# How do I calculate CFM for a structure?

This is for calculating the CFM (cubic feet per minute) of air that the blower needs to move in order to compensate the total heat loss (ventilation,infiltration,heat loss through walls, windows, etc)

The formula is: CFM=Building volume in cubic feet/Minutes Air change

Where do I get the "minutes air change number" from?

I want to verify if my ducts are properly sized for the furnace that I am getting.

The heat loss for a structure (home) is calculated with: H = Ht + Hv + Hi (1)
where
H = overall heat loss (W)
Ht = heat loss due to transmission through walls, windows, doors, floors and more (W)
Hv = heat loss caused by ventilation (W)
Hi = heat loss caused by infiltration (W)

My house heat loss was calculated by Heat2000 to be around 55kBTU/h
My furnace will be sized for 60K BTU/h
Now the blower must be able to recirculate the entire volume of air in the house in that hour. That means the heat will be uniformly distributed (according with the volume of each room) if the duct sizes are calculated correctly.

If the above are correct the CFM should be house volume/60

Is this correct?

• What kind of structure? A green house will have different requirements than a single family home. Dec 26, 2014 at 16:17
• It is for my home Dec 26, 2014 at 16:46

The "minutes per air change" is a design value chosen for the type of space and use that space has. In common situations, that number is ignored and simple rules of thumb for air flow are used instead. (Living rooms don't vary greatly in size, and the target numbers usually have a magnitude range of 2 to 4 anyway.)

However, a common design for bedroom space is 1 to 4 air changes per hour (15 to 60 minutes per air change). Living room space which is uncommonly occupied can be once per hour (60 minutes per air change), whereas a crowded smoky bar is comfortable at 15 to 30 air changes per hour (2 to 4 minutes per air change).

There is an extensive table here mostly covering commercial spaces.

• Yes I came across that one too but I believe that those values are used for ventilation not for delivering the heat via the same ducts used for ventilation Dec 26, 2014 at 20:46
• @user2059078: Eh? Delivering heat through ducts is ventilation. What do you think the difference is? Dec 26, 2014 at 20:59
• In my understanding heating is when you are recirculating the air in the house and you are compensating for heat loss due to air infiltration (that is what you want to do when you heat the house) and ventilating is when you are forcefully bringing in air from outside of the house to refresh the air. In both cases air is brought in but the proportion is different. Cooling and heating both try to maintain the air withing the building at a desired temperature, while ventilation means refreshing the quality of he air (ex a bar or restaurant or a conference room) Dec 26, 2014 at 21:11
• And here is a quote from a book I am reading as we "speak": In larger structures (e.g., commercial or industrial buildings), ventilation is necessary for both health and comfort. In houses, the amount of infiltrated air is generally sufficient to supply the ventilation requirements." Dec 26, 2014 at 21:14

From a blower door test. But you probably don't need to do that to determine your duct sizing. For that, you want a Manual D calculation done if you're interested in your ducts being perfectly suited to your furnace.

• Here is my problem: The basement was finished and renovated. I don't think that the Furnace was resized after that. I have weak air flow in basement registers and I can not see the dumpers (perhaps they are hidden in the finished ceiling close to the main duct). But they might not be there at all. So right now I can only assume that the blower does not have enough CFM to supply the entire duct system. I am just checking this and since the ducts are already there and I want to know if they would be enough for heating the house (I know I need 60K BTU) Dec 26, 2014 at 17:30
• If you want to fix this problem properly, you need the whole picture. Get (or do yourself) Manual J and D load calculations for your house. That will give you the heating/cooling loads of every room and the CFM required for ducts feeding them. This is a theoretically DIYable task but requires substantial study and potentially the purchase of two expensive books. Dec 26, 2014 at 17:39
• Alternatively, get a new modulating condensing furnace whose BTU output is close to the 60k load you've calculated and that has an ECM motor. That caveman solution may just fix your problem. Dec 26, 2014 at 17:40
• I have used Heat2000 to do the calculation. I am already ready the HVAC Audel manuals (reading what is relevant to my case). I also came across the ASRAE Manual J book but it was too complicated, I think that Heat2000 does the job good enough for me Dec 26, 2014 at 18:24
• Above I meant I am already reading Dec 26, 2014 at 18:55