I allways shoot for 400 a ton but I learned that you got to have a minimum of 300 and a max of 400 a ton.
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# Heat pumps and air conditioners are generally sized in tons. Typical sizes for single family residences are between two and five tons. Each ton equals 12,000 Btuh. It is important to note that actual capacity is not constant and will change based on outdoor or indoor temperatures. The published capacity rating of air conditioners and heat pumps is based on performance at the ARI standard temperature levels of 95 F outside, 80 F inside. www.hannabery.com/hvacwords_2.shtml
actually fellas you should design your cfm per ton around the calculated load design... example... if i had a home with alot of living biological plants ie trees open water etc.. one would design for higer for latent.. if i had alot of glass but very little lantent loads i would design for higher ensible loads etc. etc. etc. you just need to know what your cooling before you can say cfm per ton. ive had loads come in at 550 per ton and ive had some at 350 per ton.. same tonnage just different cfm per ton.. so the lesson is design first then cfm to accomadate..