CFM & Air Flow 

Sorry this took so long, but I must have missed the notification, to answer your question, we use 400 cfm per ton for return and supply, 400 in and 400 out. However, if you live in a humid climate you might slow down the air to 350 cfm per ton so that the air is in contact with the coil for a longer period and allows more moisture to be removed, or dehumidify the space. That is one of the first jobs that Willis Carrier did www.williscarrier.com/1876-1902.php.

Currently when doing testing we use the TruFlow Grid for static and cfm through the system. When doing a service call I use Fieldpiece smart probes and Joblink app. When designing a system on the computer I use Wrightsoft right draw and right duct. Links below. Wrightsoft www.wrightsoft.com/ True flow grid www.trutechtools.com/tec-minneapolis-digital-trueflow-grid-full-kit-with-all-8-adapter-plates-no-manometer.html?gad=1&gclid=Cj0KCQjwtO-kBhDIARIsAL6Lore-ISwGfbK0aJNgq-ileA79e00rCy4cf529F1rnyMrZXEtkvfvRzrwaAqqiEALw_wcB Fieldpiece www.fieldpiece.com/product-category/job-link-system/

I use Wrightsoft Universal for my load calculations. It does all the work all you have to do is put in the orientation of the home, what the walls, windows, floor and ceiling are made of and it does all the calculations for you. Here is a video of me drawing up a house in wrightsoft. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-3bdpqGEe6lA.html

The amount of ventilation is based on the volume of the space, cubic feet, times the amount of air that you usually want change in an hour. I assume you are talking about 50,000 cubic feet, a room that is 50'X100' room with 10' ceilings. If we desired 4 air changes per hour for a room this size on an industrial building we would divide 50K by 60 and multiply by 4 we would need 3.3K of ventilation. However, on a residence we are looking for air changes per hour of 0.35 which for the same space would equate to 291 cfm fan. So, as you can see it will vary according to the what the space is used for. Below is an excellent web page on this. Hope this helped. www.atlenv.com/building-ventilation-the-proper-air-changes-per-hour-ach

I am assuming you mean how much duct per pound of air or cfm. NCI calls out the following for Flex duct 5"-50 cfm, 6"-75 cfm, 7"-110 cfm, 8"-160 cfm, 9"-225 cfm, 10"-300 cfm, 12"-480 cfm, 14"-700 cfm, 16"-1000 cfm, 18"-1300 cfm, 20"-1700 cfm. These numbers are based upon a friction rate of 0.05 on the metal duct calculator. These are all approximate and can be used as a reference when sizing ducts for the air needed for the room.

I use Wrightsoft software to do my load calculations. You draw the house up in the software and then the computer does the calculations to give you the size of the equipment and loads for the rooms. The computer also will size the ducts for you, which makes our job much easier. You can do the load calculation manually, which is much more time consuming. Here is a link to a video that I did for someone who was just starting to use Wrightsoft ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-3bdpqGEe6lA.html Really the only right way to do heating and air is to do a load calculation room-to-room, then install dampers in the ducts to balance the airflow to the design. The problem is that too many HVAC contractors skip this part and then the customer suffers. This is why many contractors put in systems that are over sized, they are trying to compensate for poor design. This makes matters worse because too large equipment will cost the customer more money to run and will not satisfy the cooling or heating properly in the home.

Thanks for the reply. If I have my cooling and heating loads calcualted for each room, how would I manually calculate the CFM required in each room?

Thanks for the help

Once you know the cfm needed plus the friction rate you need to use, you use the ductulator to size the ducts to that room.

@@baileysair So if i go on a service call and customer complains that a certain room is always warm, i would have to do a load calculation just to see if i have the right size duct(s) to that room and have the proper cfm? That seems like alot.

@@biblebloopers946 Yes, it does seem excessive. But there is no other way to tell if the proper amount of air is getting to the space. However, you should ask the customer has it always been this way or did it just suddenly start? If it has just started, the first thing I would check is the ducts themselves that go to that room to see if they are crushed or disconnected. The next thing before doing a calculation is make sure that the loads in the room has not changed. For example did they just put a computer and monitor in the room? If this has been a problem from the start I would check that they have insulation above the room. If that is good I would take my flow hood and measure the amount of air that is coming out. I would also do it with the door open to the room and closed. Sometimes we find that when the door is closed the air is almost cut in half due to the return being located in the hallway (there is not enough undercut of the door to return the proper amount of air). Now a question for you, once I have that measurement how do I know what that number should be? The answer; The only way to know what that number should be is to do a load calculation. Once the load calculation is done and I compare the number it should be within plus or minus 10% of what the load calcs say. If not I need to either balance the system (if there is enough air to begin with) or install a larger duct along with volume dampers in all the ducts and balance the system. Okay, that being said there is one other thing that needs to be considered and that is how we install and run the system. What I am talking about is the location of the thermostat. Since we only have one thermostat in the home, we can have rooms that are hotter or colder. Why, because the thermostat does not know what the temperature in that room is if it is in the hallway and loads like computers, monitors, televisions, and people can cause those rooms to heat up quicker than the rest of the home and therefore before the thermostat has a chance to sense it. The solution, zoning. Hope this helped, Mitch

@@baileysair wow, that's a lot of good info. Thank you so much for taking the time to write it all out. Unfortunately, if i come across a customer who just moved in, they wouldn't know if this was always an issue or perhaps a new issue. But i hear what you're saying. First things first, check to see if any ductwork has come loose or if anything is kinked. Check to see if there's insulation over that particular room. I actually never thought of that. I'm just a service tech so that never crossed my mind LOL. So insulation over room... Got it. And then check to see if perhaps they have a high heat load in the room.

@@baileysair Assuming that your system is size correctly, assuming that when you walk into a house you have a 5-ton unit and it sized correctly. Is there at that point a rule of thumb for the size you should use for return duct and supply duct? I don't like assuming. But I'm just trying to get a better understanding of this. So my question is if I have a 5 ton condensing unit and air handler that is sized correctly, would I be able to calculate what size duct I need at that point? The reason I'm asking is I went to a service call yesterday where my airflow seemed a little crappy. I'm following up today. But I'm wondering if my return and Supply duct is sized correctly

If you are talking about the weight of the air, I checked the math and it comes out 1200 cubic feet per minute X the weight of air 0.0807 pounds per cubic feet X 60 minutes does come out to 5810 pounds over an hour. Anyhow, I was trying to show that an air conditioning system pushes a lot of air. Thanks, Mitch Bailey