Over sizing HVAC systems is all too common among contractors who think they’re doing their clients a favor.
One of the problems with over-sizing is over cycling. Over sized systems have shorter run cycles because it takes less time to bring the temperature to the desired setting. It’s like starting and stopping a car. The shorter the run cycle, the more often your system turns on and off thereby dramatically shortening the lifespan of the system.
Another big problem with oversized units, especially in humid climates, is their inability to remove humidity and condensation from homes. Large units, having shorter run times, are much less efficient at dealing with humidity. Most systems take 10-15 minutes for the coil to get cold enough to start condensing moisture and large systems rarely run for this long. Degraded humidity control can increase the potential for mold growth, allergic reactions, and respiratory problems.
Three main reasons for over-sized equipment are (1) a guess is made on the load, (2) mistakes are made in the load calculation, (3) the equipment is selected for unusual/extreme conditions such as abnormal temperatures or unusual occupancy loads (i.e. gatherings/parties). Other reasons include the use of inappropriate or inadequate “rules of thumb” such as ‘500ft²/ton, 400CFM/ton’, or ‘total cooling capacity = 13 x sensible cooling capacity’.
Furthermore, seemingly trivial mistakes such as ignoring building efficiency upgrades and assuming that the original design and installation are correct, all contribute towards inappropriate equipment sizing.
Properly sized equipment runs for longer periods while maintaining better indoor air quality as they more continuously filter the indoor air.
The right way to properly size your system is by referring to the ACCA manual J, (Air Conditioning Contractors of America). This reference takes into account things like more efficient windows, tighter building envelopes, and ventilation designs. These factors can have a huge impact on your system’s efficiency. If all or most of these are addressed in the building design, significant savings can be realized by installing a smaller system while at the same time maximizing efficiency.
An equally important consideration for system design is duct sizing calculations. (Refer to the ACCA Manual D for duct sizing), and proper duct sealing.
A duct system that is poorly designed or maintained can have a detrimental effect on the health of the people who live in the house, through the unintended distribution of indoor air pollution. Leaky ducts can reduce heating and cooling efficiency by as much as 20%.
Stay tuned for more information on the benefits of proper duct design.