Sizing Duct for new room


Old 01-19-18, 05:18 AM
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Join Date: Jan 2018
Location: USA
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Sizing Duct for new room

We added a Sunroom onto our house this last summer. I used a 3D CAD program called Chief Architect and designed the room myself, then contracted a company to build it. It’s approximately 18’ x 20’ with 8’ walls and a 14.5’ peak ceiling.I’ve designed the HVAC for it, but I want to make sure I did everything correctly, and I had a few questions.

Eventually (probably next winter), we will be adding a 30k BTU ventless fireplace, and ceiling fans for use in the summer; now though, I want to get the new ductwork put in.

We have a fully finished basement, except for the Utility room. I cannot access the supply and return trunks except from there. The supply and return trunks run perpendicular to the floor joists, and the supply ducts attach to the top of the trunk and run inside the joists to the registers. I cannot rip out a bunch of ceiling to tie new ducts in straight to the trunk (my wife will kill me). I can, however, use the open space in the tray ceiling in the basement Living room to run from the furnace in the Utility room to the new Sunroom, and only have to tear out a small section of ceiling. I have been informed from an HVAC guy (husband of my wife’s friend) that I should not tie into the existing supply trunk, I should tap into the air plenum right below. I have read online that I should not do this. I don’t trust that other guy very much. So which would be the better option for me?

System Specs:
As you will see, I have attached many photos. You will see a layout of our house with the existing ductwork and registers. Register locations are approximate, as I did not measure the exact location of each one. Next to each register is the CFM of that register. The two with the asterisk (*) are located under cabinets. The vent hole is under the cabinet and the register is placed in the baseboard, so I know the readings are not accurate.

The trunk runs and sizes are approximate, I can only see what is in the Utility room, I cannot see past that. The green notates the new ductwork I want to put in. I placed the registers about 5’ from the north wall, I assumed this was a good location.Each branch to the registers is a 6” sheet metal duct. The supply trunk is 14” x 8” and the return is 16” x 8”. The supply plenum is 12.75” x 19” and the return plenum is 8” x 24.5.”

My furnace is 54,000 BTU output, and my A/C is 2.5 ton. The furnace uses a 1/3 horsepower blower motor (see photo). Using the output of the furnace and the supply (125deg) and return (74 deg) I found that my blower is about 1000 CFM.

Heat Load:
Using an online Manual-J program (, I calculated the total heat load of our house with the added Sunroom to be 40,000 BTUs; the load of the Sunroom itself is 9,300 BTU’s (it’s not heated below); and the total cooling load of the house to be 2.52 ton (see photos). Will this require me to upgrade my A/C unit? We are going to have ceiling fans to help move the air in the summer, and I have read that an A/C unit should be as close to load as possible.

I borrowed an anemometer from work to measure the wind speed coming out of each of the register vents (using the timed average method), and then converted them to CFM using the area of the 6” supply ducts.

I calculated the CFM needs of the room using volume of 3960 ft^2 and because it has a lot of windows and glass doors, I used 8 ACH to get 528 CFM. I want to put in two vents, one on each side of the room, so I divide that in half to get 264 CFM per register.

What interests me is that my calculations state that my furnace only puts out about 1000 CFM. Doing research, I found that one can assume a 1/3 HP furnace blower can average 1200 CFM. When I add up the CFM of all my registers, it comes to 1390 CFM. Our basement is a little cooler than the main level in the winter, but I think most are. Upstairs, everything is fine. The furnace never seems to run for an extended period of time.Based on this information, should I put in a booster fan in the 10” duct to help move air into the Sunroom?

Onto sizing the ductwork:
I used an online ductulator ( with the friction loss set a 0.1 w.g. per 100’ to calculate my ductwork. First, I calculated the duct in the sunroom. Each side will be equal length. 260 CFM, 24 feet of duct (that’s per side) and two 90 degree bends calculated to a duct diameter of 7.1” (874 FPM). I assume I should size up to 8”? Then I calculated the run to the sunroom: 32’ of duct and 3 90 degree bends calculated to a duct diameter of 10” (972 FPM).

Final design:
10” run from furnace area, stays below the joists all the way to the midpoint of the south Sunroom wall, then tee’s off into 7” (or 8”) and each length goes equal distance to the registers. I will use a damper in the system to help balance it out.

I came here because I wanted to make sure everything is done correctly.
  • Should I look into upgrading the A/C?
  • Where should I tie this new ductwork into?
  • Do my calculations work out? Is there something that I missed or something different you suggest?
  • Should I put a booster fan in the 10” duct to help move air?

Thank you for your time.
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