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mi2006 07-17-2012 09:37 PM

Replacing central A/C
 
Hello,
I am planning to replace a furnace and AC in my house but so far I have been getting different opinions regarding the size of AC I should get. The first contractor actually did manual J calculation and recommended replacing my existing 4 ton unit with new seer 13 5 ton unit. According to him my house requires at least 49000 BTU to cool with all windows covered and 60000 BTU with all windows shades open. The other 2 contractors did not perform manual J calculation in front of me and instead told me that I should get a new 4 ton unit since my current ductwork cannot handle anything bigger than that. Any advice will be appreciated.

joecaption 07-17-2012 10:03 PM

How well did the old one heat and cool?
Has there been any changes to the home since it was installed? Better windows, more insulation in the attic ECT.

mi2006 07-17-2012 10:19 PM

The old furnace did fine in terms of heating the house but the AC which dates back to 1988 cannot keep up and on a hot day like today with 90F outside it could only lower tempurature to 78 with thermostat set to 73 on the 1st floor. It felt much hotter than 78 upstair.
As far as improvements, new windows and more insulation was added in the last few years

joecaption 07-17-2012 10:54 PM

Ever had an energy audit?
May pay to have one done. If they can point out your weak areas and you fix them then you could get by with a 4 ton unit.

joecaption 07-17-2012 11:01 PM

Ever had an energy audit?
May pay to have one done. If they can point out your weak areas and you fix them then you could get by with a 4 ton unit and save money.

scottmcd9999 07-18-2012 04:56 AM

I'd trust the Manual J contractor, but would be sure that the ducts will be replaced/modified as needed to handle the new system. As others have said, you might be able to tighten the house and use a 4 ton, but be sure of that before investing money. Have that energy audit done, it would be money well spent.

mi2006 07-18-2012 09:18 AM

Thank you guys for your replies. My question is how do the ducts come into play when choosing an a.c. unit

scottmcd9999 07-18-2012 11:56 AM

The duct system must be sized to accomodate the amount of air the unit moves (i.e. the CFM of the system), and it must be designed to deliver the correct volume of air to each space in the structure (and to return the proper volumes of air from the proper areas of the structure).

A duct system that is too large or small for the unit will cause inadequate air flow delivery, poor performance, poor efficiency, shortened equipment life, etc etc. Around my area, it's rare that an existing house with an R22 system will have an adequately sized return for the newer 410A stuff. When we quote a changeout, we almost always quote a return upgrade as well (in addition to any other duct mods that are needed). That puts me higher than the "magnetic sign brigade", but so be it.

mi2006 07-18-2012 02:00 PM

I had a 3rd contractor come out and measure the house. He was proficient to do manual j calculation which showed same numbers as before and also recommended 4 ton unit since the return ducts can handle up to 1700 ftm if I understood him correctly. According 5 ton unit will cool the house faster but I will have issues with coil freeze and extra moisture


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