View Full Version : back pressure
had somebody tell me today that if you don't transision from the a/h outlet but just go with a large duct (larger than opening) fan wont build up back pressure and wont work.
is that how it works or doesent work?
02-02-2006, 08:12 PM
Static pressure is the term.;)Check out manual D.
thanks coolwhip, i guess it's about time for me to start getting familiar with manual d. and i mean it too, but in the mean while i want to know who's right.
02-02-2006, 08:25 PM
air handler and duct both have to be sized.
if you have a 3 ton furnace and you install a 20 inch supply duct you will have no static pressure, get it.
yea i get it,
transitions are not important but the diameter of the trunkline is.
thank you for highlighting that for me
02-02-2006, 09:33 PM
You can purchase a copy of manual D through the ACCA and also a ductulator. If you take a class, which may be offered through one of your local supply houses, they will give you a ductulator and go over how to use it. It took me years to grasp duct design and I still feel pretty basic at it coming from a service background. Manual D was confusing to me. I forced myself to read it over and over again. Eventually some of it sunk in.
02-02-2006, 10:15 PM
Yes, transitions are important and proper duct size is also important, proper fittings are crucial to a good duct sytem. Manual D as a matter in fact is in my face everyday I do duct layout. It will show what fittings are preferred and what fittings you may have seen out there that are the worst to use based on what is called Effective Length of the fitting or a value of straight duct added to the total length of the duct system, buy the manual and read it over and over. Inspectors here in Michigan use it as a bible and even a simple 10x10 addition requires a Manual D done on the system or Friction rate worksheek before any work is started, so my point is the "Authorities" do care about proper duct and will enforce it
we replaced a 2 ton system with a 2.5 ton and now that i'm looking at the ductulator it looks like there will be back pressure---static to last to the next job. .21. while the old one was transitioned down and the new one up the main trunkline will still remain at 12x10.
thanks for bringing that to my attention.
but, if one would build a 5 ton straight shot would he still have to include 4 foot of 17x16 on both ends or could the unit run by itself? and are transisions really nessessary?
02-02-2006, 10:37 PM
Lets say you have a horizonal trunk line on a 3 ton system that requires 1200 cfm. You have 6 outlets, 2 are close to the plenum, 2 are halfway and 2 are at the end and each outlet needs 200cfm. If the trunk line were the same size all of the 30' it runs, you would get the 1200 cfm, but the first 2 outlets nearest the plenum would get much less than the ones on the ends. By reducing the trunk down as cfm is used, you keep the pressure at the first 2 outlets.
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