When to Clean or Replace Furnace Filters

January 30 2015
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replace furnace filters

When to Clean or Replace Furnace Filters

Air filters are efficient products, but they do have a limited lifespan. Even if you choose a high-quality filter (as you should) it won’t last forever, and if you fail to change it in a timely fashion your indoor air quality—and the performance of your HVAC system—will suffer as a result.

The same holds true if you have a permanent filter, which must be periodically removed and cleaned. It may last indefinitely, but if you neglect proper maintenance procedures it won’t perform up to an acceptable standard.

But how, exactly, will you know when the time has come to clean or replace your current air filter?

The answer depends, to some extent, on what type of air filter you have. If, for example, you go the cheap route and purchase a fiberglass throwaway (as many people do), you should change them every 6-8 weeks without fail. These filters won’t capture much more than 10-20 percent of the particulate matter that passes through them, but even at this low level of efficiency they aren’t designed to last long.

Pleated air filters are higher in efficiency and can last anywhere from 3-6 months before they must be discarded. But you shouldn’t take them for granted; they must be checked from time to time, to make sure they haven’t become clogged and are still working. These filters can capture approximately half of all the pollutants that pass through them (give or take 10-20 percent) and their higher efficiency can cause them to clog up faster than expected.

The same goes for electrostatic furnace filters, which will capture the widest range of pollutants among air filters commonly purchased for use with home HVAC systems. Electrostatic furnace filters are built to last indefinitely but need to be cleaned on some sort of regular basis, based on the heaviness of their workload and their absorption capacity.

Ironically, the cheapest and least efficient filters (the fiberglass type) are the easiest to manage: just replace them every few weeks and you won’t have to give it a second thought. Unfortunately, they won’t do much of anything to improve your indoor air quality, making them a poor investment at any price.

If, wisely, you’ve chosen better filters with MERV (Minimum Efficiency Reporting Value) ratings of 8 or higher (10-12 is recommended), you should learn to recognize the signs that indicate filters ready to be cleaned or replaced. When your air filters are clogged, dirty or otherwise malfunctioning, here’s what you can expect to see …

Gradually escalating utility bills

When air flow through duct work is inadequate (as it will be when filters are dirty), furnaces and air conditioners cannot function correctly. On air conditioners a lack of air flow will cause the evaporator coils to freeze up, while heat exchangers in furnaces will overheat and automatically shut off if they aren’t efficiently cooled. Clogged filters will reduce your HVAC system’s production of heated or cooled air, forcing your furnace and air conditioner to work harder, run longer and consume more gas, oil or electricity.

Frequent on-and-off cycling

HVAC equipment struggling to get enough air will inevitably shut itself on and off more frequently than normal, and will usually run for shorter periods of time when it is on. If this is happening in your home it should be fairly easy to notice, since you’ll be used to the system working one way and when its natural rhythm is disrupted the difference is bound to capture your attention.  

Unpleasant odors and a stale atmosphere in general

Rank, musty, dirty odors are a common side effect of clogged air filters. When filters aren’t working right air flow is restricted, which prevents the diffusion of odor-causing sources and makes it easier for moisture to collect inside duct work. A lack of air movement through an operating HVAC system will inevitably lead to stale air and bad smells are a common side effect of this condition.

Respiratory distress or other signs of allergic reaction

High-quality air filters remove the dust, dirt, pet dander, smoke, chemical fumes, fungal spores and other forms of particulate contamination that can trigger allergic reactions and their accompanying symptoms (runny nose, plugged nasal passages, rashes, breathing difficulties, irritation or itching of the eyes, throat or skin, etc.). But clogging reduces filter efficiency and will quickly compromise your home’s indoor air quality, putting the health of your family at risk.

Hot and cold spots (heating or cooling inconsistencies between rooms)

Beyond the reduction in volume, clogged filters cause interruptions in clean, smooth air flow. The quantity of heated or cooled air flowing from vents will be reduced but its intensity will also fluctuate, or flow more strongly through some vents than others, leading to hot or cold spots in certain areas.

Checking Your Filter to Stay Ahead of the Game

Even if you don’t notice any of signs of furnace filter or AC filter breakdown, you should still clean or replace your air filters on schedule, as recommended by the manufacturer.

But you can’t rely on that alone. The suggested time span for cleaning or replacement should be viewed as more of a guideline than a rule, and if you notice any indicators of poor performance you should pull your filter out and check it immediately. If you don’t do this your health and the health of your family could suffer and your HVAC equipment could be subject to a catastrophic mechanical failure.

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