HVAC 101: An HVAC System's Cooling Capacity
Related to proper duct sizing is duct
leakage. You can have every other point on this page covered and if
the ducts leak 30%, which happens more often than you might think,
then you'll still have a sub par system.
Title 24 rules require
a maximum of 6% leakage on entirely new duct systems and a maximum
of 15% leakage on the rest. Not all of California's climate zones
are affected. There are some loop holes. And of course Title 24 is
strictly a California requirement. But such standards are entirely
reasonable goals for any system, especially new systems. If a
contractor can't get duct leakage down to 6% on an entirely new
system that he's installing then he doesn't belong in the
Now even if you have a two ton (or any other size) air conditioner with a blower and duct system that actually delivers two tons worth of air, you'll still not get two tons of air conditioning capacity if the copper lines that carry the refrigerant (sometimes called Freon) are too small. Just as a blower can't push its rated airflow if the ducts are too small, so too an air conditioner will not push its rated amount of Freon if the Freon lines are too small. Generally speaking undersized refrigerant lines aren't as problematic as airflow problems. But nevertheless it's not hard for an air conditioner to lose as much as 5% of its capacity to undersized refrigerant lines.
Finally we come to the evaporator coil (cooling coil). An air conditioning system's capacity can be broken down into two functions: heat removal and humidity removal. (Humidity removal is actually a form of heat removal. But that distinction is not important for this discussion.) Cooling coil selection doesn't affect total capacity as much as it does the ratio of those two functions. Just as lowering the airflow will cause more humidity removal and less heat removal, reducing the coil size will do the same. Either action causes the coil to get colder and condense more water from the air. Conversely, increasing the coil size and airflow will cause the coil surface to be a bit warmer. That may sound bad. But in dry climates it's good because the system will remove more heat and less humidity. Further explanation can be found here and here. The bottom line is that your cooling coil needs to be selected in order to provide you with the correct ratio of heat and humidity removal. In some areas of the country too much of one at the expense of the other will cause an otherwise properly sized air conditioner to not cool well.
Hopefully you now understand that the relationship of the air conditioner, blower, ducts, refrigerant lines and evaporator coil is critical. Break any link of that five link chain and your air conditioner will not deliver the cooling capacity that it's rated for and that you need. Unfortunately a broken chain, so to speak, is the rule rather than the exception. Learn these things for yourself. Present your contractor with them. And you'll increase your chances of getting an unbroken chain. You may even teach him a thing or two.
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