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The good news is that there are fairly straightforward guidelines that you can follow to know when you have to have your filter replaced.
Under normal conditions when the fan is off you can expect to have a one to two-inch filter replaced every three months. For a four-inch filter, you should have it replaced every six months, and a five-inch filter should be replaced once a year.
If you keep the fan on your furnace on all year long, you will need to replace a one to two-inch filter every month. A four-inch filter every two months, and if you have a five-inch filter, it should be replaced every three months. If you only run the fan occasionally, you can wait two months for a one to two-inch filter, up to four months for a four-inch filter, and a five-inch filter should be replaced every six months.
If you keep your windows and doors open during warm weather or if your home gets dusty quickly, and you have your furnace fan on, you should replace a one to two-inch filter every month. Four-inch and five-inch filters should be replaced every two and three months respectively. If you only occasionally keep your windows and doors open, then you can use the replacement times listed above for keeping the fan on occasionally.
Having smokers in the house can also clog your furnace filter faster than a non-smoking home. If you have one smoker in the house, your filter should be changed every two months for a one to two-inch filter, every four months for a four-inch filter, and every six months for a five-inch filter. If there are multiple smokers in your home a one to two-inch filter should be replaced monthly, a four-inch filter should be replaced every two months, and a five-inch filter needs to be replaced every three months.
Just as in a house with multiple smokers, if you have animals in your house, especially if they are heavy shedders, you should replace a one to two-inch filter every month, a four-inch filter every two months, and a five-inch filter every three months.
As you can see, it is not too complicated to figure out how often you need to have your furnace filter replaced. These are just general guidelines, however. The conditions in your home may vary slightly from these scenarios, but if you have a professional come in to change your filter, they can give you a better idea of when it is best to have your filter changed.