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I own a 50 year old slip level home with 3 floors and a gas furnace that uses bottom wall vents
to distribute the warm air. I have received two bids on an ac installation.
The first company said that the house will not cool evenly if I use the existing
duct work. They recommended that they install the air handler it in the attic and create new
ceiling vents. The second company said that it cooling distribution will be the
same on the top as it is on the basement level. They recommended I use the
existing vents. I am non technical and there is a big difference in the price
quote. Any advice would be welcomed.
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One simple fact to remember is cooler air is more dense than warm air. Therefore cooler air will tend to migrate to lower levels.
The best solution for AC is the air handler in the attic and ceiling vents and new returns on the 3rd floor. The cooler air will tend to fall down to the 2nd and 1st floor. If you have a full basement the first floor should be cooler naturally. Otherwise add an AC unit to your existing furnace. You will have 2 systems and many times the upstairs one will be enough to keep the house comfortable. With the 2nd system you have backup for very hot and humid days. You may only need a window unit for the 1st floor. You can start with only the attic air handler and see how the next cooling season goes.
If you have the AC added to the existing system, you will be trying to move heavier(cooler) air up to the 3rd floor. That will create higher static pressure causing more air flow from the lower floor registers. It will not keep what will be the hottest part of the house (3rd floor) cool enough.
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