2nd Floor AC not working well


Old 06-03-02, 01:34 PM
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2nd Floor AC not working well

Just had a 5 ton Carrier unit ( split installation, outside and basement) installed in my house. House is similar to a cape style, with a small, crawl space attic. House was already ducted, but the second floor ducts are the circular, ceiling mount type. The thermostat is located on the first floor. Cooling works perfectly on the first floor, but the second floor is not cooling off well. (Probably about a 6-10 degree difference between the two.) The company that installed the AC is sending someone out to look at it, (though all dampers and ducts have been checked already) and they mentioned something about putting a back-up unit in the attic crawlspace. This sounds to me like I'd be going through the cost of purchasing a second central AC unit, which I'm not happy about.
Questions are this:
1. Any ideas about the likely cost of a back-up unit to cool the second floor? It's approx. 900 sq. ft.
2. Won't adding a back-up unit mean that the 5 ton unit that we just installed will be way too overpowered for the job, then, and end yp short cycling all the time?
3. Any other suggestions on how to "beef up" the cool airflow to the second floor?
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Old 06-03-02, 02:58 PM
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Adding a second unit now to the upstairs doesn't sound to appealing. The downstairs unit would be oversized as you thought.

The main problem with 2 story homes is temp difference between the two levels. Two systems would be ideal but since you already have one you'll need to look into duct design. The first thing to look for is how the main trunk duct gets to the upstairs, asumming there's only one, the other thing is sizing of that trunk. If there's only 900 sq.ft. you'll probably need 14" trunk and a good clean run. (I'd bet you have no more than 12" now and possibly 10") That trunk will need to be properly taken off the main system and also properly distributed once it gets to the upstairs to get decent air flow. More than likely you'll need balancing dampers in the downstairs and one in the main trunk to the upstairs to balance air from summer to winter between the two floors.

By the way, if your a/c company who installed the unit is telling you to put in a backup system, get rid of him and find someone who knows what he's doing.
Old 06-03-02, 06:03 PM
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check previous posts

there was a real good one on this about 10 days ago. the unit you have now is supposed to be sized for the whole house, adding another one will cause problems

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