ACCA Manual J

Why are heat loss and heat gain calculations important?

Achieving occupant satisfaction is the principal goal of any HVAC design. Primary factors impacting occupant satisfaction include: Filtration, temperature and humidity control, air motion in the room, adequate ventilation, interior zoning needs and energy efficient operation. Occupant satisfaction is maximized when the heating and cooling system and equipment are the correct type and size and the air distribution system is properly designed and installed. For the residential applications, ACCA’s Manual J 8 Edition is the only procedure recognized by the American National Standards Institute (ANSI) and is specifically required by residential building codes. Methods not based on actual construction detail, nor founded on relevant physical laws and engineering principles are unlikely to result in correct equipment sizing.

Problems with over-sized equipment

Over-sized equipment results in marginal part load temperature control. While the temperature control at the thermostat may satisfactory, occupants can experience many other issues associated with over-sized equipment:

• Noticeable temperature differences between rooms due to equipment short cycling
• Degraded humidity control
• Higher installation costs
• Higher operating and maintenance cost

Problems with undersized equipment

Undersized equipment will have a negative impact on the occupant’s ability to maintain optimal comfort under high load conditions. Occupants may experience other issues with undersized equipment:

• Inability to maintain indoor design temperature
• The equipment runs continuously without being able to meet the desired temperature setpoint.
• Higher operating and maintenance cost.

Manual J Verification

It is important to verify key items in the MJ8 calculation and their level of impact

• Design temperature (indoor and outdoor): High Impact
• Windows, glass doors and large skylights: High Impact
• Duct location, leakage and R-values: High Impact
• Ceilings under an attic, R-values, roof materials, roof color: High Impact
• Small skylights, infiltration and ventilation: Medium Impact
• Appropriated insulated floors, walls and internal gains: Low Impact