Expert: Richard Taylor, AIA - 4/6/2005

Question
Is there a standard/rule for determining the size of an a/c unit per square footage?  I have a 1,950 sq.ft. house in north Texas with a 4-ton unit.  I'm enclosing the garage which will add another 420 sq.ft.  I've been told by many people that this will be adequate, but now I'm hearing that a standard is one ton per 400 square feet.  This seems awful strict.  Granted, some city codes may be different than others, but is there some sort of industry rule-of-thumb?

Thank you

Answer
Hi Paul - thanks for your question.

Nope, there's no rule of thumb!  Actually, square footage doesn't have that much to do with the tonnage.  It's the VOLUME of the house that counts.  There's a big difference between a 1950 sf ranch with an 8 foot ceiling and one with two-story spaces, for example.

You also have to consider the insulation value of the walls and ceilings, the type and amount of glass, the compass orientation of the house, typical humidity levels (probably lots of that in north TX!), etc.

Finally, the distance of the manifold from the conditioned space, the type of ductwork, and the age and condition of the equipment will all have an effect on the required tonnage.

Your best bet is to have the system evaluated by a qualified HVAC contractor.  Another option is to contact your local utility - in many areas, the gas or electric company will do a free energy evalution for you that will give you some if not all of the information you need.

Having said all that, I would think that 4 tons for 2400 sf is more than enough.  Actually I'm surprised you have that much for your existing home.  In air conditioning, more tonnage isn't better.  Oversized equipment will cycle more often, instead of maintaining a steady flow of cool air throughout the day.  Frequent cycling is expensive, and results in big temperature swings.  In a humid climate this can be very annoying as the house gets cold but not necessarily dry in the summer months.  Humid air is harder to cool so the equipment works much harder.

Hope this helps!

Richard Taylor, AIA
Richard Taylor Architects, LLC
www.rtastudio.com