Q. Do I Need to Adjust the Size of My HVAC System for Spray Foam?
A. Yes, properly sizing HVAC systems in well-sealed and well-insulated homes leads to greater comfort, reduced initial expense, lower operating costs, longer equipment life, fewer callbacks, and healthier indoor air quality. One key step in properly sizing the HVAC system is using the industry standard, Manual J. Manual J software calculates heat loss from the house through walls, ceilings, leaky ductwork, and infiltration through windows, doors, and other penetrations. Manual J also helps calculate heat gain into the house from sunlight, people, lights and appliances, doors, walls, and windows. It is common to reduce the size of an HVAC system by more than 30% through proper design based on insulation and air-infiltration rates – that can mean significant cost savings on the equipment. A typical house with fiberglass has an ACH (air changes per hour) rate of 0.5-0.6, meaning 50% to 60% of the indoor air is exchanged with outdoor air every hour. Typical homes with spray foam have an ACH rate of 0.1 to 0.15, meaning 10% to 15% of the indoor air is exchanged every hour.
While over sizing equipment on the heating side leads to a loss in efficiency, over sizing on the cooling side can result in moisture problems. Air conditioners cool a house first, then dehumidify it. If an HVAC unit is too large, it will short cycle. It will turn on, cool the house down, and turn off again before removing much humidity from the air. Frequent starting and stopping increases energy consumption, makes the home uncomfortable and clammy, and contributes to mold and indoor air quality problems. Often, a house with spray foam can be sized with one ton of cooling for every 1,000sf of living space. Consult with EcoLogic for further assistance on this issue.