New HVAC system -- what to choose?

mangogirl3August 6, 2012

Hi all,

I'm a long time lurker, but really in need of help choosing the right HVAC system. The motor blower died last week on my 17+year old 90000btu gas furnace and 3 ton a/c. I've gotten three quotes (with a fourth expected tonight). I'm reasonably confident that any of the systems proposed will be fine for the main floor. My problem is the 2nd floor, which was renovated after a fire, and the air flow has never been adequate since. The guy that came out today looked in the attic, and noted that there are 3 flex ducts (6" I think) coming off of one 4x12 duct. One of them is fairly short and the airflow is fine, but the other two run into the attic and there is almost no air coming out of them. Since I'm replacing the system, is there anything that can help resolve this? I know it won't be perfect, but on hot days it's about 10 degrees warmer on the 2nd floor than the 1st floor.

One guy said they would use a 4 ton blower with a 3 ton a/c to help push the air. Does that sound reasonable?

Any suggestions are greatly apprecitated -- thanks!

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Is the 4x12 duct the main duct for the second floor? It is too small if it is. A 3 ton AC needs 1200 CFM of air. This means the area of the duct should be about 175 sq. inches. You have 48 sq. inches. This duct needs to made bigger. The flex ducts should also be checked for leakages.

A 4 ton blower would puch 1600 CFM. You should not exceed 1200 CFM with a 3 ton unit. This is not going to solve your problems.

    Bookmark   August 6, 2012 at 5:06PM
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Thanks for explaining about the blower Mike, that's what one contractor proposed.

It looks like there are two ducts for the second floor. The other one (also 4x12 I think) feeds two short metal supplies that put out decent air.

We think the problem duct is coming from a 4x12 on the first floor.

    Bookmark   August 6, 2012 at 5:20PM
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Mike is right in that you cannot compensate for too small of a duct system by trying to force more air through.

Think of it as water instead of air. If you pipe diameter (duct size) is 3 inches round, and to get the correct flow of water (air) you need a larger pipe, then trying to force the water to flow faster doesn't really solve the problem.

    Bookmark   August 6, 2012 at 7:18PM
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Thanks, forgot to ask is there a way to make the duct bigger as Mike has suggested? The 4x12 runs between the walls on the first floor up into the front attic space, then changes to the flex duct when it goes up into the upper attic and out to each bedroom. The house is a cape cod.

    Bookmark   August 6, 2012 at 10:08PM
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It may not be feasible to increase the size of the existing duct with opening up walls. Another option is to run another duct through an existing chase, or create a chase inside a closet.

    Bookmark   August 7, 2012 at 4:29PM
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one of the contractors mentioned adding another duct, but we couldn't find a place to hide it for one of the bedrooms on the 2nd floor. Might work for the room on the opposite side of the house. I really don't want to open up walls.

I don't necessarily have to do this part right now, correct? I've been without a/c for almost a week, and really want to make a decision and get it scheduled!

    Bookmark   August 7, 2012 at 5:08PM
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You can, but discuss it with the contractor so it can part of the proposal.

    Bookmark   August 7, 2012 at 5:33PM
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