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Old 06-29-2006, 04:28 AM   #1
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central air unit question

Need some help here.... have a 2 story 2500 sq ft home- my central air unit never cooled house well ( 12 years now). Finally have decided to replace it. My question is this: is there a unit out there that will realistically cool ( really get house cold) both upstairs and downstairs? Or, is it an option to install a second central air unit for the upstairs and is this even done? (having two units - one for each story?)
Sorry for my ignorance -- but anyone who can help I'd appreciate it !! Thanks!


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Old 06-29-2006, 09:59 PM   #2
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This is a case of improperly sized ductwork, and or improperly sized equiptment.

call a heating contractor to properly asses the situation.

then, as far as your options of replacement if necessary, 2 systems is a very typical installation.

Your heating contractor will fill you in on the rest of your questions.


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Old 07-03-2006, 11:44 AM   #3
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I totally agree with ACEINSTALLER. I've seen many a system installed with undersized ductwork which results in excessive noise and poor operation (contractors often sacrifice size in order to get the sheetmetal installed in the roof/floor spaces).

A single system can be designed to properly heat and cool a multilevel dwelling. However, in my experience I've never seen one that was capable of maintaining both floors at the same temperature (due to the simple reality that hot air rises and cool air falls). To achieve this, you typically need some rather larger supply and return ducts from the lower to upper floor.

Installation of two systems, one for upper and another for lower will undoubtedly provide the most efficient operation. However, the equipment cost is essentially doubled, services/condensate drains need to be provided to the upper level, etc. This is probably not the best option in your case and I'm sure when the problem is identified, you single system will operate just fine.

Your best bet is to consult a professional HVAC contractor.
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Old 07-05-2006, 02:13 AM   #4
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In a typical home of that size, one unit should do the job just fine --- if capacity and ventilation is sized and installed properly.

If you have the room and/or depending on the ducting, you could install VAV (Variable Air Volume) dampers and dump box with remote sensors. This would then automatically direct air volume to the level that needs it the most. Again, in most homes the typical system is fine however some seasonal adjustment of the registers may be necessary --- to compensate for hot air that’s rising or cold air sinking.

Given that some of my work involves “critical load” data center environments where everything is about redundancy, I wouldn’t object to two systems either. Although not exactly necessary if you’re talking about AC units, two furnace systems sure are handy if you live in a northern climate. When your furnace craps out in the middle of the night and in the middle of winter at -35F (and I’ve seen this happen a few times myself), a second (acting backup) system is a really nice thing to have! It lets you go back to sleep and worry about repairs in the morning (no emergency overtime service charges either).

Last edited by MrNoMaintenance; 07-05-2006 at 02:18 AM.
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Old 07-08-2006, 09:13 PM   #5
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Most 2 story homes IMO are better served with 2 units. You can go with variable air volumn (VAV's) and or other mechanical dampening devices, but your cost will be very high.

Deffinately get the correct size unit for your house. The HVAC company will have to fiquire out the size required depending on the amount, size, and quality of windows, r-factor of insulation, and cubic footage of home.

You may be better off installing a window unit upstairs to help out with the heat load.
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