changing return air duct size?

Would like to field a duct size question.
I am splitting up a ceiling return air box. The original was two 12" flex hoses jammed into one box. I took one out. Replace the one return vent and place it in a 2' X 2' box.
Planning to take the other one a bit further to take up the return from another room.
Before I purchased another 12" hose, can I change the duct size up to 14" mid-run? I have a bag of 14" left by the original owner.
thanks richard
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Fri, 6 May 2011 14:46:49 -0700 (PDT), snipped-for-privacy@altavista.net wrote:

    Seeing as you obviously don't feel the need for any kind of system design work to actually make it right, the answer would then be 'sure, why not ? Use what little bits of scrap you happen to have laying around. Who cares ? See if your neighbor has some extra. ' I sure can't see it from here..
--
Click here every day to feed an animal that needs you today !!!
www.theanimalrescuesite.com/
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
responding to http://www.homeownershub.com/hvac/changing-return-air-duct-size-44402-.htm steveshapiro wrote:
snipped-for-privacy@altavista.net wrote:

You might be able to, but realistically, do you really want to have two different hose sizes running through your vents?
-------------------------------------
****** ****** ******* ============================ ******* ******* ******* =========== ** ** ** ** * ** ( | ** ** ** ** ** ____/ ******* ******* ** ****** ****** ** /\_/\
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Before anybody can give you any kind of ligitimate answer, how about some really basic information....
Make/Model/Serial number of both inside and outside units length of duct run from the return in the ceiling to the unit Does the return have any fittings ine it (elbows, etc) Does the ceiling grill have a filter in it, and what type
Yes this is all important information
From what you have tols us so far, it sounds like the whole system was ethnicaly engineered and installed by the lowest bidder. FWIW, to turn chicken shit into chicken salad costs a whole lot more than if it had been done right the first time.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I have to get the info on the unit. I know it's a Carrier.
The furnace plenum to return air grill is about 6.5 ft. and no fitting (flex duct). Filter is the paper accordion 20" X 20" X 1" in a 2 X 2. Both return ducts were originally pulling air from 1 return box. My intent was to split this one box into 2 - 2 X 2.
The additional location that I am thinking of relocating is going to need another 6-7 ft of 12". With that in mind, - and I could be wrong - I thought that the effect will be negligible enough to get close to 500 cfm. What I don't know is how a mid-run change (adding a 6-7 ft. piece) will affect the flow on the transition from 14" to 12".
My thinking is this will not make a difference, since the flow would end up being restricted by the 12" duct capacity and thus the reasoning for using the unused 14" duct.
thanks richard
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I have to get the info on the unit. I know it's a Carrier.
The furnace plenum to return air grill is about 6.5 ft. and no fitting (flex duct). Filter is the paper accordion 20" X 20" X 1" in a 2 X 2. Both return ducts were originally pulling air from 1 return box. My intent was to split this one box into 2 - 2 X 2.
The additional location that I am thinking of relocating is going to need another 6-7 ft of 12". With that in mind, - and I could be wrong - I thought that the effect will be negligible enough to get close to 500 cfm. What I don't know is how a mid-run change (adding a 6-7 ft. piece) will affect the flow on the transition from 14" to 12".
My thinking is this will not make a difference, since the flow would end up being restricted by the 12" duct capacity and thus the reasoning for using the unused 14" duct.
thanks richard
---------------------------------------------------------
Assuming it has A/C, how many tons?? This will determine what you will need for return air.
400CFM per ton
Now get out your handy dandy, flex duct, Duct-u-lator and figure out how big its gotta be for the correct airflow
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

    This big :
    | |
--
Click here every day to feed an animal that needs you today !!!
www.theanimalrescuesite.com/
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On May 6, 7:22pm, snipped-for-privacy@altavista.net wrote:

.
Yes, use the 14" flex but put a damper in that run to balance the system once youre done as the 14 " run is going to move more air than the 12" . And im assuming there is adequate return air for your particular system with what you want to do.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Yes, use the 14" flex but put a damper in that run to balance the system once youre done as the 14 " run is going to move more air than the 12" . And im assuming there is adequate return air for your particular system with what you want to do.
-----------------------------------
sure it will move more air, but will it be enough?? 14 inch will almost be enough for 2 tons of a/c (maybe)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

...
Looking at my handy-dandy Trane duct sizing calculator, a 14" round will handle 1,000 cfm airflow or 2.5 tons of cooling. ; add to that a 12" and we are up to 1560 cfm or 4 tons nominal.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Looking at my handy-dandy Trane duct sizing calculator, a 14" round will handle 1,000 cfm airflow or 2.5 tons of cooling. ; add to that a 12" and we are up to 1560 cfm or 4 tons nominal.
-----------------------------------------------------------
What are you allowing for filter(s)? boots? fitting(s)? and grills?
BTW.. this guy is talking about running a pair of 12 inch into a single 14 inch... at least thats my take on it.... so your still at a single 14inch for total volume.
You only have .5in H2O *total* to work with, 300fpm across the filter, and a pressure drop across the filter of .08 - .25in H2O. Just the filter(s) can cost you up to half of your allowable SP.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

No, the original run consists of 2-12" returns (2-12" off the plenum) in one box . I am putting one return back in its original location and relocate the other one one wall over (in another room). This is the one I am thinking about changing.
So, in all it's one 12" and one 14".
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On May 6, 4:46pm, snipped-for-privacy@altavista.net wrote:

I don't know how well the ASCII drawing is going to work, here goes:
+--------+ | |-------- 12" return | | | |--------========= 12" to 14" return | | +--------+
The box is the plenum. The dashes are 12" ducts. Please note the second longer line. The equal sign is the 14" duct. The 14" is the one going across the wall into another room.
thanks all, richard
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

I don't know how well the ASCII drawing is going to work, here goes:
+--------+ | |-------- 12" return | | | |--------========= 12" to 14" return | | +--------+
The box is the plenum. The dashes are 12" ducts. Please note the second longer line. The equal sign is the 14" duct. The 14" is the one going across the wall into another room.
thanks all, richard
----------------------------------------------
You still haven't told us what size the system is to know if you have enough airflow for it to operate correctly. Without the originally requested info, we are just guessing.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I looked around and could not find the serial number on the furnace. I did find that it is a Carrier 58RAV unit size 070. I may have to look under the unit to find which 070, since it is in the attic and it was a horizontal installation
Please note that this is a heat only. Air con. is still window units. I googled and according to this: http://xpedio.carrier.com/idc/groups/public/documents/techlit/58rav-7pd.pdf
it could range from 415 to 1435 (if I am reading the table correctly) depending upon static pressure - I was looking at both 070-08 and 070-12 to get the range - since I haven't found the actual size other than what was scrawled on the plenum.
thanks
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I looked around and could not find the serial number on the furnace. I did find that it is a Carrier 58RAV unit size 070. I may have to look under the unit to find which 070, since it is in the attic and it was a horizontal installation
Please note that this is a heat only. Air con. is still window units. I googled and according to this: http://xpedio.carrier.com/idc/groups/public/documents/techlit/58rav-7pd.pdf
it could range from 415 to 1435 (if I am reading the table correctly) depending upon static pressure - I was looking at both 070-08 and 070-12 to get the range - since I haven't found the actual size other than what was scrawled on the plenum.
thanks
Ok.... so you need between 1100 - 1400cfm for that furnace, depending on the hear rise requirements.... probably between 35 - 55F. size your return(s) accordingly.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Hmmm.....seems the last 2 posts have been removed from the server... WTF??
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.