CFM and AC

AC and heating size chart:




 

 

 

Typically we need about 1 CFM of air flow per square foot of floor area of conditioned space provided that the ceiling height is about 8 feet above the floor, with a typical number of windows and doors and typical building insulation and heat gain or loss.

That is 1 sqft x 8ft = 8 cubic feet space needs 1 CFM.

You can use this to roughly calculate how much CFM needs for your space. (Tong added)

Examples of components for which air velocity (speed through the device) is particularly important are given in the HVAC Air Velocity table below.

Typical HVAC System Air Velocity Specifications (速度， 不是CFM）
HVAC Component Recommended Air Velocity / Flow Comments
Air Ducts - main trunk, typical 700 - 900 fpm Residential
Air Ducts - branch, typical 600 - 700 fpm Residential
Air Ducts - target 400 fpm Residential Ducts run through conditioned space
Air Ducts - target 400 - 600 fpm Residential Un-conditioned attic, ducts very well insulated
Air Ducts - typical 600 - 750 fpm Residential Un-conditioned attic, ducts exposed, little insulation

Definition of standard HVAC air flow rate per ton: 400 CFM per ton of cooling capacity

Typical air flow rates in CFM vary depending on the type of cooling system:
  • 350 CFM per ton of cooling capacity is required for high-latent-heat HVAC applications. (A latent heat cooling system is one that uses a cooling media that changes state - for example refrigerant that changes state between a liquid and a gas form).
  • 400 CFM per ton of cooling capacity is required for typical cooling designs
  • 500 CFM per ton of cooling capacity is required for heat pumps and sensible heat designs

Question:

My new home in Louisville Kentucky will be about 2000 square feet. How much cooling capacity in tons do I need?

Reply: rule of thumb calculation of required cooling capacity
For an average climate and building, you need
  • 1 CFM/Sq.Ft. of living space, or 1 x 2000 or a total of 2000 CFM of air flow into the total occupied building space.
Now divide the total CFM required by 400 CFM (typical air flow per ton of cooling capacity of an air handler)
  • 2000 CFM required / 400 CFM per ton = 5 tons of cooling capacity.
You need 5 tons of air conditioning capacity.But according to the chart below, the required tonnage can be a bit smaller. For example, Kentucky is in zone 3, for cooling, it should require between 22-19 btu/sqft, so for 2000 sqft home, it should require 2000x20.5 (use middle number) = 41000 BTU,  1 ton is 12000 BTU, so 41000/12000 = 3.4 ton, just go to the next available AC should be 3.5. This method is just a different way of doing calculations. This method is showing using the following table. It seems to me that this method is a lot easier. It is hidden from using CFM.



 if the air temperature is at 70 degrees and the air pressure is 14.7 lb/sq in., then this condition is called standard conditions. Under the standard conditions, 1 lb air will occupy 13.34 cubic feet. With some rather complex calculations, it is normally the rule of thumb, 400 CFM per ton is used to determining “Standard Air” Conditions Air Volumes.

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