ReplyI'm not sure what you're asking - but duct length is much less important than cross sectional area. Maybe that helps?
Regular Guest
First I must say that getting a few Manual J's and D's gives peace of mind.
I've been going over the ducting, comparing the design(s) of each. I see that the ducting has a column for TEL. Also each branch is listed as a separate measurement. It's great that this is part of the equation and I can grasp the concept as for why.
So my question is, Due to limited space in the attic am I able to streamline or cut down the length(s) of the ducting as designed? I have no problem with resubmitting a fairly detailed print or drawing of the workable routing and lengths that would result.
I understand that rules of thumb are bad and the Manual D was drawn with the TEL in mind. I even tried to self educate by crunching the room requirements as compared to the branch line vs the trunk line vs the cfm vs fpm and so on.
I am totally pleased with the engineered results but unfortunately it will be a PITA to make this work in my situation. It looks good on paper, but...
Professional Member*
I'm not sure what you're asking - but duct length is much less important than cross sectional area. Maybe that helps?
Regular Guest
The TEL is set from the Manual D. I need to chop that way down to make it fit in my small arse attic. I would have ducting criss-crossing, blocking egress, etc.
Is this doable or should I resubmit a drawing with what will fit keeping all as drawn trunk and branch dimensions and still have the same end result.
Professional Member
use the dimensions of the ducts as your guide as to what size trunk, takeoffs etc, but cut down the lengths to fit your needs.
Banned
Are just using shorter runs,or reducing the duct size??Originally Posted by regis101
Professional Member*
I can't figure out what he's asking either.
God Speed, Jim. We'll miss you - 2-26-23
Apparently he has a drawing. That we can't see. And wants an answer based on us not knowing what he see's.
Banned
Okay,read the OP again,think I understand.
Yes you can reduce the lengths,may need balancing dampers,as the shorter they are the more air they will get,but this is also the case with the original design.
You should not;
Change supply or return grille sizes or location
Add more elbows or turns
Use fittings(transitions,etc.) other then shown on the print.
Last edited by dash; 08-13-2008 at 08:17 PM.
Regular Guest
The operative word in the opening post is, length. Duct diameter is not going to be reduced. Defeats the purpose. I need that diameter to get the air into the room (s).
Can I cut down on the length(s). Will that defeat the purpose.
I think some of you read too fast
Go easy on me, I'm new
Banned
Uh did you read my post?
I used the plural of the operative word!!
Professional Member*
We're going easy on you because I think many of us suspect English is not your first language, not because you're new. Which is fine, of course. But if it is your first language - you type to fast - we don't read too fast. No-one in here could figure out what you were asking - and I said the cross section was more important than the length in the first reply!
Regular Guest
Thankx dash.
I am under the assumption that the length(s) of the ducting is part of, not just the reason, of the Man D to help get the airflow down to the rooms requirement.
As dash mentioned, a shorter run will get more air. So this means a longer run will slow it down? If so, will having to take a branch line from 20ft as drawn down to 8ft affect the airflow as described for that room.
Professional Member*
The velocity of the air is determined by the cross-sectional area of the duct.
Longer runs do impose more static pressure, but the difference between 20 feet and 8 feet is almost negligible. For the third or forth time, I lose count, shorten the runs and maintain the other dimensions the same as designed.
Banned
Man. D determines he friction rate,which determines the duct size based solely on the longest return and supply air path.So all the shorter runs tend to get more air.Man. D suggests that balancing dampers be installed at each branch take-off,to allow for adjustment due to this.
We find that the dampers at the supply grilles will take care of any required balancing.
So yes yes,shortening it will have a minor increase in air flow,likly not an issue with proper design,in all other aspects of Man. D.
Banned
If you shortened the longest runs,it could increase the total air flow,in which case you would select a lower fan speed,or add restriction to the main ducts to get the correct air flow.
We add 4 non-existent elbows to new home designs,to account for fild changes,and our installers know it,if the don't add any,we have less static which is easy to deal with,as stated above.
Regular Guest
Some knowledge. Thanks.
With all the hammering you guys, the powers that be, do, about getting the
Manuals J,D, and sometimes S, forgive me when I have a question that deviates from what has been stamped out.
And well, if it takes three or four times to get the point across, so be it.
When talking with the tin knockers at work, they mention not to make a tap at the end of a run. Instead, have a dummy leg, of sorts, as the last few feet of the (trunk) line. Acts as a buffer or bumper or something. They deal with mostly, if not all, metal trunks as done in the commercial world. Maybe another moot point.
Regular Guest
Thanks for the replies. I'm done
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