If you are building a new home, or if you’ve decided your air conditioner needs to be replaced, we hope that you are familiar with the term, ‘Manual J’. Because if you are not, you might need to talk with a different contractor, or two. (We can help you with that one. Start here: http://www.acdoctor.com/find-contractor/.)
Air conditioning has come a long way, baby, over the past few years. It is now more efficient, cheaper to operate, and does a better job of removing humidity. In fact, there is a good chance that you can manage just fine on a smaller system than the one you are replacing. So if your contractor gave you an estimate based on replacing the same size system, or a quote that’s just based on a SEER number, you are getting short-changed.
A Manual J is a load calculation that takes into account a whole bunch of factors, including but not limited to the square footage, insulation, number of windows, orientation of the home (does it face south?), height of ceilings and number of occupants.
There is a Manual J for the entire house, and a different set of calculations for a single room. The whole-house formula is appropriate if you’ll be using the same ductwork, for instance. The room-by-room calculation is appropriate if you plan to modify your ductwork.
Price should never be your guiding principle, because believe it or not (Ripley), a more expensive system can deliver higher energy bills, higher humidity and higher maintenance costs whereas a suitably large system will perform better overall and cost less to install and operate.
In fact, it’s a good idea to ask the contractor to show you the Manual J and explain the computations. You’ll have a better idea of how he’s envisioning your system. While you’re at it— you, proud owner of that new home under construction— ask your contractor to perform a Manual D. This is a set of calculations used to determine the design of your ductwork.
It takes a little longer for the contractor to work up a Manual J— which is why some may try to tell you that it’s not necessary— but remember that you’ll probably be living with this AC system until the kids have left home, or until you retire and sell the house and move to Tupelo. So isn’t it worth a few extra hours of time invested up front to get it right?