How to Calculate the CFM of a Room
- Measure the room's width and length.
- Multiply the three measurements from step 1 to determine the cubic footage of the room.
- Multiply the cubic volume of the room by the number of times you want the air to turn over or exchange in an hour.
- Divide your answer from step 3 by 60 to calculate CFM.
To calculate Air Flow in Cubic Feet per Minute (CFM), determine the Flow Velocity in feet per minute, then multiply this figure by the Duct Cross Sectional Area.
CFM = FPM x Duct Cross Sectional Area
Finding the Flow Velocity, typically expressed in feet per minute (FPM), is the first step in filling in our equation variables.To calculate CFM, you need to know the size of your heating or cooling unit in tons. Multiply this number by 400, which is the average output of an HVAC unit. Then divide by the total square footage of your home. This will give you the multiplier for all your rooms' CFM.
What is a BTU? The British Thermal Unit, or BTU, is an energy unit. It is approximately the energy needed to heat one pound of water by 1 degree Fahrenheit. 1 BTU = 1,055 joules, 252 calories, 0.293 watt-hour or the energy released by burning one match.
Heat and energy recovery ventilators
| Total area of home (square feet) | Continuous ventilation rate |
|---|
| 1,000 square feet | 50 CFM |
| 2,000 square feet | 100 CFM |
| 3,000 square feet | 150 CFM |
METHOD 2: Go by square feet + climate
| HVAC System Sizing |
|---|
| Blue | Green |
|---|
| 3 tons | 1651-2100 sf | 1601-2000 sf |
| 3.5 tons | 2101-2300 sf | 2001-2250 sf |
| 4 tons | 2301-2700 sf | 2251-2700 sf |
In the example above, using an 80% efficient furnace, the 2000 square foot home above would require a 100,000 BTU input furnace which will produce the necessary 80,000 BTU's output of heat.
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.
Rule of Thumb — The maximum size unit you should buy would be 15% over the BTUs you need for cooling and 40% of the BTUs for heating. The max for heat pumps is 25%, since it handles both cooling and heating.
Understand here that the standard practice among HVAC contractors sizing air conditioners for new homes is to use a rule of thumb. It's often in the neighborhood of 1 ton of air conditioning capacity for each 600 square feet of conditioned floor area, usually abbreviated 600 sf/ton.
Latent and sensible heat are types of energy released or absorbed in the atmosphere. Latent heat is related to changes in phase between liquids, gases, and solids. Sensible heat is related to changes in temperature of a gas or object with no change in phase.
Conversely, if you cool off some water vapor, it can condense into liquid. If it condenses on its own, it will give off (sensible) heat and get warmer. Therefore sensible heat can be felt while latent heat is the type of heat that cannot be felt.
The sensible cooling load refers to the dry bulb temperature of the building and the latent cooling load refers to the wet bulb temperature of the building. For summer conditions the humidity influence on the selection of the HVAC equipment and the latent load as well as the sensible load must be calculated.
Latent and sensible heat are types of energy released or absorbed in the atmosphere. Latent heat is related to changes in phase between liquids, gases, and solids. Sensible heat is related to changes in temperature of a gas or object with no change in phase.
Latent capacity is one aspect of scalability where you include additional performance and availability resources into your system so the system can easily handle unusual peak loads. You can also monitor how latent capacity is used in a deployed system to help determine when to scale the system by adding resources.
The specific latent heat is different for solid to liquid transition and liquid to gas transition. For example, if we want to turn 20 g of ice into water we need Q = 20 g * 334 kJ/kg = 6680 J of energy. To turn the same amount of water into vapor we need Q = 45294 J .
Latent cooling load is a measure of the amount of energy that is necessary to dehumidify the air in a building, for example, regardless of the outdoor humidity. Cooling load needs to be considered when a cooling system is being dimensioned. Latent cooling load refers to the wet bulb temperature.
Latent heat plays a very important role in refrigeration. Latent heat is the heat involved in a phase change in a material. You could use just a gas - pressurizing it to liberate heat, releasing the pressure to absorb heat. That's the latent heat of vaporization and it works both ways: boiling and condensing.
Definition of latent heat of fusion. The amount of heat required to change 1 g of a substance at the temperature of its melting point from the solid to the liquid state without changing temperature.
By calculating the heat gain from each individual item and adding them together, an accurate heat load figure can be determined. Add the results together. Calculate the heat generated by occupants, allow 600 BTU per person. Add the above together to find the total heat load.
CFM shows the volume of air entering the room per minute (as it is cubic feet per minute). TR is tonnage of refrigeration which means amount of heat extracted to produce one ton of ice. Generally we take 1 TR = 400 CFM. As a thumb rule.
