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Building Science

Converting Heating and Cooling Loads to Air Flow Needs

Here’s how Manual J software takes your inputs and gives you both the BTU/hour and cfm needed for each room

This supply register for heating and air conditioning system moves air into the room. We measure the air flow in cubic feet per minute but it's really the amount of heat moving through the system that matters.
Image Credit: Energy Vanguard

When you embark on the project of educating yourself about building science, one of the first things you encounter is the concept of heating and cooling loads. Every building has them. (Yes, even projects.) That’s why we do . We enter all the details of the building, set the design conditions, and get the heating and cooling loads for each room in the building. Here in the US we still use that yield British Thermal Units per hour (BTU/hr) for the loads. In most of the world, the result is measured in watts of kilowatts.

But then what? We don’t just turn on the spigot. We move those BTUs into and out of the rooms in a house with a fluid, typically air or water. So how do we know how many cubic feet per minute (cfm) of air will give us the right number of BTUs per hour? Well, today we’re going to do a tiny bit of math and talk about this relationship between BTU/hr and cfm. (I’m going to leave the discussion of using water for heat distribution to my friends on the hydronic side, but it’s analogous to what I’m explaining below.)

How much heat can air hold?

Matter is pretty neat stuff. It has all kinds of interesting properties that have kept scientists hidden away in laboratories for centuries. (I hear that Galileo is still toiling away in the basement of the Leaning Tower of Pisa.) When talking about the capacity of air to hold heat, the relevant property is called — you’re not gonna believe it — heat capacity. Yep. It’s a term I’ve mentioned occasionally in this space but never really defined, so let’s take care…

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  1. Jon R | | #1

    > "With heating, it's
    > "With heating, it's just need a piece of equipment that can supply at least as many BTUs as you need.."

    And distribute/mix it properly within a room and maintain proper balance between rooms (even with non-proportional room load variations).

  2. Todd Witt | | #2

    Write an Article on Delivered System BTU
    Great article as always. We encapsulate 99% of our HVAC designs in open cell foam encapsulated attics or encapsulated crawlspaces or both when 2 story homes are constructed on crawlspaces as most homes are in North Alabama. The calculation you are doing is for delivered Btu's needed. I am sure you would agree that the temperature of air that leaves the air handler is much different than the temperature of the air delivered into the room. This is especially true with HVAC systems installed in vented attics or crawlspaces. For example, the temperature of vented attics in Alabama is somewhere around 130F to 140F. When you consider a PSC air handler installed in that attic with high pressure drops due to bad duct design and installation such as undersized returns, restrictive filters, pinched and kinked flex duct, no transfers grilles in bedrooms, etc. you are not getting the proper cfm out of the system and you are not getting the proper btu delivery. On Wrightsoft HVAC design software you can easily compare the heating and cooling load for ducts located in a vented attic vs an encapsulated attic, ducts in a vented crawlspace vs an encapsulated crawlspace and so on. The best comparison I have found is that for a one story home built on a crawlspace and showing the difference in heating and cooling loads in a vented attic vs an encapsulated crawlspace. We also require variable speed air handlers in all of our homes we design and insulate. We start out with a 0.8 available static pressure and we limit pressure drops as well as total effective length of the duct system. We then don't worry much about duct leakage, duct insulation (though some is necessary in our area to prevent condensation), and air velocity.

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