Duct sizes

[alwayssomething]

I have to move some duct work during my bathroom remodel. The existing duct coming off the main trunk is 5", but when I go to my local home depot the only things I can find in that size are reducers and 90 degree angles. It seemed like all of the registers connected to 6" duct work. Is 6" standard now? should I expand my 5" duct to 6" before adding a new register? Should I cut a new 6" hole in the main trunk and just scrap the 5" stuff? will it kill my airflow going from 5" off the main trunk into a 6" pipe?

I'm hoping to heat/cool two small bathrooms (one is directly above the other, off one line.

Thanks!

[suemarkp]

A 6" duct flows more air than a 5" duct. A 6" duct will steal a little more air off your main truck than a 5" duct. Do you know how much airflow you need for the two bathrooms? A 5" would typically be plenty for one bathroom, and perhaps two if they are rather small.

If you change from a 5" takeoff to a 6", I don't think you'll hurt things much downstream, and you'll have the ability to flow enough air to keep the bathrooms comfortable. A 6" pipe usually flows between 60 to 100 CFM, so each bathroom would get 30 to 50 CFM if sharing a single 6" duct run.

Worst case, put a balancing damper on the 6" take off so you can limit the air if you're getting too much to the bathrooms.

How will you split the air to the bathrooms -- with a Y pipe or a T? Note that more air will go through the straight part of a T than the bend, so one bathroom could get much more air than the other. If this is your plan, you may want a second damper on the run with less resistance (the straight part of the Tee).