Need Help on Return Air Size
Mr Smith
We are building a new home @ about 1900 Sq. Ft. It has a 4 ton A/C unit and
I think the return air is a little small. The round duct is 18" and the
grill size is 24x24.
We have 13 registers coming off of this unit.
Is there a way for me to double check my A/C contractor?
Are there any sites on the web that might help? I have spent a while
searching with no luck.
Thanks in advance.
Mr Smith
ceiling.
Thanks for the info.
And by the way I was sold on the 12 seer Janitrol because of the scroll, 10
year comp warranty and it was $800 cheaper than anything else.
<theh...@postoffice2.bellatlantic.net> wrote in message
news:37ABDE1A...@postoffice2.bellatlantic.net...
> 18in. round duct for the return should be sufficient if it is properly
> run. Is it flex? Is it run as straight as poss? How many feet is the
> run?
> Jay
turtle4air
wrote:
> Hello,
>
> We are building a new home @ about 1900 Sq. Ft. It has a 4 ton A/C unit and
> I think the return air is a little small. The round duct is 18" and the
> grill size is 24x24.
>
> We have 13 registers coming off of this unit.
>
> Is there a way for me to double check my A/C contractor?
>
> Are there any sites on the web that might help? I have spent a while
> searching with no luck.
>
> Thanks in advance.
>
>
this is turtle.
i would call that good. 18" duct return will do fine on 4 tons.
TURTLE
--
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Exchange ideas on practically anything (tm).
theh...@postoffice2.bellatlantic.net
run. Is it flex? Is it run as straight as poss? How many feet is the
run?
Jay
Gary Edelman
4 ton approx. 400 CFM's per ton = 1600 CFM's
1600/2 = 800 sq. " of filter space.
24x24= 576 , that's a little small for a 3 ton. It will probably be
noisey & your filters will need
to be changed more often than usual but it will still work. Two 20x20's
is usually a good alternative. The 18" round will work also but a 20"
round would be what I would use for a 4 ton. If it is a heat pump it is
a little more important to up the size of the round duct.
Add up your registers. If you come up with more than 1600 then don't
worry.
4"= 50 CFM's
5"= 75 "
6"= 100 "
7"= 150 "
8"= 200 "
Gary
AllTemp
Kitchen 10,6,6 = 100 CFM
Dining 14,6,8 = 200 " "
Den 12,12,10 = 400 " "
Family
2-14,8,8 = 400
" "
Baths
3-8,6,4 = 150 " "
Bedrooms
4-10,6,6 =
400 " "
Total CFM 1650 CFM
Okay I'm adding up all my registers and they total 1650 CFM and I'm not to worry but my cutomer is worried and complain that they don't get any cooling in the den without shutting all the registers in the living room, the den is an addition to the end of the house...
Upon inspecting the plenum and the duct runs coming from it I quickly
learned that the plenum is very undersized and shortcuts were used because
of the limited access and the area of the furnace upflow model..... The
plenum consist of a 8" high 24x24 box with two (2) ten (10") inch flex
duct coming off the top without even elbows and one distributing into the
main trunk line each picking up the rear of the house and one the front....
800 CFM of duct at it's best without even allowing the kinked flex at the
plenum (no elbows) feeding a trunk line designed for 4 tons (1600 CFM)
HOmeowners always complained of lack of cooling and the return a furred
out utility wall sufficent in size but to square to round at the top to
a 12" duct 20 feet into another sq. to round into a furred out wall into
a 20x20 return grill.....
Hmmm what's wrong with this picture? All originally installed
in a tract home by a Lennox contractor.
The fix of course were installing new heating and cooling equipment 4 ton 14 seer coming off the top of the coil with an 18" 90 across aprox 10 feet to the center of the attic for heigth... Installing a distribution plenum 20x20x48 with proper duct runs with dampers to all existing stack head boots and insulating the entire project with foil FSK... An addition 20x20 filter grill will be installed and the 12" and sheet metal t 16"... Will the system work you bet ya! And it will be very quite...
So my point is totally the registers isn't going to tell you squat if
you don't know what the plenum is delivering or capable of delivering!
MDamron,AllTemp
AllTemp
available on that unit from Goodman...
MDamron,AllTemp
Mr Smith wrote:
> It is only about 6-8 ft with 1 bend in it. The R/A grill is mounted in
> ceiling.
>
> Thanks for the info.
>
> And by the way I was sold on the 12 seer Janitrol because of the scroll, 10
> year comp warranty and it was $800 cheaper than anything else.
>
> <theh...@postoffice2.bellatlantic.net> wrote in message
> news:37ABDE1A...@postoffice2.bellatlantic.net...
theh...@postoffice2.bellatlantic.net
Would you recomend a rack of bag filters?
Add up registers to determine CFM???
Can you listen to a fan to determine its RPM also?
Up size round duct for a heatpump??
Are you suggesting to calculate the needed size, then throw
that calculation out the window?
I do believe that I disagree with the above.
Jay
Gary Edelman wrote:
>
> Return should be 2 CFM's per sq. " of filter size.
> 4 ton approx. 400 CFM's per ton = 1600 CFM's
> 1600/2 = 800 sq. " of filter space.
> 24x24= 576 , that's a little small for a 3 ton. It will probably be
> noisey & your filters will need
> to be changed more often than usual but it will still work. Two 20x20's
> is usually a good alternative. The 18" round will work also but a 20"
> round would be what I would use for a 4 ton. If it is a heat pump it is
> a little more important to up the size of the round duct.
Gary Edelman
> So my point is totally the registers isn't going to tell you squat if
> you don't know what the plenum is delivering or capable of delivering!
>
>
>
I can't argue that point but I was just figuring if they set aside
enough CFM's on their branch runs at least you know they got part of it
right & did some correct figuring into the duct system. There's a much
better chance of the design being right that way. I always figure out
all my runs & let the duct crew use their calculator to figure out the
trunk line & transitions. I've been working with them for years & never
have to worry about them getting it right but that's not always the case
with every duct crew.
Gary
Gary Edelman
> 24 x 24 filter not big enough for a 3 ton??
> Would you recomend a rack of bag filters?
>
I did'nt just pull numbers out of the air. A 3 ton (1200 CFM) should
have 600 sq. ft. of filter area. 20x30 is pretty standard unless the
structure doesn't permit it.24x24 is 576, close but a little small.
> Add up registers to determine CFM???
> Can you listen to a fan to determine its RPM also?
>
>
When it doesn't sound like it's turning it usually is below 125 RPM's.
> Up size round duct for a heatpump??
> Are you suggesting to calculate the needed size, then throw
> that calculation out the window?
>
No, what I said was a 20" round would be what I would use but an 18 is
still O.K. It's a little more important to have the correct size return
on a heat pump application than a straight air because of the possible
high head pressures you might get during the heating cycle. I've seen 4
ton straight air's running on a 14x20 return for years but I guarantee a
heat pump wouldn't last with that size return.Just because something
works doesn't mean it's done right.
If you calculate .05 friction for 1600 CFM's you get 19" round & at 700
ft. per minute you get 21" round & thats for metal duct.
Most residential systems are using flex which gives a little less CFM's.
Anyways that's how I came up with my numbers. I'd like to hear where you
get yours. I'm always willing to learn from someone elses point of view
if it makes sense.
Gary
theh...@postoffice2.bellatlantic.net
> When it doesn't sound like it's turning it usually is below 125 RPM's
Makes sense to me :-)
I'll have to get back to you on my calculations. My info. is in the
truck, and I don't even look at mt truck on weekends. (besided I'm goin'
fishin')
Jay
AllTemp
distribution trunks are undersized..... if you go from a 10 inch duct to a
12 inch does that mean your now can calculate having the cfm of a 12 inch
duct , no I don't think so.....
MDamron,AllTemp
Tina944
>5"= 75 "
>6"= 100 "
>7"= 150 "
>8"= 200 "
>
>Gary
I have often seen arbitrary values mistakenly assigned to things such as
cfm,friction rate,and heat load. I never for a second ever thought I would see
it advocated in here though. This come from hackmasters handbook of
fundamentals?
HVACMAN
Gary Edelman
> I still say addding up the registers doesn't mean squat if thier
> distribution trunks are undersized..... if you go from a 10 inch duct
> to a
> 12 inch does that mean your now can calculate having the cfm of a 12
> inch
> duct , no I don't think so.....
>
I wasn't arguing with you on that point. I just give the average
installer the benefit of the doubt if
I know he's mapped out the correct branch runs & assume the trunk system
will be sized right.
From the systems I see I have more confidence in the duct crew than the
guys who start up the systems. Half of them don't even pull a vacuum or
check superheat.
Gary
Gary Edelman
I can hack with the best of 'em.
Tina944
>Does that mean you disagree with me??????
>I can hack with the best of 'em.
That means you don't ASSing arbitrary values. Not to cfm,duct size,equipement
size,load,supply outlets,returns,or grilles. And everything else I forgot.
ASSume no rule of thumb.
Tina944
HVACMAN
just thought for a newcomer "Tina's" flames were huge! She obviously has
an opinion, I don't! ;-)
theh...@postoffice2.bellatlantic.net
I still haven't taken the time to look up the specs, but I don't believe
I need to.
Gary Edelman wrote:
> I did'nt just pull numbers out of the air. A 3 ton (1200 CFM) should
> have 600 sq. ft. of filter area. 20x30 is pretty standard unless the
> structure doesn't permit it.24x24 is 576, close but a little small.
3 tons run off of 20x 20 all the time without excess filter loading.
I did not "look this up". I know it to be fact from first hand
experience.
> > Add up registers to determine CFM???
> > Can you listen to a fan to determine its RPM also?
I don't think I need to comment further on this one, however I do get
your point.
Up size round duct for a heatpump??
> > Are you suggesting to calculate the needed size, then throw
> > that calculation out the window?
I do not disagree with your calculations here. My point is that you
should go with what the calculation determines to be correct. If you
determine that "X" size duct is sufficient, then "up sizing" the duct
would give you an incorrect size.
Jay
bord...@my-deja.com
"Mr Smith" <spam_...@nospam.com> wrote:
> Hello,
>
> We are building a new home @ about 1900 Sq. Ft. It has a 4 ton A/C
unit and
> I think the return air is a little small. The round duct is 18" and
the
> grill size is 24x24.
>
> We have 13 registers coming off of this unit.
>
> Is there a way for me to double check my A/C contractor?
>
> Are there any sites on the web that might help? I have spent a while
> searching with no luck.
>
> Thanks in advance.
>
fine,but on a unit that size it would have been better to have r/a
grilles in 2 locations. (2-20x20)
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Arthur
bord...@my-deja.com wrote in message <7pftbu$2cc$1...@nnrp1.deja.com>...
BGBevill
>Arthur
Probably.