Is it okay to install filters in each grill in my home?

Multiple air filters and air filters located at the grills and registers

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Is it okay to install filters in each grill in my home?

No, it is not okay and, if you should fall victim to those Internet Advertisements promoting this foolishness, it is likely that these will cause damage to your heating and air conditioning system as well as your home.

Is it okay to install filters in each supply register in my home?

The supply registers in your home are the register into each room that is dispensing the conditioned air, warm or cold, into the room that they are serving. These register are specifically designed to project the air across the full length of the room creating enough velocity to cause a mixing of the air and thereby eliminating stagnant air spots in the room. Generally, they do a very good job at eliminating areas of the room that would not normally allow the air to move around and mix with the surrounding air.

This brings us to the problems that these additional filters cause:

1. Air stagnation and poor air quality
2. A reduction in the airflow across the heat exchanger
3. A reduction in the airflow across the evaporator coil
4. An increase in gas and electric utility bills.

Air Stagnation Causes by Filter Induced Excessive Static PressureMultiple air filters and air filters located at the grills and registers

These are not in the order of importance. Air stagnation is caused by a number of factors and the factor that we are most concerned about here is a lack of airflow. Every single filter, no matter how good, how big or how small, causes an increase in static resistance. This can be on the vacuum side of the blower motor or the pressure side of the blower motor. To measure total static resistance we add the resistance just before the blower motor and just after the blower motor. Both of these numbers are expressed as a positive number even though the vacuum, pre-blower motor, is initially a negative reading on our manometers. This number is then dropped into a chart that graphs the amount of airflow the motor is actually moving. Most heating and air conditioning systems are designed for up to 0.5 inches of water column of static pressure. The more impediments to airflow, filters are an impediment; cause an increase in static resistance and a reduction in airflow. Most systems are, without the addition of these filters, already operating in an environment that decreases their ability by having too much static resistance.

Stagnant air in the home leads to poor indoor air quality and causes an increase in problems related to allergies and asthma. There is no system designed to handle the impact of air filters tucked behind supply air registers in any residential applications.Perhaps the worst air filters that can be installed in a home heating and air conditioning system

A Reduction in the Airflow Across the Heat Exchanger

As the static pressure in the ducting increases the airflow that keeps the heat exchanger from overheating decreases. As the heat exchanger, or firebox (same thing), heats up it expands. The greater the temperature difference between the starting or cold temperature and the full hot running temperature, the greater the expansion of the metal that makes up the heat exchanger. As the furnace cools down after its heating cycle the fire box cools down and contracts. The greater this expansion and contraction is, the greater the risk for cracking and heat exchanger failure.

Take a paper clip and place it between your fingers. A heat exchanger is nothing more than carbon steel, just like the paper clip between your fingers. Now bend that paper clip back and forth as far as you can. I think you’ll find out that it will break after about 6 to 8 bends. The metal will simply give up. Now take a second paper clip of the same type and only bend it a quarter of the maximum amount you can. I think that you will find out that you will be at this exercise of bending back and forth, expansion and contraction, for a long time before it breaks in half. The reason is stress. We want your heat exchanger or firebox to be under the least amount of stress possible so that it lasts the maximum length of time between furnace change outs. this can only be performed if the heat exchanger does not become overheated because of excessive filter static resistance.

A Reduction in the Airflow Across the Evaporator Coil

I have taught hundreds of contractors over the years about heating and air conditioning. In air conditioning the three most important factors to ensure a cool comfortable home are airflow, airflow and airflow.

Airflow across the condensing coils. The condensing coils are those coils in the outside unit. These exchange the heat removed from the air inside the home to the outside air. If the airflow is compromise the heat exchange cannot take place or a reduction in the amount of heat exchanged takes place. In any case the result is a poorly running system that costs allot to operate and delivers a poor level of home comfort.

Airflow into the home. This means that if you cool or heat the air across the evaporator only to dump it into the attic or crawl you really have nothing but increase the utility bill. The air must make it back into the home after being conditioned.

Airflow across the evaporator. Perhaps the worst air filters that can be installed in a home heating and air conditioning system

If the static air resistance is increased by the presence of supply air register filters, they will always increase the static air flow resistance, then the airflow across the evaporator coil is reduced. If the airflow across the evaporator coil is reduced this can lead to several things:

1. Mold and mildew resulting in poor indoor air quality. An inability to properly exchange the heat of the home through a reduction in airflow across the evaporator coil can cause mold, mildew and bacteria growth on the indoor cool causing hazardous living conditions in the home.
2. A reduction in efficiency. No, it will not cost more to operate, but it will produce far fewer cooling BTUs and this will cause the air conditioning to operate longer will less chance of proper indoor comfort. The result is a system that operates all the time and produces little cooling. This increases the utility bills while decreases the level of comfort.
3. Ice buildup and flooding. Water forms on the evaporator coil when the coil temperature reduces below dew point, this is normal, but then that water forms ice when the coil temperature reduces below 32 degree Fahrenheit. This is not normal, but it is common. When additional resistance to airflow is built into a system, a second set of filters, the resistance is too great for maximum efficiency and is often too great to enable the system to function above freezing. The ice forms and melts and the water causes damage resulting in short circuits, transformer failure, circuit board failure, dry rot and home flooding. Bear in mind that a 3 Ton air conditioning system in our climate can remove upwards of 1 gallon of water from the air per hour. That is a lot of water that can cause damage.

An Increase in Gas and Electric Utility Bills

As airflow decreases from the increase in static pressure the ability of the system to perform the way the manufacturer designed it to perform suffers. The number of BTUs generated or removed by the heating and air condition system decreases. This results in poor performance and a lack of heating and cooling. Translate this into a longer on cycle and a shorter off cycle and you have a system that runs nearly continuously just trying to stay up with standard weather patterns. This increases the utility bills and makes a dent in your wallet month after month.

I know this has been a long article on measly little filters, but thank you for reading and understanding that they just don’t pay for themselves and they can do a lot of damage.


With one simple Tune-Up, you are able to get more comfort, lower your energy cost and actually reduce the risk of future repairs. The number one cause of system failure is dirt! Don't let a dirty un-tuned system cost you money out of your pocket!