How to clean a furnace: Should you do it yourself or hire a professional?

Keeping your furnace clean is important in order to have it run properly in winter (or year-round if you have whole-house air conditioning). A clean furnace will burn less fuel -- thus saving you money -- and extend the life of the unit.

Having said that, doing a full cleaning of your furnace is not a simple task, and requires time and effort. If you are not comfortable with taking apart a large piece of machinery, you would be better off hiring a professional to clean your furnace once a year. In fact, some furnace warranties are voided if you do not have a professional cleaning done once a year.

There is one task of furnace maintenance that anyone can -- and should -- do, however, and that's replacing the filter. Your furnace filter traps dust, dirt, allergens and more so that it doesn't impede the ability of the furnace's systems to function. A good filter also keeps pollutants out of the air so you are not breathing them. The following tips will show you how to clean or replace it, simply and easily, and we'll also discuss what is involved in the more thorough cleaning that a professional might do.

How often do I need to clean my furnace?

You should plan on an annual whole-furnace cleaning, preferably in fall before the weather turns cold. That way, your furnace will be operating at optimal levels when it's needed most.

Your furnace's filter, however, should be replaced or cleaned every few months -- approximately four times a year. Most newer furnaces have disposable filters. Some, however, use washable filters that last longer but need regular cleaning.

What kind of filter do I need?

Your furnace manual will tell you what kind of filter you need. If it's a disposable filter, the manual should indicate what size, and recommend either a pleated or non-pleated variety. Disposable filters are readily available at hardware and big box stores, and often come in a package of three or four. 

When purchasing a filter for your furnace, you will see a measurement on the package called a Minimum Efficiency Reporting Value, or MERV, value. MERV numbers range from 1 to 16, with higher numbers being better at removing pollutants from the air. A MERV rating of 6-12 is considered good for most homes. If you are a smoker or have multiple pets or bad allergies, purchase a filter with a MERV that's at the higher end. 

Washable filters are more expensive, but a good one could outlast your furnace. Washable filters are more environmentally friendly, since you won't be throwing out a used filter every few months. On the other hand, you do need to take the time regularly to clean them, and you may have to purchase them online since local stores often do not carry them.

How do I change my disposable filter?

Here are the simple steps you'll need to take to change your disposable filter:

1. Turn off the electricity going to your furnace.

2. Look on the front of your furnace and locate the service panel that houses the filter. It will be a long, narrow door that either clasps shut or has several screws keeping it shut. 

3. Open this door and slide out the current filter. 

4. There may be an arrow on the filter that shows the airflow direction. It's a good idea to draw an arrow on the outside of the furnace on that side with a permanent marker, so you know the correct way to position the new filter.

5. If you didn't note down the size of the filter before this, it should be written on the filter itself. Write this number on the door of the furnace, too, so you'll always have it handy when you need to buy new ones. The size will consist of three numbers, written like this: 12x12 x1, which indicate the height, width and depth of the filter in inches.

6. To install your new filter, look at the marking that indicates airflow and slide the new filter into place with that mark aligned with the airflow arrow you wrote on the furnace. 

7. Replace the cover and turn the power back on to your furnace.

8. Note the date you replaced your filter, so you can keep track of when it needs to be replaced. 

That's all it takes! Once you've done it a few times, you should be able to change the filter in less than 15 minutes.

How do I clean my washable filter?

Unlike a disposable filter, which has a cardboard frame, a washable filter can be submerged into water if necessary. Clean your washable filter every few months, a process that should take no longer than 20-30 minutes. Here are the steps you should take:

1. Turn off the electricity going to your furnace.

2. Locate the tall narrow service panel that houses the filter. Undo the clasp or, if it has them, unscrew the screws that are holding it shut.

3. Open the door and slide out the filter.

4. Using a brush attachment, run a vacuum over the filter. When you're done, you should be able to hold up the filter and see light coming through it.

5. Spray the filter with a mix of mild detergent and water, until the filter is saturated.

6. Using a hose or in your sink, wash off the filter, being sure to get all traces of soap and dust.

7. Set the filter in a sunny spot until it is dry. 

8. Reinstall the filter and close and fasten the filter door.

Other things to know

Washing or replacing your filter regularly will keep your furnace running smoothly and will ensure the air you breathe is clean and fresh. Doing a full-furnace clean is a more complex process that will take three to four hours. For most homeowners, it's a job that's better left for an HVAC professional.

A qualified HVAC professional will clean the combustion chamber, replace the oil filter for oil-powered furnaces, adjust the burners and test them to be sure they are working properly, and dis-assemble the blower assembly to clean it thoroughly. They will also inspect your furnace while cleaning it, and alert you to any problems that they find.

Even if your furnace is relatively new, it still needs proper maintenance, with annual cleanings and regular filter checks. This is true whether you get your heat from oil, gas, propane or even solar power. 

For more information on purchasing the best furnace, check out our furnace buying guide. If you're concerned about dirty air ducts, we can show you how to keep them clean and when to hire a professional to do so. If you live in a mild climate and are curious about furnace alternatives, we give you the low-down in our buying guide for heat pumps.