Diagnosis of a building can help designers and architects to decide which aspects should be retrofitted in order to improve the energy efficiency of the heating, ventilation and air-conditioning (HVAC) system.

The most important aspects to watch out for and recommended actions are:

• Duct leakage and seal ducts: aeroseal/tape/mastic
• Duct insulation: add insulation to ducts
• Air flows at registers: replace registers, open/close dampers, reduce system flow resistance by straightening existing ducts or replacing them with straight runs of new ducts
• Low air handler flow: replace filters, fix duct restrictions, change fan speed, replace fan with a high-efficiency unit, add extra returns in return-restricted systems
• Bad filter condition: replace filter
• Thermostat setting: raise thermostat in summer and lower it in winter to account for better distribution, mixing and envelope improvements
• Spot ventilation: replace fans if necessary; if possible, remove spot ventilation and use ducts and central ventilation
• High power consumption in spot ventilation: install a higher efficiency unit, remove/reduce duct flow restrictions, clean fan and ducting
• Equipment capacity: replace with the correct size
• Refrigerant charge: add/remove refrigerant
• Age and condition of HVAC system: clean the system and repair damage or replace the system if greater than 15 years old
• Location of HVAC system equipment and ducts: seal and insulate duct locations; if applicable, move system location
• Window A/C units: replace with central unit or improved distribution
• Multiple systems/zoning: ensure correct damper operation, check the capacity of each system/zone load calculation
• Moisture testing: improve source control; better venting in sensitive zones, fix flashing/detailing, seal crawlspaces in high humidity climates, replace windows, add insulation to walls, floors, and ceiling
• Occupant survey: asking customers to report problems; create moisture removal strategies; install new windows, change register type, airflow and location to improve mixing/remove drafts, add envelope insulation, etc.