Return question


  #1  
Old 01-31-07, 05:16 PM
J
Member
Thread Starter
Join Date: Jan 2007
Location: Piedmont
Posts: 92
Received 0 Votes on 0 Posts
Return question

Hi guys. I'm planning on running all new ductwork in my crawlspace, to accomodate a new HVAC system. I'm going to follow the original layout for all supply ducts, as the air delivery seemed good. My question concerns the return-air duct....the existing one is 8", seemed a bit small, but the furnace was fuel-oil + uninsulated. The new system will be 2 1/2 ton all-electric, and the house is 1200 SF w/ a total of (8) supply drops. The return should be roughly 25 ft. w/ (2) 90's.

Thanks for any help.
 
  #2  
Old 01-31-07, 07:26 PM
Ed Imeduc's Avatar
Member
Join Date: Aug 2002
Location: Mountain Williams Missouri
Posts: 18,386
Received 0 Votes on 0 Posts
Wink

2.5 ton heatpump..return-air duct....the existing one is 8", seemed a bit small,

Thats for sure. The unit should work at a 1000CFM. You need about a 16"round for the return or 175sq" will be good.
The 8 runs if 6" that only let 800cfm out???

When in a crawl space we always put two small registers for out and one small return in the return duct.
 
  #3  
Old 02-01-07, 04:17 AM
J
Member
Thread Starter
Join Date: Jan 2007
Location: Piedmont
Posts: 92
Received 0 Votes on 0 Posts
Ed.....thanks so much. I thought the return was undersized, and I'll take your advice to condition the crawlspace as well. I'll probably put in a second return grill, that'll give me (2) 8" returns. Thanks again!
 
  #4  
Old 02-01-07, 05:16 AM
airman.1994's Avatar
Member
Join Date: Dec 2005
Location: VA
Posts: 5,795
Received 8 Votes on 8 Posts
Make sure you seal all duct!
 
  #5  
Old 02-01-07, 07:56 AM
J
Member
Thread Starter
Join Date: Jan 2007
Location: Piedmont
Posts: 92
Received 0 Votes on 0 Posts
Yeah, definitely. I'm planning to screw all duct lengths together, wrap with metal tape, then mastic the whole connection, before wrapping all w/ duct wrap. Another follow-up question: any concerns about having some return air from the crawlspace migrating into the house?....it gets pretty damp down there sometimes, probably has some light mold growth here and there <which I'm planning to remediate pretty soon, as well as install a moisture barrier>. Should I be very worried about it? I'm guessing that conditioning the crawlspace will itself dry the air to acceptable levels....

Thanks again
 
  #6  
Old 02-01-07, 11:27 AM
A
Member
Join Date: Jan 2007
Posts: 32
Received 0 Votes on 0 Posts
Originally Posted by Ed Imeduc View Post
2.5 ton heatpump..return-air duct....the existing one is 8", seemed a bit small,

Thats for sure. The unit should work at a 1000CFM. You need about a 16"round for the return or 175sq" will be good.
The 8 runs if 6" that only let 800cfm out???

When in a crawl space we always put two small registers for out and one small return in the return duct.
Ed- Im not sure if you have posted these before but can you share with us the calculations (simpler the better!) that you use to see how much sq inches you need of return duct in a given square footage house? Also, the calculations used to find out how many vents (and how big the ducts need to be) for a given room size would also be helpful. I apprecaite the help!

Nick
 
  #7  
Old 02-01-07, 01:42 PM
Ed Imeduc's Avatar
Member
Join Date: Aug 2002
Location: Mountain Williams Missouri
Posts: 18,386
Received 0 Votes on 0 Posts
Wink

Nick
Its not the home as to what or how the size of the duct will have to be. You have to run a heat loss and a AC load on the home first. Now we know what size the units will be and also what CFM we have to work with. Then we go from there as to the duct size. That we need in the home and how many runs we need .
If you just want to play with it...... A 60sq " per ton is very tight 70sq" per ton good and 80sq" per ton very good. This would be like the duct at the unit for the supply and the return
 
  #8  
Old 02-02-07, 10:30 AM
A
Member
Join Date: Jan 2007
Posts: 32
Received 0 Votes on 0 Posts
Originally Posted by Ed Imeduc View Post
Nick
Its not the home as to what or how the size of the duct will have to be. You have to run a heat loss and a AC load on the home first. Now we know what size the units will be and also what CFM we have to work with. Then we go from there as to the duct size. That we need in the home and how many runs we need .
If you just want to play with it...... A 60sq " per ton is very tight 70sq" per ton good and 80sq" per ton very good. This would be like the duct at the unit for the supply and the return
Ed- Where do you find the info on how much tons your furnace is? I looked at mine and the only thing I see is 93% efficency Amana brand.

Nick
 
  #9  
Old 02-02-07, 04:44 PM
Ed Imeduc's Avatar
Member
Join Date: Aug 2002
Location: Mountain Williams Missouri
Posts: 18,386
Received 0 Votes on 0 Posts
Wink

For the furnace look for the Btu's on it there. You also want to check out and see what the Cfm on the blower is. When we say tons that is just for the AC.
 
  #10  
Old 02-02-07, 09:50 PM
C
Member
Join Date: Jan 2007
Location: LI, NY
Posts: 62
Received 0 Votes on 0 Posts
Originally Posted by joshmack View Post
Ed.....thanks so much. I thought the return was undersized, and I'll take your advice to condition the crawlspace as well. I'll probably put in a second return grill, that'll give me (2) 8" returns. Thanks again!
This is not the same as a 16", no? One 16" will net you about 200 sq" but 2 8" will only get you about 100". Im in a similar situation with 3 6" return lines that seem to be inadequate for my 1200 sq ' house.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: