Air Conditioning Problems

The air conditioning in our new house doesn’t seem to work very well. What could be wrong ?

Problematic air-conditioning systems abound. According to a recent study, says Chris Green in Energy-Smart Homes (winter 2008, $ 8. 99, www. finehome building. com ), 95 percent of new air-conditioning installations fail in regard to operating efficiency, with more than 70 percent of systems improperly sized or installed.

Because air-conditioning systems integrate refrigeration, air distribution, and electronics, there are lots of opportunities for mistakes.

The top three reasons for poor air-conditioner performance are 1. Improper sizing (1. 5 to 2 times too large is common ).

2. Improper installation.

3. Poorly designed and installed duct systems. Heat naturally moves from a higher energy level (warm ) to a lower energy level (cool ). Heat that accumulates within a house will not leave on its own unless the heat sources (the sun, people, appliances ) are removed. Help comes in the form of air conditioning, which uses refrigeration combined with ventilation to push heat uphill, or move it outside, where it’s even warmer.

Residential air-conditioning systems are made up of indoor and outdoor units connected by a pair of pipes that circulate refrigerant in a loop. By manipulating pressure and temperature, the indoor unit absorbs heat by blowing warm indoor air over a cold coil. The heat is released to the outdoor unit, which houses a compressor (which compresses refrigerant and itself generates heat ) and a condenser coil and fan (which dissipates the heat to the outside ).

In addition to cooling, air conditioners serve another function: They dehumidify the air. In the same way that moisture condenses on the side of a cold soda can sitting outside on a hot day, air conditioners wring moisture from warm humid air as it is forced across the indoor unit’s cold evaporator coil. Once past the evaporator, cool dehumidified air is delivered to the rest of the house — unless there’s a problem. Approximately two-thirds of all residential air conditioners are too large. According to Bruce Harley, a heating, ventilating and air-conditioning consultant with Conservation Services Group in Westborough, Mass., oversize units will cool your house, but they’re not necessarily designed to run efficiently.

The first problem is that they dehumidify poorly. Oversize units satisfy the temperatures at the thermostat so quickly that only a little moisture has time to condense on the evaporator coil. This is known as short cycling, and it’s more of a problem in humid climates. If cycles are very short, moisture on the coil can evaporate back into the house before it drains away.

Second, air-conditioning units are least efficient when they start up. It can take 15 minutes to reach operating efficiency, so oversize units run more short cycles and more time is spent running in the least efficient part of the cycle. As a result they use more energy; costs to operate them run 20 percent to 30 percent higher than for a proper size system.

Finally, at an installed cost of around $ 1, 000 per ton, oversize systems cost more.

The right size system is not a rule-of-thumb amount derived from the square footage of a house. The most efficient way of calculating the proper size for a residential air-conditioning system is found in Air Conditioning Contractors of America’s Manual J — Residential Load Calculation by Hank Rutkowski. It’s a methodical approach to arrive at room-by-room cooling loads for sizing ducts and whole house systems.

Manual J takes into account solar-heat gains, which don’t peak in all rooms at the same time. It also includes the house’s orientation to the sun and shading, which greatly affect the cooling load as well as the insulation values of walls, ceilings and floors. Window types, locations and specifications as well as internal heat gains (people, lighting and appliances ) are figured in. Another reason for poorly performing systems is faulty installation — incorrect refrigerant levels, low airflow and poorly designed and installed duct systems.

Condenser units arrive from the factory with the correct amount of refrigerant for a given length of piping, usually 15 or 25 feet, to connect the indoor and outdoor units. Refrigerant levels often are wrong because the line length in the field can vary and technicians frequently don’t make adjustments according to the manufacturer’s recommendations.

According to Armin Rudd of Boston-based Building Science Corp., if the levels are up to 20 percent low, there’s some loss of cooling. More than that causes an unacceptable loss of cooling along with frosting of the evaporator coil and eventual complete loss of cooling. If refrigerant levels are too high, there is again loss of cooling with possible damage to the compressor.

The speed and volume of air moving through air-conditioning systems is often incorrect, partly due to mismatched indoor and outdoor units, which occurs more often on retrofits than on new installations because only the exterior compressor / condenser unit typically is replaced. Also, airflow at the evaporator coil often is low because it usually isn’t tested, so no one actually knows what it is.

Fan speeds at the evaporator coil should be around 400 cubic feet per minute per ton of cooling capacity. Slightly lower fan speeds improve dehumidification.

Tied to airflow and directly affecting it are duct design and installation. Ducts are the least expensive part of the system and frequently are given short shrift. A properly designed duct system begins with determining the cooling load for each room (not based on square footage ) which can vary greatly. Duct runs need to be as short as possible and insulated, and should be installed within air-conditioned space. Ducts should also be sealed. Leaky ducts waste energy and might draw dust, spores or combustion gas from a gas appliance back into a house.