The optimal sized equipment is the smallest defensible size. This means using a method recognized as comprehensive and accurate. The ACCA Manual J does this, for more on Manual J. Here are the benefits of using this method.
Provide specified comfort and humidity control at design and part-load conditions.
The nature of residential hvac equipment is that they operate either on or off. Over-sized equipment provides too much capacity when it is turned on. For example, a furnace once on is providing much more heat than is needed to compensate current heat loss. This causes the indoor temperature to rise quickly often overshooting the desired temperature. Then the furnace turns off, allowing heat loss to cool the house and reduce the temperature. This oscillation of temperature will be perceived as discomfort. Some thermostats have tried to deal with this by turning the furnace on and off quicker. However, this leads to faster wear and tear on the furnace.
In the summertime, the air conditioner removes heat and humidity. That water dripping off of the cooling coil is humidity from the house. In most areas, during most of the time, the normal operating time of the air conditioner can keep the humidity at a comfortable level. But an over-sized air conditioner runs less frequently, so less humidity is removed from the indoor air. The thermostat senses temperature not humidity, so it is no help. High humidity throughout the house may be a indication of an over-sized air conditioner.
Provide reasonable comfort at outdoor design conditions.
The Manual J procedure uses the 99% and 1% weather data for your location. This means the actual temperature is the same or higher than the winter design temperature 99% of the time based on statistical weather information. The actual temperature exceeds the summer design temperature only 1% of the time. Using this data, prevents equipment over-sizing due to infrequent temperature extremes. It may seem the equipment may not be able to handle rare extreme temperatures. In reality, this is not true. The homeowner can often take measures to help reduce heat gain or loss during extreme weather. For example, draw blinds or curtains on hot or cold days or adjust schedule so appliances are not used during peak summer afternoon. If existing equipment cannot maintain comfort yet is sized for these temperatures, it may be due to degradation of efficiency due to poor maintenance or increased infiltration due to unknown openings in walls or ductwork.
Optimize installed and operating costs.
Quite simply, larger equipment has a higher cost. The initial higher cost for larger equipment is obvious. The operating costs over the life of the equipment is also greater for larger equipment. Assume you are only meeting heat gain and heat loss, with the same running (steady-state) efficiency of equipment, then size doesn’t matter. But there is far more to the situation. Larger equipment will be starting and stopping more frequently. Efficiency and energy use is higher during the start sequence of the equipment. When it stops, residual energy in the equipment is lost. So even if run time efficiency is the same, energy use is higher for larger equipment due to more frequent starts and stops.
Improve reliability of equipment.
Most of the wear and tear on equipment is the turning on and off. As explained in the paragraph above, larger equipment cycles on and off more frequently. Modern HVAC equipment is designed to be reliable while it is running. Much of the wear and tear on equipment is during startup. For example, an air conditioner compressor needs oil to operate and the motor needs cool refrigerant. During startup most of the oil has dripped to the bottom of the compressor, so the initial few minutes the compressor has an insufficient amount of oil. Until the operating temperatures are reached, the motor is not being cooled as designed by the cool refrigerant. Yet during startup is when the motor pulls much more current resulting in heat.
Reduced size of ductwork.
The amount of air delivered tends to be lower in smaller systems. Ductwork sized to lower airflows is smaller which leads to lower material cost and in some case lower installation cost (smaller ducts are easier to fit into tight spaces).
Reduce load on utility system.There have also been discussions of the utility limiting your power
Simply, larger equipment puts a larger load on the utility system. This is especially critical for summertime cooling. Residential air conditioners running in the middle of the afternoon place a large load on the electric system. This leads to higher billing. Some utilities have gone to charging more for summer afternoon power.
Demonstrate “due diligence” in a court of law.
If a dispute erupts between you and the homeowner, you need to be able to show proof you conducted business in a professional manner consist with the trade practices. Sizing equipment based on a rule of thumb or even your past experiences alone is not currently accepted trade practice. Providing an accurate Manual J provides the proof to support and justify your actions.
These numerous benefits of completing a Manual J calculation are great. Go here to have your ACCA Manual J Analysis done for you.