How to Service a Furnace

Furnaces are tricky to say the least. They can be your best friend or the bane of your existence depending on how well they’re cooperating and how much you’re paying for them. Evil or not, they’re an absolute necessity if your home depends on one for heat during the cold season. The reality is that contemporary furnace models are a far cry from the cast iron fire pits lurking in the shadowy basement of every horror movie ever made. However, the idea of maintaining a furnace is still just as daunting to your average homeowner, which is why we decided to create a step-by-step breakdown of how to properly service a furnace.

What is Furnace Maintenance?

The first thing you should know about furnace maintenance (AKA furnace servicing) is that it’s not the same thing as furnace repair. Maintenance refers to the relatively cheap upkeep performed on a regular basis regardless of whether it’s needed or not. Repairs are an exponentially more expensive outcome, the avoidable consequence of a furnace that’s too far gone for a quick fix and the second to last resort before replacement. So while distinctly different from one another, maintenance and repairs are directly correlated in that more of the former will ultimately result in less of the latter, and vice versa.

Furnace Parts 

Before you do anything to your furnace, it helps to familiarize yourself with its various parts and their respective functions. Most standard gas furnaces consist of the following main parts:

Heat Exchanger – a heat exchanger separates any potentially toxic gases from the air that is blown through your furnace and into your home

Pilot Light or Hot Surface Igniter – pilot lights (older furnace models) and hot surface igniters (newer models) both serve the same purpose, which is to ignite the gas so it can burn and produce heat 

Burner – burners combine the gas and air to create the steady flame that will supply your heat

Draft Hood – draft hoods are responsible for drawing air into your chimney to regulate your furnaces air flow to a constant. 

Gas Valve – the gas valve controls the furnace’s gas flow and is responsible for shutting the gas off if the safety switch fails 

Heat Shield – a heat shield is designed to protect your furnace from overheating prematurely

Flame Sensor – a flame sensor is used to detect heat on newer models that use a hot surface ignition system

Limit Switch – a limit switch is responsible for safely limiting and shutting off the gas flow in the event that your furnace becomes too hot

Furnace Filter – like A/C air filters, furnace filters are responsible for keeping dust, debris, allergens and other contaminants out of your furnace to keep them from entering your home’s air supply

Circulating Fan – circulating fans are responsible for pushing the air through your furnace and into your home

Gas Pipe & Drip Leg – a gas pipe is responsible for feeding gas from your supplier into your furnace, while the drip leg is responsible for collecting condensation within the pipe. 

How Often Should You Service Your Furnace?

The general rule of thumb is that you should have your furnace fully serviced by a professional at least once a year to ensure its safe and proper function. All of the steps listed below and their corresponding parts can be checked as often as you care to do so, and most can be addressed as needed upon your inspection. However, some of the smaller tasks like replacing your furnace filter and cleaning around your vent covers should be completed at least once every 1-2 months.

Tools & Materials Needed 

Below is a diagram listing all the tools and equipment you will need to complete all of the steps listed in this article, with accompanying imagery for reference.

Step-by-Step Furnace Maintenance

Below is a list of steps that will walk you through the overall process of furnace maintenance and servicing. Above all else, it is important to always err on the side of caution whenever you’re dealing with gas furnaces. While we’ve already identified some steps and circumstances that are best left to professionals, it is ultimately up to you as a homeowner to determine what you can and can’t do. If you don’t feel 100% comfortable doing something on your own, this is one case where it’s better to not try at all than it is to try and fail.

1) Turn Off & Remove Chamber Door

To get started with servicing your furnace, turn your furnace off by flipping the electrical power switch. Once you’re sure that the furnace is off, lift the combustion chamber door up and off its hatches. If your furnace has a burner cover, you may have to take a few screws out to remove it. 

2) Inspect Burner Flames

Turn the furnace’s power switch back on and activate its burners by turning your thermostat up slightly so you can inspect the flames coming from the burner. A safe and clean flame will burn with an even blue tint, while a yellow tinted flame means your burners are dirty, which could be dangerous. If you see a yellow flame, don’t panic, but act quickly and don’t inhale. Switch the furnace off and call a professional. If the flame is blue, turn off the furnace anyway and proceed with the next step. 

3) Vacuum Out Burner & Blower Compartment

Make sure the power switch and your gas valves are both turned off. Using a shop vac, carefully vacuum out the burners, then remove the blower door and vacuum out the blower compartment. Gently vacuum out any and all cavities and crevices, removing whatever dust, debris and soot you can reach. 

4) Remove and Clean the Blower

With the blower door off, use your screwdriver and socket wrench (if needed) to remove the blower cage and take the blower out of your furnace so you can clean it. Use a vacuum and a small brush to very gently and very carefully clean off the blower blades. Do not disturb the blower’s wiring or counterweights, and clean as thoroughly as possible so you don’t throw off the blower’s delicate balance. 

5) Change the Furnace Filter

As mentioned, changing your furnace filter should be done on a regular basis every 1-2 months. A dirty filter means dirty and potentially dangerous air, so be careful not to fall behind on this one. Assuming you have a new furnace filter handy, simply open the filter latch, remove the old filter and carefully insert the new one. 

6) Blow Dust Off the Pilot

To clean off your pilot light or hot surface igniter and remove dust from sensitive, hard-to-reach areas, use a plastic drinking straw to carefully aim and blow the dust out. Use an abundance of caution when doing so with a hot surface ignitor as they are hyper-sensitive and break easily, which is why you shouldn’t touch them. If you don’t feel confident that you will be able to complete this step properly, it’s best to skip it all together and let a pro handle it. 

7) Clean the Flame Sensor

Make sure your flame sensor isn’t coated in any residue, as this can prevent your furnace from lighting properly. Remove the flame sensor by pulling it out of its bracket and clean it off with an emery cloth before putting it back on. 

9) Inspect the Drive Belt

If your blower is belt-driven as most are, you will need to inspect this drive belt thoroughly to ensure there are no cracks or frays. If you find either, you will need to look into replacing your drive belt, which is typically something that you should let a professional handle, as they will need to tension the belt properly. 

10) Lubricate Bearings

If you have an older model furnace, your blower may have motor bearings and blower shaft bearings (2 of each) that will need oiled every so often. To do so, clean of the bearings and surround areas, remove the bearings, add 2-3 small drops of machine oil and return the bearings. Make sure you use your machine oil conservatively and don’t over-lubricate.

11) Adjust Dampers

If your furnace uses the same duct system as your air conditioner, you will need to adjust the dampers on these ducts to account for the change in weather whenever your furnace takes over for the season. The seasonal settings on these dampers should be marked off, so adjusting them should be a synch. Just remember to switch them again whenever you go back to cooling your home instead of heating it. 

12) Check for Leaky Ducts, Corrosion and Proper Combustion Gas Flow 

Turn your furnace back on and turn up the thermostat so your burners come on, then conduct a backdraft test by holding a lit incense stick with the smoke stream beside your furnace’s draft hood, checking to make sure the smoke flows directly into the hood – if it doesn’t, you may have a problem. Listen for leaky air ducts and check with a flashlight around all metal parts for any signs of corrosion. If you notice any of these issues, you will most likely need a professional fix.

Call Us Whenever You Need Help

At Maichle’s HVAC, we’re here whenever you need us and more than happy to help however we can. Whether you’re looking for an expert to answer questions and walk you through it over the phone or a trusted professional to come in and take over, Maichle’s has you covered. Give us a call at (302) 328-HVAC or check out our heating services to learn more. Thanks so much for reading, and good luck with your furnace this winter!