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Proper Sizing for Correct Air Flow
Home Knowledge Hub Proper Sizing for Correct Air Flow
24 Apr 2023  /  by admin   /   0 Comments

Proper sizing is a “catch-all” wording that covers the correct sizes of heater, air-conditioner, ductwork, venting and exhaust for the heating and cooling system of any home or business. The reason for proper sizing is to permit the optimal flow of air through whatever structure is being heated, cooled or ventilated. Incorrect sizing of any of the components of the overall system can result in obstructed, decreased and generally inefficient flow of air (heated, cooled or treated) throughout the structure, thereby resulting in discomfort for the occupants of the structure.

Purpose of Proper Sizing:

Air and water are very similar in that they are both fluid because they flow. If you take a funnel with a half inch neck and try to pour 5 gallons of water down that neck, it will spill all over because the ability of the neck to handle the amount of water flow into the neck is less than the ability of the neck to receive the water flow. Air reacts in exactly the same way. There must be a correctly sized heater flowing air into a correctly sized air ducting system in order for the flow to be received by the duct system without it back-flowing. If the system is too restrictive to the amount of air being pushed into the system, the air will flow backwards into the heater and out the sides of the heater. This is not good in that it will cause the heater to go into safety shutdown.

A reverse problem is having too great a size of ducting that causes a severely reduced flow below what the ducting being able to handle a far greater amount of air flow into the duct but there is a lesser amount of air being pushed into the ducting which results in not enough actual air pressure in the ducts to heat or cool the structure. Imagine, if you will, your own home. There are certain heating/ cooling vents that seem to put out more than some other ones. Perhaps it is the first-floor vents that really do a great job but the second-floor vents just do not seem to put out what the first-floor vents are putting out.

The key to even heating/ cooling air distribution is air velocity. Maintaining a consistent air velocity will allow for all the air vents to pass the same amount of air and thereby maintain overall comfort levels within the structure. This necessarily consistent velocity is accomplished by a steady reduction in the size of the air handling ductwork which allows the air to flow at a steady rate throughout the home or work area.


My house has great heating/ cooling on the first floor but not on the second floor. Is there anything that can be done about this? Yes, but within bounds. It all depends on the accessibility of the ductwork. Some of the ducting could be replaced or restricted to increase flow in another area of the ductwork and thereby level out the system as a whole. It is best to contact a professional for advice.

My system used to work great but now the air flow is a lot less than before…why? If airflow from a vent is not what it used to be, firstly, check the air filter, perhaps it needs to be changed or cleaned. This is most likely the cause, however, if after cleaning or changing the air filter air flow is still not what it formerly was, the blower could be dirty or there may be a restriction in the ductwork itself. Operating a heating/ cooling system without a filter will clog up everything in the air flow eventually. There are other items that may cause this flow reduction but again, it is best to have a professional take a look.

My system never worked good even when the house was new. What can I do? In a circumstance like this, there is a possibility the air system was incorrectly installed or engineered. This is a problem that must be looked at by an air flow system professional.

I want to add an addition to my home and tie it into the existing heating/ cooling system I currently have. What do I need to do or watch out for?

This is a very good question because it will take into account all of the air flow system. As we have discussed earlier, flow balance is very important because you do not want areas where you have no air. The entire system would need to be analyzed because it will need to be modified to meet the new demands placed upon the system.

As an example: My mother had a single-story addition build onto her house and the builders simply tapped into the end of the existing ductwork of the house to provide heating/ cooling for the new addition. There was good flow at the end of the old duct but the addition never was warm when it was cold out, neither was it cool when it was hot out. There was never enough pressure for the addition because you cannot simply cut some new lines into the existing ductwork and expect everything to function perfectly. All it succeeded in doing was to unbalance the existing ductwork.

Final air-flow note: Air-flow and ductwork is a science and not something to be “cobbled” together. Certain air pressures are needed at certain areas of the system for the system, as a whole, to function correctly. Additionally, a properly functioning system saves money and saves wear and tear on the components of the system. Be well!

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