Recently I was checking a rental after the tenants had moved out.  I always check the HVAC filter because tenants never change them even though it is written in their lease to change it every 30 days. This tenant had installed a “smart” thermostat.  Supposedly the thermostat came with instructions that the filters only need to be changed every 90 days.  And the thermostat included an app that would tell the occupant when the filter needed to be changed.  Their defense was since the app did not say the filter needed changed then it did not need change.   It sure looks like it needed to be changed to me.

First, what is the definition of HVAC.  It originally stood for Heating, Ventilation and Air Circulation.  After Willis Haviland Carrier began experimenting with the laws of humidity in 1902 to solve an application problem at a printing plant in Brooklyn, NY the air conditioner was invented and AC became air conditioning.

Air filters are a central, but often under-appreciated, part of a central HVAC system. They don’t just filter out bits of pollen and dust that would otherwise circulate through the home and lower indoor air quality. They also provide a first line of defense against larger objects such as bits of loose insulation being pulled into the system where they could cause damage or present a fire risk. If you don’t change your air filter regularly, it can turn against you. Clogged air filters are the number one cause of HVAC system failure.

All the air handled by the HVAC system passes through the air filter at one point or another. As the filter catches more and more of the natural particulate pollution of your home – dust, mold and fungal spores, pet dander, fabric fibers, etc. – the fine mesh through which air passes becomes denser. This means that if you don’t change your air filter regularly, air can’t pass through as readily.  This makes the blower fan in the HVAC system work harder to push air. When it’s working harder, it’s drawing more energy, leading to higher bills.  And It’s also more likely to wear out from the strain.  The last thing you want is the HVAC system to fail in the middle of a Phoenix summer.

A secondary issue is when the system has difficulty in moving air is your living areas may not get all the air they need. This means poor home comfort for you.  The temperature sensors which regulate when the HVAC system turns on and off might not ever register the temperature needed to signal the system to power down. That’s another source of expense, which puts more strain on the fan motor.  Because the heated or cooled air can’t travel out of the furnace, heat pump, or air conditioner as easily, the system runs the risk of overheating or freezing up. Then there will be an unwelcome change in indoor temperature the unit stops working.

The additional material on the air filter itself can provide a space for moisture to gather, encouraging the growth of mold or bacteria colonies. Not only do these form an even stronger barrier to circulating air, but if they colonize the wrong side of the filter, they can begin to introduce more pollutants and allergens into the indoor air. And if they get into the HVAC system and form colonies there, it could spell a lot of trouble for the entire system.

Because air doesn’t circulate as quickly, particulates can settle in ducts and on household surfaces when they’d usually be carried into the system and filtered. It’s a lot simpler to change your air filter than it is to schedule a duct cleaning, but dirty ducts can result in an ongoing drain to your system efficiency and a long-term source of pollutants in your air supply.

When to Change Your Air Filter
It’s best to change your air filter every month or two, especially if you have pets or live in a high-pollen area. Air filters are generally inexpensive and changing your air filter is a simple task that doesn’t require a professional hand. In most cases, you should simply be able to slide the old filter out and slide a new one in.

Check your unit’s documentation to determine what size of filter you need.  Buy the cheapest filters like the green one in the picture below.  Don’t buy the one in the picture on the left that has a tight mesh.  This filter restricts too much air flow resulting in a situation a lot like a clogged air filter and will cause more problems than is solves.  Never, ever run your system without a filter as it can result in dirty coils that require an AC tech to clean.

Was the Thermostat smart? 
If the tenant was relying on the thermostat to tell them to change the filter, then the thermostat failed them.  And the tenant did to realize not changing the filter was costing them far more in electric than the cost of the filter. Rather than relying on the thermostat simply set a reminder on your calendar of phone to check the filter once a month.