- Q. Some hvac contractors in this area of Virginia size cooling systems using an indoor design temperature of 78ºF. I’m under the impression that a system should be designed for an indoor temperature of 75ºF. Is there a standard interior design temperature for cooling systems?
A.Martin Weiland, manager of technical services at ASHRAE, responds: Both the ASHRAE residential load calculation method described in the 1997 ASHRAE Fundamentals Handbook and the ACCA Manual J are based on an indoor temperature of 75ºF. I am not aware of a system-sizing method that uses a 78ºF indoor design temperature.