HELP, is my main trunk line fatally oversized?
Jim
all the labor for everything from the ground up. I think we had help
one day for about 3 hours when we set the trusses. We were experienced
enough in carpentry, concrete work, roofing and electrical.
We got through the plumbing by having a long telephone conversation with
a plumbing contractor friend during which he answered my questions. But
the air conditioning duct work was different. I had watched fiberglass
duct being installed many times and the assembly went well. The
installation was completed and passed inspection. I installed the air
handler unit and the condensing unit outside since I had some
refrigeration experience. The electrical and control work was OK.
Then, when we were ready to "start up" the system, I was told that much
of my duct work installation was too big. I guess I always thought
"bigger is better." Specifically, I was told that my starting duct
running two directions from the main plenum was nearly double what it
should be. Our 2 1/2 ton system produces 1125 CFM and we have a 9 inch
by 24 inch main trunk running about eight feet each way, then reducing
to 9 inch by 18 inch (again both ways for about 1 feet each way for a
total of about 36 feet. This is duct work in the conditioned air
running through the center of the house. We had started framing a drop
ceiling to enclose this duct work. All the drops are in the walls and
the total square inches of the grills is 900. Anyway, this person tells
me that if we don't reduce the size of the trunk by about 33%, the
system will not work properly. He said the air would build up in the
large trunk line and not reach the outlets. What about this??
Jim Coleman
mailto:87...@cheerful.com hotmail address is primarily used to collect
junk mail.
Russell Sahagian
Jim wrote in message <35B93FF9...@hotmail.com>...
>My wife and I have been building a home together and have basically done
>I guess I always thought
>"bigger is better." Specifically, I was told that my starting duct
>running two directions from the main plenum was nearly double what it
>should be. Our 2 1/2 ton system produces 1125 CFM and we have a 9 inch
>by 24 inch main trunk running about eight feet each way, then reducing
>to 9 inch by 18 inch (again both ways for about 1 feet
>Anyway, this person tells
>me that if we don't reduce the size of the trunk by about 33%, the
>system will not work properly. He said the air would build up in the
>large trunk line and not reach the outlets. What about this??
>Jim Coleman
>
>mailto:87...@cheerful.com hotmail address is primarily used to collect
>junk mail.
How big is the house?
It sounds like the right size trunk to me but did you say fiberglass? How
about the runouts, what type, size; the dampers; the coating in the
fiberglass; the type of transitions; where did you get the info about the
reduction of the trunk line?
Who instructed you for the installation; was the system designed with air
flow and distribution in mind or did you use the box pressure "method"?
----------------------------------------------------------------------------
------------------
And most importantly, how is the return run. Please describe in detail.
++++++++++++++++++++++++++++++++++++++++++++++++++++
If you answer these and any other pertinent design Q's, I'm sure other
designers will pipe up...(no pun intended)
I'm gonna try to say this nicely, you can't know everything. Please respond,
I'll try to help.
P.S.: (Don't frame it yet!)
----------------------------------------------------------------------------
Russ Sahagian, sales, HVAC
fars...@flash.net
----------------------------------------------------------------------------
eter...@my-dejanews.com
Jim asked:
> I guess I always thought
> "bigger is better." Specifically, I was told that my starting
> duct running two directions from the main plenum was nearly
> double what it should be. Our 2 1/2 ton system produces 1125
> CFM and we have a 9 inch by 24 inch main trunk running about
> eight feet each way, then reducing to 9 inch by 18 inch
> (again both ways for about 1 feet {snip}
> Anyway, this person tells me that if we don't reduce the size
> of the trunk by about 33%, the system will not work properly.
> He said the air would build up in the large trunk line and not
> reach the outlets. What about this??
> Jim Coleman
>
> >mailto:87...@cheerful.com hotmail address is primarily
> used to collect junk mail.
"Russell Sahagian" <fars...@flash.net> replied:
> How big is the house?
The house is 1160 square feet within the walls.
> It sounds like the right size trunk to me but did you say fiberglass?
The duct is "Certainteed 1" EI 475" basic 1" fiberglass with foil covering for
air conditioning and does not have the expensive interior sealing coat now
available. All the trunk lines are nine inches in depth with the variables in
the width. I noticed that I had written on the reduced duct that it extended
"1 feet" each way from the larger duct. It should have been "ten feet".
> How about the runouts, what type, size; the dampers; the coating
> in the fiberglass; the type of transitions; where did you get the
> info about the reduction of the trunk line?
As mentioned, there is no special coating in the interior of the duct. All
the drops are short runs out the side of the trunk line. There is one 14 X
6, four 12 X 6, two 10 X 6 and three 6 X 6 located approximately half on each
side of the air handler plenum which is 14 X 16. There are no dampers other
than the lever type shutter/dampers in each of the sidewall registers which
have three fixed air flow patterns. The person who advised me to reduce the
trunk lines nearest to the air handler is a 20+ veteran air coditioner
mechanic, just a friend (not hired by us). He also advised us to install
turning vanes above the air handler to divert the air flow both ways which
seems reasonable. He also suggested that we install reductions in the
openings for the registers to make the net opening compatible with the square
footage in the rooms and gave us a formula for the number of square inches of
these openings based on the size of the room they serve and the total cfm of
the system. He said this would improve the effect of cooling the room by
increasing the velocity since the registers are on the inside walls versus
the outside walls of the room.
> Who instructed you for the installation; was the system
> designed with air flow and distribution in mind or did you
> use the box pressure "method"?
Unfortunately, there were no instructors. While libraries have pretty good
volumes on plumbing, electrical, etc., there were no books on "the art of
designing and assembling fiberglass ducts. The manufacturer gave us
directions on assembly and reducing, but the balance was "bigger is better".
> And most importantly, how is the return run. Please describe in detail.
++++++++++++++++++++++++++++++++++++
The return is one common in this area, a large enclosure under the air
handler with a 20" X 20" return air grill to a side wall. The unit is almost
perfectly in the center of the home. The doors to rooms with registers are
cut above the floors to allow return flow when the doors are closed. Having
been in construction all my life here, this all seems to be common for small
homes.
> If you answer these and any other pertinent design Q's, I'm sure
> other designers will pipe up...(no pun intended)
>
> I'm gonna try to say this nicely, you can't know everything.
> Please respond, I'll try to help. P.S.: (Don't frame it yet!)
Not knowing everything is true. I'm 68 years old, retired on a very limited
income and tried to get an air conditioning system with the $1000. we had
available. We apreciate yor help.
Jim
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eter...@my-dejanews.com
Sonofdawra
>by 24 inch main trunk running about eight feet each way, then reducing
>total of about 36 feet. This is duct work in the conditioned air
>running through the center of the house. We had started framing a drop
>ceiling to enclose this duct work. All the drops are in the walls and
>me that if we don't reduce the size of the trunk by about 33%, the
>system will not work properly. He said the air would build up in the
>large trunk line and not reach the outlets. What about this??
I'm answereing from home and do not have my duct calc stuff here, but I can get
a reasonable estimate of what will happen. If you use the following formula,
you can begin to approximate what will happen:
CFM = velocity of air in ft/min x cross sectional area of the ductwork is
sq.ft.
Actual CFM will actually be somewhat less due to the frictional losses of the
ductwork. Following through this line of thought and wanting about 600 ft/min
in the return air duct and 800 ft/min in the supply duct on residential
equipment, your trunkline should be able to deliver about twice the CFM that
your equipment can deliver. This will cause the velocity of the air in the
trunkline to be reduced and the internal static pressure to be reduced.
With reduced static pressure in the trunkline, your ability to move air into
the branch runs might be reduced too much. If it was mine, I would start up
the equipment and see what happens. Since apparently you have everything all
installed, I wouldn't go to the effort of changing things until I at least
tried what I had.
Jerry Weber
duct. What goes in must come out. If you decrease your duct size from what
it is you will have less air flow.
Some systems are design with a supply air plenum which is essentially and
oversized duct. This provides a more balanced air flow for the all the
branches coming off the plenum without the need for balancing dampers and
more efficiency loss.
You are right. The bigger the duct the better for air flow efficiency. The
only negative is the higher first cost and more space take up by duct.
Jerry Weber
Jim wrote in message <35B93FF9...@hotmail.com>...
>My wife and I have been building a home together and have basically done
>all the labor for everything from the ground up. I think we had help
>one day for about 3 hours when we set the trusses. We were experienced
>enough in carpentry, concrete work, roofing and electrical.
>We got through the plumbing by having a long telephone conversation with
>a plumbing contractor friend during which he answered my questions. But
>the air conditioning duct work was different. I had watched fiberglass
>duct being installed many times and the assembly went well. The
>installation was completed and passed inspection. I installed the air
>handler unit and the condensing unit outside since I had some
>refrigeration experience. The electrical and control work was OK.
>Then, when we were ready to "start up" the system, I was told that much
>of my duct work installation was too big. I guess I always thought
>"bigger is better." Specifically, I was told that my starting duct
>running two directions from the main plenum was nearly double what it
>should be. Our 2 1/2 ton system produces 1125 CFM and we have a 9 inch
>by 24 inch main trunk running about eight feet each way, then reducing
>to 9 inch by 18 inch (again both ways for about 1 feet each way for a
>total of about 36 feet. This is duct work in the conditioned air
>running through the center of the house. We had started framing a drop
>ceiling to enclose this duct work. All the drops are in the walls and
>the total square inches of the grills is 900. Anyway, this person tells
>me that if we don't reduce the size of the trunk by about 33%, the
>system will not work properly. He said the air would build up in the
>large trunk line and not reach the outlets. What about this??
paul milligan
>Oversized duct will not cause you any problem.
Please go out and buy yourself a copy of Manual D before
attempting to render advice on duct design. Pay special attention to
the sections on velocity.
I have snipped all your errant advice from this reply - there
is nothing there worth responding to, as it's all wrong.
Jeez....
Paul
I have employment openings in Raleigh/Durham, NC
HVAC, controls, engineering, programming, CAD etc...
Email me if interested
>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~
pjm@(remove this part )pobox.com
My WWW site is at http://www.pobox.com/~pjm, featuring free HVAC software.
The Sci.Engr.Heat-Vent-AC and Alt.HVAC FAQ is at http://www.elitesoft.com/sci.hvac/
Jerry Weber
Are you saying that if this guy reduces the size of his main duct he will
improve the airflow? I see it as a give and take when you use a larger duct
and let the velocity decrease. There is a loss in letting the air velocity
coming off the fan blade decrease and then farther down stream use static
pressure to increase the velocity. However, the air flow friction loss is
greater with the higher velocity. It varies by the velocity squared and the
length of duct. So it depends on the length of duct that you have the lower
velocity since the loss due to velocity changing is a single event.
I am familiar with ASHRAE duct design and Carriers duct design manual. Who
puts out the Manual D?
Jerry Weber
paul milligan wrote in message <35bcee81...@news.concentric.net>...
Jim
morning. If the large ducts will be any less cost efficient to run, we will
cut and paste. If not we will frame. Lets' take a vote - how many for cut and
how many for frame?
Here is a jpg file of the duct layout:
http://www.angelfire.com/fl/Sumter/images/print.jpg
here's a picture of the house:
http://www.angelfire.com/fl/Sumter/images/home.jpg
Sandy and Jim in Central Florida
Jerry Weber wrote:
> Paul,
> Are you saying that if this guy reduces the size of his main duct he will
> improve the airflow? I see it as a give and take when you use a larger duct
> and let the velocity decrease. <snip>
paul milligan
>Paul,
> Are you saying that if this guy reduces the size of his main duct he will
>improve the airflow?
Very possibly ( I would not presume to answer yes or no on the
specific case without doing proper calcs myself :-) ).
> I see it as a give and take when you use a larger duct
>and let the velocity decrease. There is a loss in letting the air velocity
>coming off the fan blade decrease and then farther down stream use static
>pressure to increase the velocity.
Static regain is indeed the normal way. Duct can certainly be
sized at various design velocities and statics - I would greatly
question, though, the design of any system that starts out hugely
under or over sized - how likely is it that it is actually part of a
carefully thought out plan, as opposed to a SWAG ?
>I am familiar with ASHRAE duct design and Carriers duct design manual. Who
>puts out the Manual D?
ACCA, same folks as Manual J. I'm sure the other methods are
just as effective, in fact all of them are based on ASHRAE, if I
recall.
Jerry Weber
I would definately frame. The larger duct will give you better air flow.
Jerry Weber
Jim wrote in message <35BC8199...@hotmail.com>...
Jerry Weber
After seeing his layout I would say that his air flow would be decreased
if he reduced the size. I contend that everything I said in my first post
is correct.
The velocity of air is like the speed of a car. When you make a turn you
have to decrease (stop) the velocity in one direction and accelerate it in
another direction. The faster you are driving the more energy it takes to
make the turn. With air flow it is like using regenerative braking. Most
of the velocity energy is converted into static pressure when the air stops
flow in one direction and then the static pressure accelerates the air in
the other direction and is converted back in velocity energy. However,
there is some loss every time there is a change. So the higher the velocity
(smaller duct) the higher the loss.
Jerry Weber
paul milligan wrote in message <35bf0b36....@news.concentric.net>...
Big "G"
surface area of the duct itself increases friction on the air and reduces
the airflow. With the correct sized duct, most all of the air is moving in
the same direction of the duct, and you won't have the efficiency losers
such as swirls and cross currents inside the duct.
I'm not even a HVAC pro and this is quite obvious to me...
Gavin
Jerry Weber wrote in message ...
Bob Pietrangelo
velocity will be shit.
Jerry Weber wrote:
> Oversized duct will not cause you any problem. Air cannot build up in the
> duct. What goes in must come out. If you decrease your duct size from what
> it is you will have less air flow.
>
> Some systems are design with a supply air plenum which is essentially and
> oversized duct. This provides a more balanced air flow for the all the
> branches coming off the plenum without the need for balancing dampers and
> more efficiency loss.
>
> You are right. The bigger the duct the better for air flow efficiency. The
> only negative is the higher first cost and more space take up by duct.
>
> Jerry Weber
>
Russell Sahagian
Jim wrote in message <35B93FF9...@hotmail.com>...
>My wife and I have been building a home together and have basically done
>all the labor for everything from the ground up. I think we had help
<snip>
> Anyway, this person tells
>me that if we don't reduce the size of the trunk by about 33%, the
>system will not work properly. He said the air would build up in the
>large trunk line and not reach the outlets. What about this??
>Jim Coleman
>
>mailto:87...@cheerful.com hotmail address is primarily used to collect
>junk mail.
Sheesh...
I suspect the runouts are inappropriately designed for the expected flow
ratios.
In other words the take off ducts would probably work better if they were
round, and made to attenuate properly. (that means larger at the area where
it joins the trunk, to offset the "natural" slowing down, or restriction at
the take off)
Another problem you have is the friction. Yes you have reduced velocity
with the oversizing, probably 15-20%, although this isn't fatal in and of
itself. Then add the large friction component and you have reduced air flow
efficiency by another 10-15%. Then add to the mix the size and type run outs
or take off's and you have reduced even more. This combination is fatal to
air flow.
The problem cannot be corrected with a simple fix. You will have to improve
the installation procedures somewhere. I suspect you may gain a few
percentage points from turning veins. Although not more than 10-15%. Then
you may gain a few points from reducing trunk line size but do not reduce
below 8"x18".
Additionally you may gain a few points by installing the "expensive"
coating you eliminated during the installation. These and a few other tricks
of the trade may allow for an operable, albeit imperfect air distribution
system. Probably 80% efficient at best.
Recommendation: use metal, let a journeyman install it. Sorry if this sounds
like an admonition.
Alternative recommendation: a larger air handler, more CFM.
Russ, HVAC guy.
Balance
<fars...@flash.net>
Jerry Weber
What matters is the cfm and velocity at each outlet. The less pressure
the fan sees the more cfm it will put out. The larger mains will present
the fan with less pressure and so the cfm will be greater.
If you need a higher velocity at a particular outlet to get a greater
throw then you size that outlet with a smaller neck and diffuser.
Jerry Weber
Bob Pietrangelo wrote in message <35BE8FBC...@concentric.net>...
>How can you say this? I just added up all of his runs and his trunks. His
>velocity will be shit.
>
>Jerry Weber wrote:
>
>> Oversized duct will not cause you any problem. Air cannot build up in
the
>> duct. What goes in must come out. If you decrease your duct size from
what
>> it is you will have less air flow.
>>
>> Some systems are design with a supply air plenum which is essentially and
>> oversized duct. This provides a more balanced air flow for the all the
>> branches coming off the plenum without the need for balancing dampers and
>> more efficiency loss.
>>
>> You are right. The bigger the duct the better for air flow efficiency.
The
>> only negative is the higher first cost and more space take up by duct.
>>
>> Jerry Weber
>>
>> Jim wrote in message <35B93FF9...@hotmail.com>...
>> >My wife and I have been building a home together and have basically done
>> >all the labor for everything from the ground up. I think we had help
>> >one day for about 3 hours when we set the trusses. We were experienced
>> >enough in carpentry, concrete work, roofing and electrical.
>> >We got through the plumbing by having a long telephone conversation with
>> >a plumbing contractor friend during which he answered my questions. But
>> >the air conditioning duct work was different. I had watched fiberglass
>> >duct being installed many times and the assembly went well. The
>> >installation was completed and passed inspection. I installed the air
>> >handler unit and the condensing unit outside since I had some
>> >refrigeration experience. The electrical and control work was OK.
>> >Then, when we were ready to "start up" the system, I was told that much
>> >of my duct work installation was too big. I guess I always thought
>> >"bigger is better." Specifically, I was told that my starting duct
>> >running two directions from the main plenum was nearly double what it
>> >should be. Our 2 1/2 ton system produces 1125 CFM and we have a 9 inch
>> >by 24 inch main trunk running about eight feet each way, then reducing
>> >to 9 inch by 18 inch (again both ways for about 1 feet each way for a
>> >total of about 36 feet. This is duct work in the conditioned air
>> >running through the center of the house. We had started framing a drop
>> >ceiling to enclose this duct work. All the drops are in the walls and
>> >the total square inches of the grills is 900. Anyway, this person tells
Marc L O'Brien
:
: I have employment openings in Raleigh/Durham, NC
: HVAC, controls, engineering, programming, CAD etc...
: Email me if interested
sheesh paul, what happened, all your staff do a runner on you ?
John Tan
I sense an impending earthquake coming :-)
paul milligan
wrote:
>paul milligan <pjm@see_my_sig_for_address.com> wrote in article
>:
>: I have employment openings in Raleigh/Durham, NC
>: HVAC, controls, engineering, programming, CAD etc...
>: Email me if interested
>
> sheesh paul, what happened, all your staff do a runner on you ?
If I _had_ staff, I would feel sorry for them :-)
Paul
I have employment openings in Raleigh/Durham, NC
HVAC, controls, engineering, programming, CAD etc...
Email me if interested
paul milligan
>> sheesh paul, what happened, all your staff do a runner on you ?
>>
>
>I sense an impending earthquake coming :-)
Nah, Marc's just an old temblor... :-)