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jlvanals 06-09-2010 12:02 PM

New A/C Unit not working, contractors not assisting, Please help
 
Ok the specs: for my ~1900 sq. ft. house in New Orleans we installed a split level A/C system consisting of two 3-ton (16 SEER) systems. All the duct work, etc was new as of April because the house didn't have central A/C up until that point. My downstairs system works fine, it cools to 75 any time of day, all day, no problem. My upstairs system has been a huge problem.

It will only cool to 75 from about Midnight until 8ish in the morning. Anytime other than that it can't get down below 80, even running in second stage the whole time. Now, I do have an uninsulated attic and the ventilation up there isn't ideal (going to fix that soon), but shouldn't a new 3-ton system that is only responsible for cooling about 900 sq. ft. be able to handle a temperature differential of more than 10 degrees (it rarely gets above 95 in New Orleans and, in any case, the system STILL cant cool to 75 when the night time temperature drops to 85 outside by around 10 PM)???

Please help. The contractor is pressuring us to pay him and I just dont believe a 3-ton unit should be struggling to cool 900 sq. ft., even on a really hot day. What the heck should I do?

beenthere 06-09-2010 01:03 PM

I've never seen a 1900 sq ft house with 6 tons of A/C.

Call the contractor, and have him remove everything.

Then get one that knows how to do a load calc, and then installs the right sized system.

jlvanals 06-09-2010 01:11 PM

The main reason we went with a higher ton A/C system is because within the next few years we're going to add a 12x16 addition onto the house and the upstairs will eventually be around ~1200 sq. ft, which would make 3 tons about right given our very old house as I've read that 400 sq ft. per ton is about right on a house that doesnt have a tight thermal envelope. So are you saying there's no way this should be struggling to cool this square footage?

beenthere 06-09-2010 01:38 PM

After you add that addition on. Your A/C will still be over sized.

Its probably struggling because the duct work is under sized for it.

That pathetic 400 sq ft per ton rule of thumb isn't accurate on 2 year, or 200 year old houses.

Insulating the attic would help. Along with venting it. Since your attic could be exceeding 150 when the sun is out.

I have 160 to over 200 year old farm houses that are 1600 to 1800 sq ft that i only installed 2 or 2.5 ton A/Cs in. And they can cool the house down to 74 when its 95 outside.

If a contractor sized and installed 6 tons in your house. He should almost be shot for being an. I won't finish that statement
Maybe 80% of your walls are actually windows. Then it might need that much A/C.

mikethe ductman 06-09-2010 09:02 PM

Here in Ga. we go about 600 sq. foot per ton with 8'ceilings
I have seen 40 days straight of temps over 90
Now when I lived in Ohio I would never put the same tonnage in a house up their that I would do down here.
Try this, on a hot day around 2 or 3 o'clock go upstairs and put your hand on your ceiling just so you know what your A/C is upagainst.

beenthere 06-10-2010 04:55 AM

Quote:

Originally Posted by mikethe ductman (Post 453853)
Here in Ga. we go about 600 sq. foot per ton with 8'ceilings
I have seen 40 days straight of temps over 90
Now when I lived in Ohio I would never put the same tonnage in a house up their that I would do down here.
Try this, on a hot day around 2 or 3 o'clock go upstairs and put your hand on your ceiling just so you know what your A/C is upagainst.

Yep. The first floor never needs as much cooling per sq ft as the second floor.

During the hotter parts of the day. When the attic is at 135 The ceiling of a 900 sq ft second floor if uninsultaed can gain 58,500 BTUs an hour. But if it was insulated to even just an R19 value, it would only gain 3,079 BTUs an hour.

jlvanals 06-10-2010 11:40 AM

That might be the other problem: my house has 12 foot ceilings upstairs, a hallway that is entirely casement windows, and at least 3 windows in all of the bedrooms (3).

beenthere 06-10-2010 12:30 PM

Do a load calc to find out what capacity you really need.

Red Squirrel 06-10-2010 12:44 PM

A non insulated attic is probably a big part of the problem. I would put down R60ish worth of insulation in there. Not TOO expensive of a project and should not take more then a few days, or you can go with blown, but probably more expensive. Ventilation is a big factor too.

mikethe ductman 06-10-2010 12:53 PM

DUDE!
Its like buying a new car and riding around on a 100 degree day with the windows down and taking the car to the dealer and saying the A/C ain't worth a crap because inside the car the temp won't drop to where you want it.
Insulate your attic and make sure it is vented GOOD.
Sounds like your HVAC contractor did his job now its your turn to do yours.

operagost 06-10-2010 04:12 PM

A radiant barrier in the attic is another option. Since you're heating far more than cooling, it may be just as effective as the insulation.


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