Shouldn’t the size reduction be installed immediately after the 5th takeoff? I know that the sketch is not to scale, but the sketch shows it midway between the 5th and 6th takeoffs. It might be helpful to illustrate, or mention, that the transition can be placed a short distance after the 5th takeoff to save materials.
I am adding 800 sqft to my home (1,400 existing + 800 new). Got quotes from 5 contractors so far. They are telling I need to upgrade my furnace and AC. The recommendations range from 3 ton to 5 ton.
Are they doing a actual load calc? If not dont pick them.. Also Dont pick 1 unit to do it all. No house can use 5 Tons. Maybe 2 2 ton units but never 5 tons out of 1 unit
Question. Is it ok to reduce the duct work (let’s use your example) after we take first 3 branches, when the duct work is in unconditioned space to reduce the use of the materials and reduce the surface area of the duct work. So we can save few dollars on insulation and metal and also few BTUs?
Eduard Khachaturyan yes, you can reduce the size as much as you want. Just make sure you keep your velocity under the maximum limit. My observations have been when people use rules of thumb to size ducts, they ignore velocity limits and we end up with an undesirable increase in static pressure.
Question: I have 17 feet of 6 inch duct supply to a room. After 17 feet, it elbows down and to the right, travels 2 feet straight, then elbows up and into a transition piece that feeds the room. Do I need to reduce the 6 inch to 4 inch OR simply get rid of the elbows and install a 45 set about 4 feet before the transition piece into the room?
Hi Ed I have a few questions for you. 1. After you reduced in the example.. won’t your last few runs “run out of velocity” on the end? (Just like when you took off your velocity dropped in the beginning?). 2. Is it acceptable when using round pipe to reduce 4 sizes if need be? I.E. let’s just say that I started with a 16” round pipe and at half my velocity I needed to reduce to a 12” pipe. Would it be acceptable to put 2 reducers together and go from a 16” to a 12”? Thank you
Question 1. No. Velocity can not be to low in a branch run if you follow the guidance of Manual D. If CFM is correct the velocity if air going through the terminal box and grille is what we care about. Question 2. Yes. Manual D is about having enough fan pressure to overcome all pressure drops, having ducts large enough to not allow velocities to go above maximum values and using dampers to get the proper volume through branch runs. There are a few more but thay are my top 3.
It's your choice. The #1 rule to follow is whatever CFM volume you are moving through the duct (in that section) should not exceed the velocity limit as per table N3-1 Page N13 of ACCA Manual D (it says 700 FPM for a return duct). Lower velocity will never hurt your perforce, but it can impact your cost. I size my ducts following the conservative path listed in the table mentioned above. Check out ACCA.org, click on the education & events tab to see where I'm doing my next virtual or live training event.
Hey if I put supply grills straight into duct 24x10 does the grill size let say 10x6 have the same cfm as say a 10x6 square duct not sure on how to size with grills going straight into trunk line thanks for any response
