Helpful Tip

AA HEATING & COOLING Maintenance Tip

About The Air Filter In Heating and Cooling System

The air filter in heating and cooling systems resists dust in the air, not only making indoor air clean but also preventing dust from entering the inside of the machine, especially in the evaporator coil (which is the main part of cooling system). If the dust accumulates inside the evaporator coil, the cooling system’s efficiency is reduced. The compressor withdraws the liquid refrigerant, which is incompletely absorbing the heat and is not fully vaporized. Over a long period of time, it is easy to damage the machine. With the low efficiency of the cooling system, it uses more electricity to lower the temperature, so the electricity bill gets more expensive. In the winter, the hot air made by electric or gas heaters goes through the gap of the evaporator coil. If these gaps are clogged with dust and restrict the air circulation, a mechanical failure will happen. So, when we make the choice of an air filter, what is the best choice? Is filter material better when it is denser?

When selecting the air filter, we not only need to consider resistance to dust, but also think of the amount of air flow through the heater exchanger and cooling evaporator coil, both of which should be mutually balanced. Usually when we make the choice of an air filter, everyone thinks that the denser filter material is better while ignoring the necessary amount of air flow that should be in a normal heating and cooling system. In actuality, the air filter with denser material holds more dust and the air resistance is also larger. Because of the dirty air filter, the amount of air flow will be reduced. There is not enough air flow, which causes a problem of heating and cooling systems. If the lack of air flow conditions exists for a long time, the life of the compressor will be greatly shortened.

In the heating and cooling industry, there are the standards for the manufacture of air filters. The density of filter material is set from MV1 to MV20 (or MERV). The higher the number is, the denser the filter material is. For the general user, I recommend using an MV7 or near MV7 air filter (relative to 3M air filter, about 700 MPR). In addition, the thicknesses of air filters are usually 1, 2, 4 or 5 inches. As the air filter becomes thicker, if you keep the size and ventilation ducts of the machines unchanged, you can choose a denser filter material. Because of the thicker size, the ventilating area is increased and the amount of air flow to maintain the original flow is unchanged. On the contrary, if we use a thinner air filter, we must choose a less dense air filter. If we want to reduce the original thickness of the air filter, we also have to use one with a lower density. It would shorten the replacement time interval if we have been using a thin air filter but would like to use a denser one (for example, in order to prevent allergies). Another point to note is that we can only put in one air filter at a time. Do not put two or more to meet the thickness you want. The average family needs to replace air filters every three to six months. It should never get to the point where the air filter is fully covered by dust. If there is a house being built or construction near your house and the amount of dust in the air has increased, it is necessary to replace the air filter more often than usual.  If you are decorating either your house or your basement with dry walls, it is recommended to use plastic paper to isolate the dust. Seal the heating and cooling air intake in your work area with tape temporarily, and do a daily inspection or timely replacement of air filter. Having one air filter is enough for an entire heating and cooling system. Air filters do not need to be placed at the inlet and outlet. If this is done, it will only cause more trouble.

Air filters are the cheapest parts in a heating and cooling system. Regular inspection and replacement will not only clean the air inside the house while extending the life of the machine, they will also save money for you. Please do not underestimate the role of air filter maintenance!

The tips above are solely from my personal experience as owner of AA Heating and Cooling. If you have any questions, comments, or concerns, please email me at

Daniel Zhou

DO NOT immediately turn on your air conditioning when returning to your home on a very hot day!

Every summer, I receive several calls from people who have just come back from vacation. They say, “I just turned on my home’s air conditioning, and there’s no cold air coming out, and the outside condensing unit/heat pump isn’t running. Before we left the house, everything was fine. Why isn’t the machine working now?

And every time I go to check it, I find that the capacitors (parts of the machine that helps with start-up) were broken.

Why were they broken and is there any way to prevent it? The answer is yes.


1. SIMPLEST METHOD: Turn on the fan first and let the air flow indoors. Wait until sunset and then turn on the air conditioning.

2. Another method is to decrease the outside condensing unit/heat pump’s condensing temperature. Before turning on the air conditioning, bring the water hose by the condensing unit/heat pump. Connect the water hose to a sprinkler and face the sprinkler to the sides of the unit(See image). The water should be able to hit the unit, acting similarly to rain, except that it’s coming from the side instead of the top. This decreases the condensing temperature. Please do not let the water strength flow to the point where it bends the aluminum fins on the unit. While the water is spraying, move the switch to cool position on the thermostat, and the outside unit should start running. When the room temperature decreases to 80 degrees or less, you can stop the sprinkler.

3. To the people using the programmable thermostat: set the highest room temperature to 80 degrees or below 80 degrees. Even if the temperature outside is above 90 degrees, the chances of the capacitor breaking decreases significantly.


The accident was caused by high temperatures.

Before going on vacations, some families will either turn their thermostats off, or they will increase the room temperature to above 80 degrees or higher. When they come back and the outside temperature is above 90 degrees and the inside room temperature is above 80 degrees , the refrigerant pressure inside the air conditioning pipes is directly proportional with the temperature. When the temperature is high both inside and outside, the condensing unit/heat pump pressure is high. The electricity current/voltage is also high.

1. For example, when you push a car that’s not moving, this motion requires more force than pushing a car that is already moving. This is similar to a condensing unit/heat pump starting condition. Pushing a car forward on an upward slope requires more force than pushing a car forward on a flat surface. This is similar to a condensing unit/heat pump working pressure and load with the temperature. Low temperature is similar to low slope, while high temperature is similar to high slope.

2. When the temperature increases, the electricity device weakens.

1 + 2  = more chances of broken capacitor

Some families that use a programmable thermostat (thermostats that can be set according to the time of day) meet the same situation. For example, one customer sets the morning temperature to 85 degrees, and 78 degrees at 3 in the afternoon. The highest temperature outside is 96 degrees that day. In the afternoon at 3 when the room temperature goes close to 85 degrees, the condensing unit/heat pump starts, then the capacitor breaks because of the high load.

The suggestions above are based solely on my job experiences. If you have any questions or notice any errors, please email me:

Daniel Zhou

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AA Heating and Cooling has been licensed, bonded and insured since 2001(HVAC License Number: H0001206). AA Heating and Cooling has been oAngie’s List since 6/10/2010.