Reviewed and Revised on 10/30/2013
Sizing the air conditioner close to the actual load provides better humidity control, and typically results in energy savings when compared to an oversized air conditioner with the same performance characteristics. Better humidity control results from a higher percentage of run time during which the coil is operating at its coldest temperature (the colder you get the air, the more moisture you pull out), while energy savings result from less start and stop operation and higher comfort levels due to better humidity control. High humidity often results in lower thermostat settings. Reducing the number of cycles may also result in longer equipment life. However, close sizing also comes with requirements. Since there should not be large, excess capacity on the hottest days the system must be properly maintained—most importantly, keep it clean. Also, energy efficient features of the house that reduce air conditioning load (e.g. proper ductwork, windows, insulation, and sealing against infiltration) must actually be done and done correctly. Have the capacity and quality of the existing duct work evaluated, because the ducts may not be able to handle the added airflow for heating and cooling if a change to a heat pump is made.
Over-sizing generally results in higher equipment costs, but will allow faster pull downs, and can overcome (from a room temperature standpoint) some mistakes such as leaking ductwork, poor AC maintenance, poorly insulated ductwork, improperly installed or missing insulation, and excessive infiltration. Note that not all of these examples are due to the AC contractor.
To assure proper sizing, the load must be calculated—not just estimated based on square feet. The standard for residential air conditioning sizing is ACCA (Air Conditioning Contractors of America) Manual J, or a method based upon Manual J. Load calculations are based on the exact area, orientation, and type of construction for each component of the building envelope, as well as the heat given off by the lights, people, and equipment inside the building, etc.
Application of Manual J is generally considered to result in some over-sizing. The capacity of the unit should be its capacity at the design conditions. In general, as outdoor temperatures increase or indoor temperatures decrease, the capacity of an air-source unit decreases. Full capacity should only be needed during the hottest part of the hottest days. Variable speed units offer more flexibility than single speed units, but are more expensive and more complicated.
The cooling efficiency of a heat pump or an air conditioning system is rated by the Seasonal Energy Efficiency Ratio (SEER). The SEER is defined as a ratio of the average amount of cooling per unit of electricity used. Federal regulation mandates a minimum SEER 13.0 for most residential air conditioners manufactured after January 23, 2006. Efficiency of some systems can be as high as SEER 20 or more. Chances are that older homes have units with SEERs less than 10.
Note: SEER rating is based on equipment performance in the Virginia climate. Some equipment may not produce the listed SEER in actual operation in your climate region.
The Sensible Heat Ratio (SHR) describes the moisture removing capability of air conditioning systems. A SHR on Heating, Ventilation and Air Conditioning (HVAC) equipment of 0.7 means that 70% of the air conditioning load is devoted to cooling and 30% to removing humidity. It is critical that the HVAC contractor accurately estimates the humidity, or latent load. It is important to note that many high SEER units have poorer humidity removal capacity, so verify system performance before purchasing, and ask the HVAC contractor to provide written confirmation. Keep in mind that with air conditioners, which operate based on room temperature, humidity is not controlled directly, and any humidity control is a by-product of temperature control.
This is where it gets a bit complex. The SEER, the SHR, and the system tonnage (a measure of the system’s size) must be in balance, so that problems don’t occur with indoor air quality. Systems without an adequate SHR, or with inaccurate tonnage, cool without removing moisture. This results in cool but "clammy" homes. Typically, when an AC system fails to provide desired levels of dehumidification, the owner response is to lower the thermostat setting. Every degree of thermostat lowering increases the cooling bills by approximately 3-5%.
An oversized air conditioner on the other hand cools a home "too" quickly and does remove the moisture very effectively
Central HVAC systems have a component called an air handling unit or AHU. The advantages of placing the AHU in a conditioned space include: it is in a more benign environment; a central location can minimize duct lengths and optimize air flow; there is easier access for maintenance; and any leaks occur in conditioned space.
An often-neglected area of AC installation concerns the placement of the outside unit (condensor). Manufacturer recommendations for proper clearance distances should be followed to ensure there is no blockage of air flow from the unit. Also, do not vent a clothes dryer within 10 feet of the outdoor unit, because dryer lint will cling to the condensing coil, lowering both the system’s efficiency and service life.
The HVAC contractor should ask you the following questions to help properly conduct a comfort analysis and system design for your family and home:
You should also realize that many of the same questions should be asked when determining which HVAC system should go in a new home. In addition…