Adding on to cold air return
 

I am trying to figure out if this will work, and thought I would ask those knowledgeable about HVAC systems. I added an office to my home. To access office I have to go out my family room, walk 8 feet through my garage, go through the former back door of garage, which is now office. I had two heating ducts in family room, I sealed one vent off and and hooked that run into the new office. (It helped out for the family room, as this room was overkilled, 12x12 room, Too hot in winter, too cold in summer).

I never got around to putting cold air return into office, I always cracked a window and it worked so so. Now I want to fix this problem, can I just cut a hole in plenum at top of furnace at cold air side, and install duct work to office? My heating duct is 6 inch round, do I need 6 inch cold air return?

Total run of cold air return duct will be 20 feet from furnace.

Yes that will work.
You can also create a more comfortable flow of warm air circulation if you can duct that return air opening in the office to somewhere near the floor.

I was wondering to, if instead of tapping into the plenum if I could go into the attic and install a saddle tap on the existing run of return line and go off of pipe up there.

Also do I need to install a filter box in my office where the return line comes in, as it will bypass my existing main filter.

As long as it's a main return air duct in the attic it should be OK. If it's a small sub run to another room...No.
I'm not getting why you think your return air duct from the office will bypass your furnace air filter?

My filter is in the ceiling in hallway inside main living area. Sorry about the crappy drawing. :laughing:

you will need to add a return filter grill for the office

Yes, according to your drawing, that would be fine.

Your central return duct is most likely already too small. Run the new return all the way to the furnace. Your heat and A/C will work better for the whole house that way.

If I cut into the plenum (on the side of it, in top picture) what kind of fitting would I use at that point of takeoff, and being as the squirl cage is pointing up towards existing cold air return, is cutting into the side going to work for correct air flow. Thanks.

Use a top take off.

The squirrel cage shouldn't be pointing toward the return, it should point toward the supply.

It's been a long Friday, thanks.

I have virtually no room on the top of plenum for top take off. Is it do able on the side with the right angles for air flow, or is it not going to work that way. Thanks.

Cut the top take off in the side, and just have it facing upward/in the direction the new return is coming from.

everybody has a good post. I would use a 8Inch run instead.This might help remove unwanted air out of the office a little bit better!

Thanks for the great advice everyone! I am going to get started on this before Summer.