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FAQ: How do you calculate return air cfm?

Example: You determine a return grille pressure zone required 340 CFM of return. It’s a 1600 CFM system with 200 CFM of outside air (200/1600 = 12.5% of outside air). Take 100%-12.5% to find a multiplier of 87.5%. 340 CFM of return air x 87.5% = 298 CFM.

How much return air do I need?

If your home is 2000 square feet, you can calculate your HVAC needs the same as you would for a 1600 square foot home. Assuming one ton of cooling capacity can cool 400 square feet of your home, you’ll need about 5.0 tons of air conditioning capacity. Multiply this by 12,000 BTUs, and you’ll get 60,000 BTUs.

How do you calculate return vents?

Calculate the square root of the total. (Example: 4 (ton unit) x 144 square inches = 576 squared. The square root of 576 is 24. Therefore, your return air duct and grill size will be 24 by 24 inches.)

How do you calculate BTU to CFM?

Take the energy transfer rate in Btu and divide it by 100,000 to convert it to CFM. Take the result and add it up to 96.7. Divide that result by 60 to figure out the gas transfer rate.

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How do you size an air return grille?

Calculate the area in square inches you need for your filter grille. For proper operation, plan on 2 CFM per square inch of filter grille area. So if your furnace puts out 800 CFM, you need 400 square inches of filter grille area.

Can you have too much return air?

Having several return vents (ideally one in every room, but even two or three is better than just one) creates consistent air pressure. If you have one return vent, your home is fine. Keep the doors to each room open so air can properly circulate.

What size should the return air grill be for a 3 ton AC unit?

A 3-ton heat pump has approximately 1200 CFM of air flow. To calculate the filter grill size, divide the airflow, (CFM), by 2. One filter grill or the total area of all grills must equal at least 600 square inches of area to adequately supply the system with the air needed to operate.

What happens if there is not enough return air?

If there is not enough return air available, your HVAC system will not heat or cool properly. If not enough air is brought back, your HVAC system will not be able to keep up with temperature demands. In some cases, two returns may be necessary to provide enough return air.

How do you calculate air flow in a pipe?

By multiplying air velocity by the cross section area of a duct, you can determine the air volume flowing past a point in the duct per unit of time. Volume flow is usually measured in cubic feet per minute (CFM).

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How do you measure HVAC CFM?

The calculation for CFM involves dividing the total volume of the space by the air exchange interval. To calculate room volume, you measure its length, width and height in feet and multiply these. Dividing this number by the change rate gives the required airflow in CFM.

How do I know if I have enough return air?

Try this simple test when your HVAC system is running: get a piece of tissue paper and dangle it about six inches from one of the return vents. You should be able to see it being gently pulled towards the vent. Next, place it up against return vent and see if the air suction holds it in place.

How do you calculate CFM for duct size?

Duct CFM Calculation Formula

  1. Room CFM = (Room load/Whole house load) ✕ Equipment CFM.
  2. 24,000 BTUs ÷ 12,000 BTUs in 1 ton = 2 tons ✕ 400 CFM per ton = 800 CFM.
  3. Room A = (2,000 BTUs ÷ 24,000 BTUs) ✕ 800 CFM.
  4. Room A = 66.67 CFM.
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