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sjthompson 08-31-2010 08:22 PM

Changing duct size
I have a 1500sq ft home with a rheem 3 1/2 ton unit and 07eamgr furnace. If I have 5 6" ducts and 2 8" ducts spliting into 2 " ducts could I chande one of the 8" to 10" spliting into 2 8" ?

beenthere 09-01-2010 03:40 AM

If thats all the duct work you have. You need a lot of duct work alterations. Besides just increasing one of the 8" to a 10, and suppling 2-8".

sjthompson 09-01-2010 09:53 AM

Well its a small house and the distribiion box is centerd so the runs are only like 15ft max except the kitchen which is 25ft 8" y into 2 6" ducts one carrying another 10 ft. Dunno maybe my stuff is wrong for the ac I have can tell you that if I open the dampers all the way that its like a mass air rush through each vent except the longest one. Home was built in 1974 hvac new in 2003 except ducts and box. I swaped the nasty solid pipe for flex same sizes and lengths.

beenthere 09-01-2010 04:43 PM

Good example of an undersized duct system.

sjthompson 09-01-2010 07:47 PM

please people help me out guys I don't know what I should do. The bedrooms will blow way harder than the livingroom and kitchen but should I up duct sizes for oppisit side of house that would help balance right?

Here are my sizes
livingroom 20x20 cathedral to 14ft 2-12x6x6
kitchen 20x10 2-12x6x6
master bedroom 15x13 12x6x6
2nd bedroom 12x13 12x6x6
3rd bedroom 11x10 12x6x6
guest bathroom 12x5 8x4x5
master bathroom 12x5 8x4x5
closet split off of master bath 8x4x4

beenthere 09-01-2010 08:52 PM

Is this 3.5 ton replacing a 3 ton unit?

A 5X12 floor register. Can have a supply of 115 to 150 CFM, and not be loud.

What rooms are over cooling?

sjthompson 09-01-2010 09:03 PM

bought the house with the 3 1/2 ton no floor vents only attic. Furnace is 60,000 btu if that matters.

bedrooms will overcool if dampers are open all the way.

beenthere 09-01-2010 09:12 PM

Your 25' of 8" then 2-6" with one being 15' long. The 6" supplies, are too small if you/they want 100 CFM from it.

6" flex, is good for around 75CFM. Any more then that. And you increase the static pressure the blower has to work against.

Most likely. Your whole duct system is undersized. Including the distribution box.

Are any of your supplies loud?

beenthere 09-01-2010 09:13 PM


Originally Posted by sjthompson (Post 494593)
bedrooms will overcool if dampers are open all the way.

Add another supply to your hottest room/rooms.

sjthompson 09-01-2010 09:28 PM

yes they are the ones in the bedrooms and guest bath. My box has space for another run That would be it though. So should I up all my 6" runs to 7" and bump up my 2 long runs to 10" I don't know this is a pain in the butt. I know that if I set the ac to like 72 it will freeze you out like crazy in the back end of the house that is also where the 20 x30 return air is.

another question is my return air supposed to be noticably load when my ac kicks on?

beenthere 09-02-2010 04:40 AM

None of the grilles/registers should be loud. That is another sign of undersized duct work.

Often, doing things right, is a pain in the but.

Increasing all supplies, except the bathrooms to 7" will make balancing easier. And yes, increase the 8" to 10".

Its easier to balance a system with duct work that is sized right, or atleast close to right. Then one the is undersized.

Might want to add another return. That will stop or decrease the noise from your existing return. And increase the amount of air, you can get to your hottest rooms.

sjthompson 09-02-2010 09:12 AM

how would I do that there is no room for another return

sjthompson 09-02-2010 12:36 PM

ok i finally got my info on my unit.
75,000 input
60,000 heating cap.
11x6 blower
2-12x16 return air cabnetsi have a 20x30
3 1/2 ton condensating unit
24x24x36 plenum
house in 1500 sqft
1372 sqft of cooling space
rest is closets
so what do you think on my duct sizing now?

beenthere 09-02-2010 04:04 PM

2-12x16 return grilles are about big enough for 2 tons of A/C. Unless you want lots of noise from them.

A;most always room for another return. But sometimes just need to enlarge the existing.

sjthompson 09-02-2010 06:41 PM

My 20x30 is actually more cubic inches than I need for my unit.

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