View Full Version : Adding larger branch ducts to existing system
06-03-2005, 09:44 PM
I read a post that said that there is a rule of thumb for sizing ducts with 20-30 sq inches needed for every 6 inch branch duct. I have a branch trunk that is 8x18 in and this serves 4 [6 inch metal] ducts. The main branch duct then tapers to 8x10 inches and 3 [ two long 24 ft 6 inch combination hard and flex ducts and one short 3 ft long 6 inch hard duct] ducts are served by the 8x10 section. If I understand the rule this duct system is at the low end of acceptable size. 8x18=144 and this gives about 20 square inches per 6 inch duct[ 7 ducts]. I had poor air flow from the last three ducts farthest away from the furnace. I opened up the drywall and found that two of the flex/hard duct runs had problems. One long duct was disconnected from the main trunk and the connection between the hard and flex duct had a loose connection with gaps. The second long duct had almost the same problem and was nearly disconnected from the trunk and had a gap where it connected to the flex duct. The 3rd duct at the end of the branch trunk has a big gap where it meets the supply vent.
I was thinking that reconnecting the ducts, sealing them with mastic and converting the flex portions would be all the fix I would need. The airflow is greatly improved just by connecting the ducts back up.I told this to one of the contractors I am considering and he suggested on the phone that I could upsize two of the long 6 inch runs [ one from the 8x18 and one from the 8x10 section] to 7 inches to get more flow into the farthest room. Based on the rule of thumb I think that upsizing the ducts to 7 inches may not help. On the other hand I have high static, .5 on the return and .35 on the supply with a dry coil and maybe increasing my duct size would help. I would appreciate your comments. Is this where a ductulator helps? The alternative would be to run a new 8 inch pipe off the plenum to supply the two long runs. The problem with that would be that the new 8 inch trunk would have 3 to 4 ninety degree turns to get to the long runs.
06-03-2005, 10:28 PM
larger pipe always helps -- sounds like you need to run a load calc to see how much cfm you need, then calculate just what size pipe and fittings will deliver that-- having gentle turns is better than 90 degree, hard right, --
"make it big, bury it deep"
You've probably fixed it just hooking the ducts back up and sealing them.
Man. J and is what you need,not rules of thumb.
I'd look at correcting the high return ESP,as this will allow more "fan" ESP to move the supply air,though maybe not enough.
06-04-2005, 07:19 PM
Will HVAC calc give me a room by room load? Trying to find a way to increase the return air. Found and unblocked one of the wall returns but I probably need even more return. Might be able find a way to increase the return by opening more basement ceiling up but not optimistic.
A lot of the techs here use it,and only have good things to say about it,so I'd think it does have room by room.
How many cfms are required for your system,heating and cooling?
Return duct and grilles sizes?
06-05-2005, 03:14 PM
When ESP is measured should all the supply registers be open?
Here is my return data on the large furnace Carrier 119k btu input 80%. Serves about 2600 sq ft of ranch style house. A/C lennox size unknown because label was bleached out by the sun. I have been told there is a wiring diagram for a 4 or a 5 ton on the unit and most contractors assumed it is a 4 ton.
Filter aprilaire 2200 on the side of the furnace
Return drop 24x8
Main return trunk 25x10
Trunk A 12.5x9 [outside dimensions panned return]feeds main trunk . Seems three returns from the laundry room dining room and edge of the kitchen feed this trunk with grill openings of 13.5x 5.5 and 11.5x5.5 and 11.5x5.5 in. [ weak flow]
Trunk B 19x9 [O.D. panned return] feeds main return. The part of this return farthest from the furnace is 12x8 and this 12x8 metal duct is fed by three 11.5x 5.5 in. grill openings using the wall cavities in the 3 bedrooms. These three grills barely can hold up a kleenex when the fan is on. The return trunk then enlarges to enlarges to 16x8 in. and receives two more returns from the office [also weak flow]and basement with 11.5x5.5 grill openings. Then it goes into the 19x 9 inch section .
Trunk C consists of a short approx 12 foot trunk that basically has panning across two adjacent joist spaces for a dimension of about 30x9 in. This serves a grill opening of 29.25x5.5 that spans two stud bays. This return pulls hard.
Trunk D is a short trunk as well[about 12 feet] that is about 10x30 and serves a grill opening of about 29.25 x 5.5 that spans two stud bays. This also pulls hard.
The return drop has an 9.5 by 5.5 in. open grill on the side of it. This also pulls hard.
The small furnace 71k input Carrier 80%. A/C size uncertain but the contractors think it is a 2 ton. Serves a north facing sun room of about 400 sq ft. I realize it is oversized. The filter is an aprilaire 2200. The main return is 18x8 and the return drop 1s 24x8. The return drop has an open grill on its side which is 11.5x5.5 in.
The sun room is open to the ajacent room so it doesn't really pressurize. There are six supplies and two returns.
The two return grill openings are 11x5.5 in. Weak flow here too. I can't size the return trunks since they are either enclosed in drywall or in a non accessible crawl under the sun room.
No load calcs done so far. I have hesitated to do it myself since I think I could easily make a mistake and I have read that HVAC calc doesn't allow for figuring a partial crawl space.
The 8 inch round flue is unlined. The contractor said He could line it with at most a 6 inch liner. He also said this would not be adequate for the 120k and the 40 k 80% furnaces he proposes and the 40k water heater that I currently have. He says because the chimney is mostly inside that I won't rot out the chimney if it continues to go unlined. He said I might get some water running out the cleanout door in the basement on a very cold day. I had to tuckpoint the mostly inside chimney about five years ago. About six feet of chimney stick out the roof. The mason did a good job and it is holding up well. The two 80 % furnaces were installed in 1983 or 1984. Since the chimney has held up so far pretty well I suspect that he is correct about not needing a liner. My question is, does a two stage furnace condense more because it is sending less heat up the chimney. I want a two stage furnace so I can have a vs blower. Have you seen chimneys survive without a flue liner?
I have no idea how many cfms I need for my system.
[Edited by heetseeker on 06-05-2005 at 03:17 PM]
ditch the flex on the long runs pipe them
Powered by vBulletin® Version 4.2.0 Copyright © 2013 vBulletin Solutions, Inc. All rights reserved.