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Fall Furnace Maintenance Guide

Keep your furnace running efficiently and safely and prevent the hassle of breakdowns with a few simple maintenance procedures. We'll walk you through a series of simple steps that will keep it in tiptop shape. The entire maintenance operation takes less than three hours and costs only a few dollars

By the DIY experts of The Family Handyman Magazine

Furnace maintenance basics

When it comes to furnaces, an ounce of prevention truly is worth a pound of cure. To help you avert the hassle of your furnace’s dying or simply not putting out enough heat—just when you need it most—we’ll walk you through a series of simple steps that will keep it in tiptop shape. The entire maintenance operation takes less than three hours and costs only a few dollars—pretty cheap insurance.

In this article, we’ll focus on natural gas and propane-fueled furnaces. The maintenance tasks involving the blower chamber also apply to oil furnaces; however, oil furnace combustion chambers are very different and should only be worked on by professionals. Heat pumps, on the other hand, work more like a central air conditioner than like a furnace, so we won’t deal with them here.

Routine furnace maintenance and cleaning don’t require special skills. If you’re handy with a few basic hand tools, you can do it. We won’t be doing tricky or potentially dangerous stuff like adjusting the gas burners. Leave that for a pro. See “Symptoms That Call for a Heating Professional,” for more details.

We should warn you that your furnace may look somewhat different than the one we show here. If you don’t feel confident about taking some of the steps we show, skip them. And pay close attention to the safety precautions in this article, in your furnace service manual (if you can find it!) and posted on your furnace. Even if you follow our maintenance steps, call in a heating professional for a thorough furnace checkup at least every three years (Look under “Heating” in your Yellow Pages.)

Follow Photos 1-9 for a complete guide to furnace maintenance basics.

Tip: If you’re faithful about changing your filter, you won’t have to clean the blower (Photo 5).

Forced-air furnace

Forced-air furnace

Figure A: Gas Furnace Details

A forced-air furnace has four main sections: (1) the blower chamber; (2) the combustion chamber; (3) the return duct; and (4) the supply duct. When your thermostat calls for heat, the burners will kick on and begin to heat up the heat exchanger. The heat exchanger contains all the dangerous gases produced by combustion and vents them through the exhaust stack. When the heat exchanger gets hot enough, the blower starts. The blower pulls cooled air through the return duct, passes it over the warm heat exchanger and returns the warmed air to the rooms. Furnaces vary quite a bit in design, so yours may be somewhat different from this illustration. If confused, consult your service manual or a heating professional.

Note: You can download and print Figure A from the Additional Information section below.


While working on your furnace, do not remove burners, stick anything into the pilot orifice or make adjustments. Misaligned burners can pose a serious hazard by allowing gas to build up before the burner ignites, causing a flash fire. Poking a sharp object into a pilot can widen the orifice, turning the pilot into a flamethrower.

Other maintenance chores: belt drives and ducts

Some homes have older furnaces with belt-driven fans, which may need maintenance (Photos 10 and 11). Sometimes dampers in ducts need seasonal adjustment (Photo 12) or air sealing (Photo 13). And make sure your furnace doesn’t cause your gas water heater to backdraft (Photo 14).

Tip: If your furnace has a standing pilot (a pilot that burns all the time), turning off the gas to the furnace when the heating season is over will save you as much as 5 percent per year on your gas bill. To relight the pilot, consult the instructions on your furnace’s gas valve.

Carbon monoxide alarm

Carbon monoxide alarm

Carbon Monoxide Alarm

Install a carbon monoxide alarm on each floor. If you already have these alarms, test them. Carbon monoxide is an odorless, colorless gas sometimes produced by oil-, gas- and wood-burning appliances (furnaces, stoves, fireplaces, etc.). If this gas spills into your home in high enough concentrations, it can be fatal. Plug carbon monoxide alarms into electrical outlets or directly wire them to the electrical system. Do not install them in utility rooms, garages, kitchens or bathrooms.

Symptoms That Call for a Heating Professional

Symptom 1: Short cycling

When your furnace runs for only short periods (less than three minutes) before shutting off, the problem is called short cycling. This happens when the thermostat is out of adjustment or when the heat exchanger overheats and the burner automatically shuts off to prevent damage.

Symptom 2: Irregular flame

Properly functioning burners have fairly even rows of flames. If the flames are uneven or lean toward the back of the furnace, call in a pro. It could be a sign of dirty burners or a cracked heat exchanger.

Symptom 3: Odd noises or rumbling

While rumbling and popping aren’t cause for concern in a hot water or steam heating system, they shouldn’t be present if you have forced-air heat.

Symptom 4: Chronic illness

Frequent headaches or flu-like symptoms can be a sign of combustion gases leaking from a cracked heat exchanger or carbon monoxide leaking from an exhaust stack. With these symptoms, have your heating system checked out even if your carbon monoxide alarm remains silent.

Symptom 5: Soot deposits

Soot is a fine black powder that collects when combustion is incomplete. Its presence may indicate that your burners need adjusting or that you have a cracked heat exchanger that needs replacing.

Additional Information

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Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

    • Socket/ratchet set
    • 4-in-1 screwdriver
    • Shop vacuum

Small stiff brush, fine-grit emery cloth.

Required Materials for this Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.

    • Furnace filter
    • 1/2-in. drain line, 2 ft.
    • Drinking straw
    • Lightweight machine oil
    • Metal tape

Comments from DIY Community Members

Share what's on your mind and see what other DIYers are thinking about.

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September 08, 1:58 PM [GMT -5]

The homeowner will be well advised to heed ACFXR,s advice. The project depicted is not valid for more than 95% of gas furnace owners.

October 04, 3:43 PM [GMT -5]

When removing your blower, either mark the connections or know how to read a schematic. Once this decision is made, remove the blower by shutting off the power to the unit, removing the panels, and start by disconnecting the wires. Most upflow models, you will only have to deal with the power or 110-120v connections. Some will have a connector, where others will need to be disconected from the circuit board. The upflow models will usually have some low voltage (24v) connections to disconnect. Again, label the connections if you cannot read a schematic. The low voltage parts you will find in the blower section and on the blower itself will be limit switches. Most limit switches are the simple bi-metal variety, and will come in two forms, automatic and push-button resettable. Sometimes, the switch will be mounted at the top of the cabinet and I have seen the reset button sticking out the top of the unit, which can cause confusion when hunting them down to find the "open" in the low voltage circuit. Anyways, once you have removed the wires from the blower connections on the upflow model, you can usually remove two screws on the front of the blower rails and slide it out. There are many models, but most are simular, as there is not many ways to hang the blower and allow the maintenance that will be required.

On the downflow models, the blower will be at the top of the furnace, and the electrical panel will almost always be mounted in front of it as well as the vent pipe on the 80% models, and even the 90%+ condensing units, using PVC pipe. The control panel will be mounted with a few screws to facilitate it's temporary relocation, while you remove the blower to clean or replace the motor, etc.... Using care not to damage or cut any wires, move the control box and vent pipe out of the way and use duct tape or other means to secure the furnance's control box or electrical box(whatever you decide to call it) out of the way, and again you will usually find two screws that need to be removed to be able to slide the blower out on it's rails.

Once you have the blower out, you need to decide how to clean it. If it is really bad and has not been done in years, you might even consider taking it to the local carwash and spraying it clean. I would just use clean hot water, as the pressure will get the job done, or if it is not that dirty, you can simply use your own hose and water pressure to get the job done. Here is the part that can start an argument. What to do with the motor when washing with high pressure water. In my opinion, do nothing and just wash it, motor and all. Yes, it will get full of water, and yes it will dry. This should not hurt the motor, and if it does, then the motor was due for replacement. I always get this when doing this to the condenser fan motor, which is an outside, in the elements, motor. It should be able to hold up to some water, and so should your blower motor. If you are in a hurry to get the unit back in service, use compressed air to facilitate drying time.

Now re-install the blower unit, put in the two mounting screws tight, but not too tight as you don't want to strip them out. Put the control box and vent pipe back in on the downflow or in some circles, they will call it a "counterflow" furnace, and on the upflow, you will not have to deal with that issue. Then replace the connections as you have removed them. If you wrote the connections on the blower and then power washed them off, you will need to read the schematic, and if you don't know how, you will get a chance to learn here, as you no longer have your connections wrote down. Relax, it is not hard to do and it will make you more comfortable reading the schematic, when you do something else on your furnace. The main thing on the new "direct drive" furnace blower moters, is finding the common, usually white wire and hooking it up to the common connection on the furnace. With that out of the way, you will just have the different speed connections to deal with. You cannot go wrong here, but you may not get the correct rpm you want for the season it will be running in. You will want the fastest speed hooked up to the cooling connection. This is usually the black wire, and you will want the slower speed for the heating season, and this could be just about any color. Pick one if you cannot read the color code on the motor that is now mounted in your furnace and hook it up to the low speed connection for the heating season. Make all you other connections, and turn on the furnace. If the blower speed is too fast, pick another color wire and hook it up, till you get the slow speed and you will be done. A lot of the time you can closely inspect the connectors and see the two that were used and limit your selections. Make sure you do not use the blower switch on the thermostat to test run the blower, as it usually uses the high speed connection, and you will not be changing the right speed connection. Even if you get it wrong, it will function fine, it just may be too fast for comfort, and the noise level will be too high, as you don't need the high air flow that is needed for cooling. You can always change the speeds until you get it to the speed you are comfortable with, as there is no right or wrong here on speed selection.

Always keep safety in mind, as you are dealing with household current, and this can kill you in the right conditions.When working on projects, always make sure you know the power is off. Buy or make a tester, it is easy to do. If this scares you, then maybe a contractor is in your future. If you are comfortable working with household current, then there is many things you can do around the home and save a lot of money that you would use to hire contractors. With some common sense, and sites like this one, you should be able to work on, service, and repair most things on a normally installed natural gas or LP furnace. There is not a nuclear submarine, so anything you do can always be fixed or replaced, so just get'r done.

One last thing I would like to stress..... Fire up your AC or furnace well before you need it. Especially your AC. If you are like most people, and wait until you cannot stand the heat any longer before you turn on the AC to save money, you will find out what contractors charge when they are very busy and you have to wait maybe days before they can send out a tech. There is always several hot days before summer really kicks in, so use one of these days to test your system to make sure everything is up to snuff. Same thing with your heating system, fire it up and make sure everything works, so you are not in panic mode when the day comes that the heat index is up to broil and you cannot find a contractor that has room on their busy schedule to come out and service your unit. People think that the AC unit costs so much to run that I have had people complain that I am running it too long during a service call. Think about it....... It only costs about 3 bucks a day for the average system, so what do you think it will cost to run it for an hour? What do you think a contractor will charge for an hour??? Food for thought..........

Motor Mouth

October 04, 12:36 AM [GMT -5]

I think the project is outdated as the experienced tech noted, but I also think the comment went too far in another direction, as there are things that a DIY can do to the modern furnace without causing "thousands" of dollars in damages. Most furnaces don't cost thousands of dollars, and the furnaces are not as complicated as was indicated. There is no reason why a DIY'r couldn't remove a dirty blower and clean it. They could also clean most areas in the furnace that look dirty, after an inspection of the burner area. It should be obvious that one should be careful around the ignition element, and the control board, but they are not overly complex and a competent person should be able to complete most of the service the unit will need. which mainly requires keeping it clean. There are many repairs and checks that can be done by someone who feels up to it. Replacement of the igniter is a fairly easy and straight forward task, and is one that seems to need replacement every few years as they will crack and quit working. Cleaning the flame sensor probe is not rocket science either, and it will cause all kinds of issues that may confuse even some techs. A simply cleaning of this metal rod, will fix many issues of the ignition sequence, and can be done my most anyone if they are careful of the igniter. So we can see two extremes, and I think there is a lot of middle ground, if the DIY'r feels up to the task.

I think there are many checks a DIYr can make on a furnace that is not operational, as most of them will be provided by a series of LED signals the circuit board will keep flashing. Just read the door panel to find out what the LED flashes mean, and if it is something the DIYr can handle, then by all means, they should do it. It is hard to cause "thousands" of dollars in damages to a furnace if you use caution and even the worst you could do is damage the circuit board, which should not cause even one thousand dollars in damage, unless you have an unscrupulous contractor that will overcharge for parts and labor. With the prices contractors charge today, people that feel they can do the work themselves, should do the research on webpages like this, ask questions, and then proceed. Repair parts can be bought on line, and a lot of help is available. If you can install components in your computer, you surely can work on a furnace, as there is not anything that complicated in a basic gas/LP furnace with an AC package included. One of the worst things about todays furnaces, is the fairly simple, but expensive control board. If you do have one go out, I would look for a used one on ebay, as some of them boards are getting very over-priced

Now I wouldn't think it to be a bad idea to have a contractor do a fall checkup and cleaning evry 5 years to make sure that you didn't miss any areas. It is always good to have a pro back you up, and most HVAC shops will have a tuneup special to clean your furnace, and then try to sell you some parts like when you have your oil changed at the spiffylube and they want to sell you a new air filter/PCV valve/extra service, etc. On a furnace, it is hard to do this, so the gig is usually to try and sign you up for spring and fall cleanings every season, so they can keep you as a customer, and expect you to call them whenever you need a repair. To be fair, this is a good thing for the people that do not feel comfortable working on the furnace, or don't have the time. They should get regular service on the equipment to make sure it is functioning properly and efficiently.

Common sense goes a long ways here, and that should be obvious.....

October 03, 5:39 PM [GMT -5]

As a licensed contractor and inspector with 30+ years of experience, I would caution any homeowner about getting into this as deep as is shown on this "step by step" The furnace that is shown is outdated, as inshot burners are the norm on modern furnaces, belt drives are obsolete as well as standing pilots, there are many items that are critical that must be properly set, aligned, grounded and tested utilizing instruments that are not normally available to the average homeowner.On a modern furnace one wire that "comes loose" during a process and is improperly landed back on the circuit board can lead to dangerous conditions and costly repairs. Unless you have significant knowledge of electrical circuitry, fuel burning appliances and combustion processes as well as proper sequence of operation for all components of a furnace, I would suggest that you spend the money for a professionally trained technician to do your maintenance. I have been on too many service calls where the homeowner tried to save a few dollars and ended up spending thousands. The DIY for a furnace should be limited to filter replacement.

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Fall Furnace Maintenance Guide

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