# Thread: trying to understand airflow of existing system

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## trying to understand airflow of existing system

I am trying to learn enough about HVAC design principles to understand the state of my HVAC.

Today, I am focusing on airflow on the second floor of a 2-story traditional colonial house over an unconditioned basement.

I am about to install a foot of blown-in insulation in the attic which will make future access to ductwork difficult. While surverying the attic for the insulation install, I have finally also looked at what I suspect is an unbalanced duct design.

The east-most room is over a garage and is 5-degrees colder in the heating season and about 5 degrees warmer in the cooling season. On the other extreme is a bathroom consisting of two small rooms, each one blasted by a short duct run from the nearby supply plenum.

Here is what is in the attic:

Air Handler: Trane XV80 Furnace Model TUD060R9V3K4
w/ 12 speed blower with 825 CFM maximum

Return Plenum:
R1 (D=10-IN, L=1.5-FT)
R11 (D=8-IN, L=3.5-FT, A=535 SQ-FT, V=4600 CU-FT)
*R12 (D=8-IN, L=7-FT, A=180 SQ-FT, V=1430 CU-FT)
R2 (D=10-IN, L=20-FT)
R21 (D=8-IN, L=5-FT, A=180 SQ-FT, V=1430 CU-FT)
R22 (D=8-IN, L=2-FT, A=190 SQ-FT, V=1240 CU-FT)
**R3 (D=8-IN, L=20-FT, A=320 SQ-FT, V=2560 CU-FT)

*R12 is positioned at the top of a stairway in a hallway
** R3 is the return for the room over the garage

Supply Plenum:
S1 (D=12-IN, L=14-FT)
S11 (D=8-IN, L=26-FT)
**S111 (D=6-IN, L=6-FT, NORTH SIDE OF A=320 SQ-FT, V=2560 CU-FT)
**S112 (D=6-IN, L=12-FT, EAST SIDE OF A=320 SQ-FT, V=2560 CU-FT)
S12 (D=8-IN, L=22-FT, SOUTH SIDE OF A=380 SQ-FT, V=3300 CU-FT)
S2 (D=8-IN, L=17-FT)
S21 (D=6-IN, L=11-FT, A=SOUTH SIDE OF 180 SQ-FT, V=1430 CU-FT)
S22 (D=6-IN, L=13-FT, A=SOUTH SIDE OF 180 SQ-FT, V=1430 CU-FT)
S3 (D=6-IN, L=24-FT, A=140 SQ-FT V=1200 CU-FT)
S4 (D=6-IN, L=20-FT, A=8 SQ-FT, V=16 CU-FT)
S5 (D=6-IN, L=17-FT, A=180 SQ-FT, V=1430 CU-FT)
***S6 (D=6-IN, L=6-FT, A=40 SQ-FT, V=320 CU-FT)
***S7 (D=6-IN, L=4-FT, A=35 SQ-FT, V=280 CU-FT)

** S211 and S212 supply the problematic room over the garage
*** overheated and overcooled bathroom

The XV80 was installed 1.5 years ago to replace a 20-year old furnace.
The furnace Nominal Capacity Output(BTUH) is Stage 1=48,000 and Stage 2=31,200. The furnace is coupled with a XL16i Heat Pump (outside: model 4TWX6024B1000AA, inside: model RXC031S3HPC0) with Nominal Cooling Capacity of 24,000 BTUH. The natural gas furnace to heat pump cut over temperature is set to 45-degreesF so the furnace is most often used for heating. The contractor used flex duct to connect to existing tubular sheet-metal ducting. All returns and supplies are located on the ceiling.

I registered HVAC-Calc Residential to calculate a current heat loss of 25,000 BTUH and heat gain of 18,000 BTUH on the second floor. I calculate that the added attic insulation will reduce heat loss to 18,000 BTUH and heat gain to 12,500 BTUH.

After browsing this site, I understand that measurements should be taken by a contractor to confirm airflow and pressure. I'd like to learn enough to understand the implication of the measured values when I get that done.

I'd appreciate suggestions for questions to ask and answer that will help improve airflow as well as identify qualified contractors.
Last edited by RobG; 02-18-2007 at 06:46 PM. Reason: updated info

2. Originally Posted by RobG

After browsing this site, I understand that measurements should be taken by a contractor to confirm airflow and pressure. I'd like to learn enough to understand the implication of the measured values when I get that done.

Try checking out www.nationalcomfortinstitute.com there is a wealth of information on how your comfort can be improved as well as finding someone in your area.

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Post some pictures ,sounds like a mess,label pictures to match what you have posted.

The required cfms (aitr flow) for each room has to do with the amount of outside wall,windows ,attic and floor(if not on a slab.

You can compare those factors and you'll see the room over the garage doesn't have large enough ducts compared to others.

Use the link in davidr's post.

You need a room by room man.J,the Man. S ,and then Man. D to redesign the duct system,it takes more then just sq inches of duct comparison to get it right!!

4. Professional Member
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then before insulating:
seal all joints & seams of ductwork,
be sure bends are smooth with large radius

have dampers to adjust airflow near the branch takeoffs -- set em per max cfm requirements per load calc -- & required air

insulate the floor between the gar & bdrm -- I did --

put any wiring NOT in the middle of the insulation

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