AC Ductwork & Supply Sizing / Ceiling locations

15 years ago

I had a system put in 10 years ago. I have a split level home and it was installed in the upper attic with a trunk line that drops down into the lower attic and feeds the main level (Kitchen/LR/DR). There was 2 ceiling registers for the LR, 1 in the DR and 1 in the kitchen. Basically the LR was the length of the DR and kitchen combined. Never got really cool down there but we had old leaky windows, crappy insulation. Big 11' window on the east side of the house that would warm things up in the afternoon and non-ic recessed cans that I love were buried in blow in fiberglass insulation.

I am doing a full remodel of this floor. Right now this floor is mostly gutted. New cans will be Air tight IC cans. All new windows and doors. Plan is to also install a front covered porch that would shield the sunlight. I am also planning on dense pack cellulose in the walls and blown in cellulose in the lower part of the attic.

The trunk line is pretty much run high just under the bracing. Otherwise there would be no access to this lower attic through the hatch. This ductwork stole space from the entrance so its tight as it is but can get in if really needed. The floor dimensions are 26'x26'. The kitchen and DR and now 1 room. There was a beam put in between the LR and old DR that is flush. The only doorway is between the Hallway and the kitchen. The hall will be open to the LR, with a half wall. There is the one bearing wall between the LR and kitchen.

Right now there are 4 flex ducts coming off the trunk line. 3 of which are on the sides of the end of the trunk. There are 3 6" lines and 1 7" line. The 7" line was feeding the existing kitchen. There are no straps supporting the ducts and I can see that the airflow is majorly restricted on the 90 degree bends into the ceiling registers. Plan is to use new flex duct with higher r-value. Strap it and also pull the duct wrap off the trunk line(s) and seal with mastic. Fun DIY job.

Question is if the duct sizes are adequate for the space and where is the best positions for them to be placed in the room. Do I use 4? Do I use 2 larger ones centered? Should the duct sizes be larger? Want to correct these issues while the ceiling is ripped down and before I get to the point of having the insulation contractors come.

below is a pic of the layout. Black boxes are current approx locations of the ceiling supply registers (There used to be walls and doorways separating the rooms. The blue lines are approx duct connections. The translucent red/orange rectangle is the approx trunk location. I can take other pics of the actual trunkline, etc later and post if needed.

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Comments (2)

ryanhughes
15 years ago
How big is the trunk line leading down to those four take-offs? What types of transitions does it have from the lower attic to the unit? In addition, can you describe the ductwork in the upper attic?

The proper answer is that you should have both a "Manual J" load calculation done and size the ductwork per "Manual D." Balancing dampers can and should be added, if not already in place, to properly distribute the airflow according to heat gain throughout the first and second floors. Balancing is very important for both existing and new installations of equipment/ductwork.

Assuming the 6" flex ducts are all carrying 75 cfm (with sharp 90-degree bends into the boots, could be less) and the 7" is carrying 100 cfm, you may only be getting around 325-350 cfm (max). With a kitchen and two living spaces, it is very likely you need more air down there, and that's why it isn't comfortable. Best to do the proper calculations, but increasing the duct sizes and number of registers should help if the trunkline can support more airflow -- important to keep in mind that this may impact the comfort of the second floor. Another reason why balancing of any system is very important. Design airflow is almost never equal to actual airflow measured.

More info needed to give you particular advice. Hope posters here share their professional experience and thoughts.
toddimt
Original Author
15 years ago
ryanhuges,

I have been looking for a HVAC person that will do manual J & D calculations and sizes equip, duct properly as wel as does duct sealing. Everyone seems to want to do shortcuts so I am trying to fix some things myself. If there was someone in NJ I would be glad to talk with them.

As far as answering your questions. I went up into the attic and did some measurements. The system starts in the upper attic.

Return Duct is 2'H x 18"w x 30" L in Hallway. Connected via 16" flex duct to the filter then to the

Trane TWE040E13FB0 Variable Speed Air Handler.
Supply then Connected to a 4' Long x 24" H x 12" W Duct. Which tapes at the 4' mark down to 12x12.

On this 4' long piece of duct are 5 takeoffs(on the sides) for flex ducts. 1ea 7" to the master Bedroom, 1ea 5" to a bathroom, 2ea 6" (I'm pretty sure but is on the other side of the air handler so move to read) to two other bedrooms and 1 7" flex duct that runs through a MB closet to the FR below.
There then is a 45 degree fitting.

The supply then connects to a 4' long 12x12 duct. There is another 5" duct (on top) to another bathroom.

This connects to another 45 degree fitting. This connects to a 5' long 12x12 duct. There is another 5" duct that was placed in the hallway since it used to be hot there. I have since insulated the attic hatch and stuff so this may not be needed anymore.

This connects to a 90 degree fitting which then starts the drop into a closet to transition to the lower attic/level.

There is approx 16" of 12x12 duct drop until there is another 90 to carry the duct into the lower attic.

This duct is approx 12' of 12x12 duct. This is where there are 4 takeoff's 1 ea 7" and 3ea 6".

There are no dampers that I see. Would be able to tell more when the duct wrap is removed to seal the ducts. All of the ceiling registers appear to be 8x8's except for the bathrooms.

Photos below are in the duct run sequence listed above.
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