On Monday, August 13, 2012 1:34:50 PM UTC-7, wrote:
A friend has a house with two gas furnaces and AC.

The downstairs one is OK, the upstairs which is based

in the attic is very marginal at best. The biggest problem

is that there is not enough air flow. It's really only a

problem in AC season, when it takes a very long time to

cool. It has to run most of the day just to keep it at

85.



It would appear that

is due to a number of factors. The way they did the

ducting is far from optimal. Right off the furnace the duct

makes two 90deg turns, then feeds down a long duct off

of which come flex hoses that feed 5 bedrooms and two

baths. The furnace is located close to one long side of

the house. Three of those supply hoses have to go

up close to the peak of the roof, going across to the

other side of the house. They did that to avoid going

straight across the attic floor, leaving space for walk-in

accessibility, which is good, but it comes at a cost.

The return ducting is good. The AC itself appears to

be working, the air coming out is 15 deg cooler than

going in, closer to 20 deg right at the unit.



So, looking at this, I can see some possible improvements.

Obviously doing something with the ducting would be

beneficial. But I also looked at the blower and right now

it's a 1050 RPM 1/3 hp. I'm thinking part of the solution

could be to go to a 1650 RPM, 1/2 hp, which should be

a drop in replacement. I know it's not a total solution, but

even if the ducting stuff is straigtened out, given the length

of the runs, etc, I think having the more powerful blower

would still be part of the solution. Being that the blower

motor is easy and cheap to do, I wonder if it makes sense

to just try it and see how much it helps?

Just one question: When you or your friend says that “the upstairs which is based in the attic is very marginal at best” do you have the downstairs on? What I mean is: Are you considering the house as a whole before you make your judgment? In other words: If you don’t have both the downstairs AND the upstairs air conditioner on at the same time the hot air is simply going to rise and make the upstairs hot.