HVAC Study

When you go to some sites shopping for ac system, you often see some of the recommendations combine condenser, evaporator coil and furnace altogether, and most importantly in each of the combination, the recommended evaporator coil size will be bigger than the condenser unit. For example, a 3.5 ton condenser unit most likely comes with a recommended 4 to 5 ton evaporator coil. This is done on purpose so that the whole system will be very efficient. So the rule of thumb is that the evaporator coil size should be slightly bigger than the size of the condenser unit.

Normally on the AC condenser unit, you will see some numbers for LRA and RLA, you should have these for both compressor and fan motor. Here are what they mean: LRA - Lock Rotor Amps.   The current you can expect under starting conditions when you apply full voltage. It occurs instantly during start up. This is normally a lot higher than RLA. For example, 104 vs 19.2 RLA - Rated Load Amps The maximum current a compressor should draw under any operating conditions. Often mistakenly called running or run load amps which leads people to believe, incorrectly, that the compressor should always pull these amps. FLA - Full Load Amps Changed in 1976 to "RLA - Rated Load Amps". 

AC and heating size chart:       Typically we need about 1 CFM of air flow per square foot of floor area of conditioned space provided that the ceiling height is about 8 feet above the floor, with a typical number of windows and doors and typical building insulation and heat gain or loss. That is 1 sqft x 8ft = 8 cubic feet space needs 1 CFM. You can use this to roughly calculate how much CFM needs for your space. (Tong added) Examples of components for which air velocity (speed through the device) is particularly important are given in the HVAC Air Velocity table below. Typical HVAC System Air Velocity Specifications (速度， 不是CFM） HVAC Component Recommended Air Velocity / Flow Comments Air Ducts - main trunk, typical 700 - 900 fpm Residential Air Ducts - branch, typical 600 - 700 fpm Residential Air Ducts - target 400 fpm Residential Ducts run through conditioned space Air Ducts