AC Return: 2nd floor, or 1st floor?

[hank42]

Hi,

I recently added a 2nd floor to a 1500sf ranch in the NE. The AC has been relocated to the attic above the new 2nd floor, but the return has not been connected, and some of the supply ducts still need to be run.

The 1st floor is basically one big flowing room - no walls per se - there are supply ducts on the perimeter ceilings from the attic AC.
There is a 400sf open to below area w/balcony/catwalk on the 2nd floor (over living room/starway/foyer) with supply ducts. There are 4 bedrooms with undercut doors and a supply duct in each.

There is a 24x36 shaft going from the attic to the ceiling of the first floor - to be used as a return if needed.

My question is this. Where should I locate the return?

a. In the ceiling of the open to below space, utilizing it as a "funnel" to suck all the hot are from the first floor up?

b. On the first floor, allowing the cool air from the 2nd floor rooms and open to below to fall and get sucked back to the attic through the shaft? (theoretically creating great circulation)

I know that going with option a. will force the AC to take hotter air and work harder to cool it off. But it will at least evacuate that hot air from collecting on the 2nd floor - probably creating an over all stable temperature in the whole house.

With option B, since the return will be sucking up the cooler air from the 1st floor, the AC will likely have to not work as hard, and I should get decent circulation from the 2nd floor supply ducts dropping cool air down to the 1st floor.

Any suggestions?

[Thatguy]

"Location of returns should be towards the interior of the home. Air supplied at the perimeter cools or heats, and then returns towards the center. If you can cool or heat the perimeter, then you can cool or heat the home."

"Whether or not this return air duct is adequate depends on how many such returns were installed in the building and whether or not the return air capacity is balanced with the air handler and air supply ducting."

"During the heating season, the warmer indoor air rises up through the building and escapes at the top either through open windows, ventilation openings, or leakage. The rising warm air reduces the pressure in the base of the building, forcing cold air to infiltrate through either open doors, windows, or other openings and leakage. During the cooling season, the stack effect is reversed, but is typically weaker due to lower temperature differences."

[hank42]

"Location of returns should be towards the interior of the home. Air supplied at the perimeter cools or heats, and then returns towards the center. If you can cool or heat the perimeter, then you can cool or heat the home."

Check. But doesn't address top to bottom location.

"Whether or not this return air duct is adequate depends on how many such returns were installed in the building and whether or not the return air capacity is balanced with the air handler and air supply ducting."

Check - One return, One return only sir. (Except for the undercut doors)

"During the heating season, the warmer indoor air rises up through the building and escapes at the top either through open windows, ventilation openings, or leakage. The rising warm air reduces the pressure in the base of the building, forcing cold air to infiltrate through either open doors, windows, or other openings and leakage. During the cooling season, the stack effect is reversed, but is typically weaker due to lower temperature differences."

Not really applicable, my heat is hydronic and quite toasty, but the reverse stack effect is kind of what I am counting on if I were to locate the return on the 1st floor.

Still looking for pros and cons of putting the master retrurn on the 1st floor vs the 2nd floor.

[jnaas2]

If the return is for air conditioning put it in the second floor, if its for heat in the first floor. Since warm air rises in the summer your upper floors will be hot unless you pull the warm air and cool it and return cooled air back . If you have radiant heat there is no need to put the returns at floor level

[hank42]

Thanks jnaas2. That's what I was lookin' for.

[jadnashua]

In my house, I've found that the return in the ceiling upstairs helps keep the entire place more even. Note, if you have a door on the stairway or often keep rooms closed off, you may need returns in each to get adequate flow.
It also helps if you keep the fan on rather than in the auto position to keep things more even plus it helps with dust, etc. because the air goes through the filter more often.

[hj]

Heating and cooling returns are completely opposite, and since few systems are installed to satisfy both with a means to shut off the unused one, you have to decide which season it the longer and install the returns for that mode, and let the other one work at less than optimal efficiency.