cold air return

[jvtran]

is it possible to have too many cold air return registers?

I have a ranch with 1500 sq ft on main level and 1500 sq feet in basement. main level has cold air registers in each of 3 bedrooms and one in the main area of the house.

basement will have one large family/rec room, bathroom, and bedroom. how many return registers should i have?

[LazyPup]

In a forced air system the supply air is under a positive pressure while the return air is drawn back by a negative pressure. In theory the total cross sectional area of return air for any given space should equal the cross sectional area of the supply. In practice, due to the lack of postive pressure the internal resistance of a return duct is greater than the resistance of a supply duct, therefore the combined area of return grilles should be 10 to 15% greater than the supply duct for the same given space to compensate for internal duct resistance.

In order to acheive a well balanced system the return ducting must be calculated with the same degree of care an planning as the supply otherwise you will end up with excessive return in one room, which would cause short cycling the heated air right back into the system, while another area may not be able to get supply air because there is no adequate path to remove the cooler air.

Proper location of return air ducts is also vital. The returns should be on the opposite side from the supply to insure a good cross flow of air within the space. Often it is better to have more than one return in a larger room to insure there will not be drafts or stratificaton of the air within the room.