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HVAC Duct Work Calculator?


Guild of Calamitous Intent
May 21, 2020
Member Number
The Natural State
Anyone recommend a duct work calculator app? Download, online, Android, cheap or free?

I've been all over the internet, I can figure out the tonnage and BTUs needed, I can calculate CFM needed for each room. I can calculate how many CFM a given size duct can support over a given length.

I can't find anything that allows for adding branches.

I'm working with a limited space and need to figure out if I can get sufficient airflow to my various rooms. I've seen some online calculators, but nothing like what I'm looking for. I'd like to be able to put in my available CFM or tonnage, with X diameter, X length, X number of branches, that are X diameter, and X length, and find out what the CFM at each register is. Knowing when to step down in size on the length of the run would be handy too.

Anyone DIY HVAC and have any tips? I'm not opposed to paying subbing out our HVAC work, but I need to figure out what I'm wanting to do is even feasible.

Anyone have experience with high velocity HVAC systems? I can use a lot smaller duct work with those.
I just want to point out that this is one of many threads I won't participate in because I don't know anything about HVAC. Up until 2010 I thought that a/c took more energy than heating.
Not as simple of a solution as I was looking for, but I'm fairly proficient with excel.

I did my calcs via spreadsheet per room. It's not an exact science, you are going to have some too big and some too small unless you custom build each duct. I used mostly round flex duct and built as little as possible. Most furnaces are two speed, and your building for worst case. 99.9% of the time you only heating a few degrees at a time. Just don't go way under and you will be fine.
There is too many factors for any generic online calculator to help. You have to figure the area of each room you plan to control, windows in a room that allow radiant heat from the sun, or doors that open to the north in the winter, how well each area of your house is insulated. I am a huge fan of putting a vent as close to the stove/ oven as possible for summer time cooking. Also remember metal pipe helps with air loss compared to flex hose, so if you have to do a main supply all the way across your house I would highly suggest running a solid metal pipe (insulated of course) and then T out with your flex hose to each individual room. Also, another factor return air plays a huge factor in keeping from having dead spots in your house, Pretend your house is a system of tunnels, the further your go down a tunnel from your return the harder the system has to work to pull that dead air from those areas farthest from the return, so the best way to fix this is add an air return at the end of that corridor.

I am pretty sure I could keep going, it's no simple procedure to figure.
IEATRKS84 nothing willy-nilly, but I'm not tying do do anything too complicated. The primary issue is, I have pretty tight constraints on the size and location of duct work holes I can have in my second floor I-joist. I need to do some calculations to make sure I can accomplish my HVAC needs, with the duct size limitation, without completely redesigning the 2nd floor.

I've got Revit, it provides some MEP templates and even automated HAC design, but not as detailed as I'd like.

I do tend to over think things.
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