Heating and cooling for finished attic space
I live just west of Atlanta Georgia and in the process of finshing approx. 750 sqare feet of attic space. It is basically two rooms. One is over the garage and the other is over heated and cooled space. I plan on putting a lot of care and money to make sure it is insulated properly to make it as air tight as possible. I just got my first estimate today. The gentleman was referred by a friend of mine and has many years in the business. He walked me through the plan and explained everything to me. He told me that the room over the garage was going to need the most air due to the unfinished garage and the heat of the roof. He quoted me a price for a 2.5 ton unit. I spoke with another professional about the tonage and he thought that was to big seeing that it is 1 ton per 300square feet. The gentleman that quoted me the job also told me that he is not a big fan of flexible duct work and always builds is on rigid duct work. Does this sound like an over kill on the tonage or is it about right. He also quoted two prices. The first one with a 13 seer rating and the next with a 14 seer rating. The 14 seer rating was 1000 dollars more and came with a variable fan handler. Any suggestions. I will be getting more quotes. I need to know if a contractor quotes me a 1.5 ton than I don't need to use him. Thanks for you advice.
Sounds a little high to me but he might see something I can't see. Do a load calculation on it and go from there.
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Thank you for your reply. When figuring out the tonage with 9 foot ceilings, how many tons per square feet. I have seen some charts that say 1.5tons per 600 to 700 sqare feet in the region of the country I live in. Is this about right?
There is never a time when tons per sq ft is correct. That's why the only right way to size is with a heat gain study like the Manual J.
We have a floor plan very common in the older section of our suburb. In replacing equipment in these homes, we've used, 2, 2.5 and 3 ton equipment. Same sq ft, same house. All sized correctly.
Thumbs are good for alot of things,sizing A/C's is not one of them. Rule of Thumb does not work.
When you are talking with contractors regarding tonnage, the size recommended should always be dictated by a load calculation.
And, efficiency costs money.
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there have been several threads about heating and cooling bonus/attic/garage rooms..
usually hvac is oversized due to high heat gain in these areas located within
insulating and air sealing the walls is the key
what you have is simply put a box surrounded on all sides by attic temps
if it is 100 degrees outside by mid afternoon the attic is 130 degrees
attic walls that are shared with living space walls should be insulated
better than exterior walls...but it just doesn't happen in the real world.
if you first insulate these walls then seal them for air tightness it will
change the dynamics of the space to be heated and cooled.
its all about air and thermal boundries.
once these improvements are made it will be much more affordable to heat
and cool this space. 300 sq ft is out the box...fix the space..then buy
correctly sized hvac.
oversizing will cost you in higher tons to buy, operate and will not allow
for proper dehumidification.
efficiency does cost money..and upfront money. but it is an investment that
pays back long term when done correctly.
best of luck.
southface institute has good information on air sealing.
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