duct sizing

Paul S_3

 i would like to know if anyone has any charts for duct sizes in relation to a/c tonnage....for example if a home has a 3 ton a/c system ....and customer wants to up size....a way to check to see if duct work is big enough?...     

RJ_4

ductulator

I would invest in a Trane Ductulator, about $15.00 on line, than I use roughly 400 cfm per ton

Techman

duct size

If the duct was "properly sized" for a 3t system then no way will that duct be properly sized for a 3 1/2 t or a 4t system. Close may work! If the 3t was "properly sized" for the house then no way will " a large tonnage unit work properly.If the duct size was screwed up and the tonnage was also screwded up then maybe/might be. A KindaSorta "rule of thumb" (here we go again) ,400CFM per ton for AC ,450CFM per ton for heatpump. Now this is a "starting point", supply trunks are sized   at .10 " water colume and return ducts are sized using .05-.06-.08" w.c. Using the "Ductalator" will do the rest. So, 3t=1,200 CFM @ .10"w.c. is 15' round or 20" x 9.5" or 12" x 16" and return trunk will be 17"round at .05" w.c. or10" x 26" or 12" x 20" Knowing how the duct size was determined is  USUALLY 1 of two different duct methods . #1-Equal Friction or #2- Velocity Reduction. This is a "start".

JStar

Ducts

There are no rules of thumb for ductwork. None!



The CFM depends on your sensible load. The static pressure depends on blower ability, restrictions in the duct system (coil, filter, etc), and total equivalet length.



There are no rules of thumb for ductwork.

Paul S_3

thanks

so basically you guys are saying is for a  new install/ or existing i should invest in a ductulator ..... thanks again Paul S 

Techman

Here we go again!!

JStar, what size duct is used for a 3t AC ,no hot water coil, no 4-5" thick filters(just the basic 1'' air filter that comes w/the unit), Equal friction method please. Normal velocities. Carrier AC Co. taught me " duct sizing" back in 1974 at a 2 week Carrier Basic AC School and then in 1979 at a 1 week" Carrier Heat Pump Specialist Certification School" they taught "duct sizing" again , but, I'm sure things are different today! In February 1976 Fedders AC CO had a 3day "Fedders Factory Authorization Certification Class" which included some duct sizing Then in  March 1976, a  Fedders 3day "Heat Pump Specialist Certification Class" and a R.S.E.S.( 2day class 1985)  also had "duct sizing classes" that I went to and in 2000 I went to a L.I.P.A(Long Island Power Authority ) I day "Residential Air Flow and Refrigerant Chargeing Class", but what do they know! Back in the day, each mechanic had to be "Carrier ( or others) H.P Certified " AND State certified (State of Ct.) as a  H.P. Specialist(1978) in order to "touch "a H.P., Those tests included a little Duct knowledge. And then in 1987 I went to a Ruud "Heat Pump Certification Class " for 2 days which included some "ducting" But, what the heck! I guess I missed a lot.

JStar

Duct

0.1 inches of static is only appropriate for a total equialent length around 100 feet. Every good ductulator should have a chart to find the adjusted static per different TEL. using 0.1 for wvery job is like using a 1/2" gas line for every furnace no matter what size.

Spence

Duct Sizing

On a new installation, lets assume your load calculation requires 1150 CFM. You then look in the fan table of your chosen unit to see what ESP the selected unit will give you as close to 1150 as possible on medium speed. If that ESP is .60 with a filter, you subtract .06 for your registers and grilles and other components. If the ID coil is .21, and you have no other components, then you have .33 left for your duct system. If the TEL of the ducts is 250' your duct design pressure is .33 x 100/250, or .132. Now you can look at your duct calculator and determine what size duct will give you 1150 CFM at a pressure of .132.



If you would like, I would be happy to discuss existing system methodology as well.

Techman

duct sizing

In my R.S.E.S. Service Application Manual (SAM) "The Equal Friction Method Of Duct Sizing" shows an example of a system requiring a 1000CFM ,using a blower  on medium speed w/  0.5 total ESP ,then the very 1st reduction in ESP is the Supply Trunk  rated at 0.10 ' w.g.  and the 2nd reduction in ESP is the Return Trunk rated at 0.05" w.g. . Carrier shows the same ST and RT  pressure readings. I'm checking ACCA but I think its the same. Maybe you are going CCW and I'm going CW to get to the same point. I like my math better, its what I was taught and its what I know. Now I DID NOT say the ENTIRE SUPPLY DUCT SYSTEM was sized for 0.10" w.g. I DID say the ST (0.10)/RT (0.05) were.  I would like to hear about duct methodology, sound interesting.

Techman

duct sizing

Spence

Sizing Methods

I believe you are dead right. I am an ACCA disciple and am not that familiar with EF, yet we are looking for the same answer, which is the maker's blower determines the duct size parameters. I use ASP and TEL to determine the design friction rate, while you use ASP and EF.

Spence

Existing Duct Systems

The first step is to determine your ESP across the blower inlet and outlet. Next, determine the pressure drops across both supply and return duct systems and subtract them from your ESP. The result is what that blower "sees" in the existing ducts, fittings, registers and grilles. Your first early warning is if either the SDs or the RDs are at .20 IWC or above. Your second warning is if the blower "sees" .30 IWC or more. I believe you can imagine what is happening here; if you need 1345 CFM for your new equipment for example, and your blower needs .50 IWC to reach that number, then you're already at .80 IWC. That is crazy-high, and we haven't added a filter or coil! Yikes! Is it no wonder that nationally, duct systems are performing at 57% of their capacity? There is good news, however. Since the SDs are usually hard to modify, the RDs are more easy to get to. The blower doesn't care whether you relieve the pressure on the supply or the return.



Here is a little secret for determining ESP. Since ESP means at the blower inlet and outlet, you can't leave the blower door open on the RA or accidentally drill a hole into your condensate pan on the SA while taking readings. First, check your furnace. My preferred maker gives me a hole in the blower compartment, so I don't have to drill one. perhaps your favorite brand does too. Rather than risking damage to the ID coil, take out the high limit card and tape the opening around your tube. That gets you exactly where you need to be (inlet and outlet) and before or after any pressure drops from components. When drilling holes for the coil and filter, I had my one and only flash of brilliance. I bought magnetic business cards to hide them.

Techman

Ducts

Hi guys. Spence, I wasn't being sarcastic by saying that I would like to hear about duct methodology. So ,fire away,but don't use too many big words, OK, I carry a toolbox around all day. I have a book called "air diffusion dynamics" theory,design and application by Ralph G.Nevins, PhD.    DUDE I get confused so easily! I got the book in 74' and have not even come close to "mastering it". But, I keep trying!



How about this 3t system has a TEL of 100'.!

Spence

Existing Ducts

Do you have PowerPoint on your computer? The above tests take about 10 minutes. It might be easier to see it rather than reading, and, when I made the slides, I didn't use any big words. I would be happy to send you a copy to any e-mail address you wish. If you don't have PP I can convert them to .pdf format. Also, I'm happy for you that you carry a toolbox all day. I have to drag mine.

TonyS

Heres what I use

It works well and is quick and easy. Its in PDF so print all you want.

Spence

Duct Sizing

I applaud your efforts in putting this together; you obviously spent a lot of time on this. However, your procedure has to come after you pick the blower, not before. Your plan assumes the blower will deliver a certain desired CFM, yet it will not because there is no account for system pressure drops, nor is there a reference to the blower performance table and corresponding external static pressure. I have no doubt your system works, yet if you take into account these critical issues, you certainly will be at the top of your game.

Techman

Spence.

LOL here, you'll send me your stuff when you get your next non-14hr day . Really ? Summers happening!!! So maybe Halloween ? LOL w/ you!!!!!! At your convience, my friend!

Techman

Duct vs gas pressuse

The gas press has to be the same if there is one appliance or 10 . The pipe size changes. If the 1st gas appliance is a big 1,000,000 BTU boiler the gas pipe is kinda big, then the next and only other appliance is a 5,000 BTU gas lamp . Then if the "proper sized gas pipe" is used on both devices, the pipe sizes are different but the duct press (I mean gas press)stays the same.

Spence

Air Versus Gas

You are right if your comparison is gas pressure to duct friction rate. FR is from blower outlet to inlet, yet many disregard pressure drops which can occur at any point along that stream. If your blower is rated for 1000 CFM at .50 IWC, then that is your FR. If you add a filter at .08 IWC, then your FR is .42 IWC, and so on. This is one of the reasons you can't read a duct calculator directly, saying for example "an 8 x 12 duct gives me 500 CFM at .10 IWC." No, it doesn't.

Techman

Ductalators

Going back to my very first reply to this thread - a real good one by the way PaulS.-  somewhere along the line I remember saying something along the line of "this is a starting point" and I finished with "this is a start". My R.S.E.S. -S.A.M.- Manual starts out with those #s - 0.10"w.g. and 0.05."w.g. THEN they go on to talk about "The Equal Friction Method" then Equivalent length, then TEL, then     "CORRECTED PRESSURE DROP" and a few more things. There is even a section "DUCT ROUGHNESS CORRECTION FACTORS" but I figured PaulS. didn't care about that part, yet! So lets all get together and have a "Burn the Ductalator Party" ! Gotta go on a service call.

Techman

Passing thru

Back here for parts. Check this out.

Spence

Duct Calculators

Don't burn RJs Trane calculator; it has TELs of most common residential fittings on the back!