty for online purchases. Many do not cover internet purchases, regardless of what the seller tells you. Learn more about the dangers of buying a furnace online.

How do you clean a gas furnace?

There are two types of cleaning for your gas furnace. Basic and surface cleaning, and then a contractor style cleaning.

Basic Cleaning –

  • Turn off the breaker to the furnace
  • Take a moist washcloth and wipe down the entire exterior surface of the furnace. This will keep the unit free of dirt and other foreign objects that could attract moisture and cause the casing to rust prematurely.
  • Open the front panel of the furnace and using a shop vac, clean out any dust or debris inside the unit.
  • Inspect the unit for corrosion, water, or any other odd things that look as if they do not belong and clean them.
  • Remove the furnace filter from the base of the unit and replace with a fresh filter.
  • Vacuum out the furnace filter housing.

Contractor Furnace Cleaning –

A contractor will handle the rest from here! Most will:

  • Clean and inspect ignitor
  • Clean and inspect gas valve
  • Clean and inspect the burners
  • Clean and inspect the duct attached to the furnace

Gas Furnace Guide recommends that everyone with a gas furnace have a seasonal maintenance contract to have the furnace serviced every year before the heating season. Contact a local contractor to set yours up today!

How big of a gas furnace do I need?

The size of a gas furnace you need is very different for each home, where you live, and several other important factors! In fact two identical size homes in the same town can use two completely different sized gas furnace!

The only true way to properly size a furnace is with a heat load calculation. In most cases, you should simply ask your hvac contractor to include the load calculation as part of their estimate.

We also added a guide on how to determine what size furnace you need.

Is a load calculation required to buy a gas furnace?

All reputable contractors will perform a load calculation on your home before recommending a furnace size!Some states DO REQUIRE a load test to be included in order to get a permit and inspection. Companies who regularly perform a load calculation are considered more professional than their counterparts and tend to know more about what they are doing as it relates to your heating needs.

That said, many others will skip over the heat load and simply replace your furnace with a similar sized unit.

Is it hard to repair a gas furnace?

Repairing a part on a furnace is actually fairly easy and a simple mechanical process.

Diagnosing the problem and troubleshooting can take years of experience and training.

How much does a gas furnace weigh?

Gas furnaces weigh between 75-250 pounds. The variance is related to the quality, size, type, efficiency and features your furnace has built into it.

Can I paint my gas furnace?

Yes. You can paint the outer surfaces of your gas furnace.

Why is my gas furnace leaking water?

In most cases, if you also have an air conditioner system, it is the evaporator coil, or cooling system that is leaking condensate water. Call a professional to inspect and diagnose why the furnace is leaking. If you have a higher efficiency condensing furnace (90or higher AFUE) water is the natural by-product of the cleaner burning systems. The exhaust or flue on these furnaces is made from PVC pipe and tends to build up a bit of moisture while running.

How often does my gas furnace need to be cleaned?

At least once each year by a professional. Many manufacturers will require this in order to keep the warranty intact.

How much does a gas furnace cost?

Gas furnaces can range from $400 for low end, low btu units, to more than $3000 for high end communicating furnaces by the high-end of the quality scale like Carrier furnaces or Trane gas furnaces.

Factors affecting the price of a gas furnace:

  • Gas furnace brand
  • Size of furnace
  • Gas furnace efficiency
  • Gas furnace features installed from factory
  • Market conditions

These factors DO NOT include the installation costs, which have far more variables! Click Here to get 3 Free Gas Furnace quotes!

Why aren’t all gas furnaces sold online?

Many furnace manufacturers or brands refuse to allow their equipment to be sold online. This is due to a combination of factors, but most common is the huge liability that would be assumed. There are simply too many technical and safety factors that must be considered before installing a furnace.

How do I pick the right gas furnace?

Call a proven HVAC Professional to walk you through all the choices in front of you, for your specific home. In most cases, you can get free estimates from almost every company!

What do the flashing lights mean on my furnace?

There are many different meanings for the flashing lights on a circuit board. If you have your gas furnace handbook, you can review the codes within the book. If you no longer have the book, check with your furnace manufacturers website support pages, to locate your specific furnace manual.

Can I buy replacement parts online?

Yes! Once you have determined the exact parts that are bad, you can find several places to buy replacement parts online.

Do Gas Furnaces still use a pilot light?

This is a tricky answer. Some do and others don’t. In most cases, if you have an older gas furnace (More than 10 years old) you may very well find a standing pilot light. (light your furnace pilot light) In newer system, solid state ignition is used, which means no pilot light.

How long does a gas furnace last?

A properly installed and maintained gas furnace should last between 15-30 years! Hows that for a wide open answer? There are several variables that come into play. The climate in your area, how much use the furnace gets, how often you have it services, etc etc. Learn more about when to replace a gas furnace.

Which Furnace parts break most often, and when?

A properly maintained gas furnace is still going to have breakdowns, there’s just no way around the fact that mechanical parts can break.  The most common breakdowns, and the timeframe for the breakdown (varies by usage) are:

  • Hot Surface Ignitors – 4-7 Years
  • Flame Sensors – 4-7 Years
  • Blower Motors – 5-8 Years
  • Draft Inducer Motors – 8-15 Years
  • Heat Exchangers – 15+ Years

Note** – If you do not have your gas furnace maintained at least once a year and change your filter regularly, remove 4-15 years from the above estimates. A dirty filter for 90 days can suffocate the system enough to cause premature and catastrophic failure!

Does my furnace need permits or inspection?

Permits and inspections may or may not be required and it varies state by state. Most states however DO REQUIRE a permit or inspection

You really need to check with your local town or city to find out if permits or inspection are required. You can ask your contractor, or even better… call your natural gas provider and ask them! Learn why gas furnaces are inspected.

Do you really need to shut off the gas, to clean or replace the flame sensor?

Realistically, No. As long as you trip the breaker so there is no way the furnace can turn on, the unit itself has fail-safe mechanisms in place to assure the gas valve will not open. (Hint, the flame sensor is one of them) I would venture to estimate that if you poll 10 service technicians, none of them turn off the gas when working on flame sensors.

Note** Turning off the gas should only take 5-10 seconds of time, since there should be a shut-off valve right next to your furnace! It’s just one extra safety step that is recommended to assure your safety when working on a furnace. In other words, even though its not required, its worth the extra 5-10 seconds for the added safety.

Where Can I Find an Old Heil Furnace Manual or Literature?

Fortunately, Heil has a GREAT resource for finding old product literature, maintenance guides, owners manuals etc. Go to: and use the menu on the left side of the page to find your product information.

What is 2-Stage Heating and Why Do I Need It?

2-Stage furnaces have a gas valve that operates at two different (some are 3-stage now as well) settings. They are higher efficiency than single stage furnaces due to the lower amount of natural gas used used during the low fire sequence. Most 2-stage furnaces will always start in low mode, and run for a set period of time to try and satisfy the need for heat. If, after that time, the temperature has not been satisfied, it will go into high heat mode and provide higher output to warm the area. Most 2-stage furnaces are intelligent enough to recognize the need for high or low heat when they start, based on the temp difference on the thermostat and actual air temp in the home, to know which setting to start in.Learn more on the Furnace Features Comparison page.

Ask a Gas Furnace Question of your Own

[contact-form 2 “FAQ”]