What Size Air Conditioner Do You Need?

Under- or oversizing can cost you performance, comfort and efficiency related issues when it comes to both the furnace and the air conditioner. Undersizing will, of course, result in the unit being unable to meet the desired temperature setting. Aside from the issue of your comfort, undersizing the equipment and/or material will most likely lead to the premature failing of the units. This being said, it is as important to make certain that the units are not oversized, either. It is rather a quite often observed misconception of consumers that the more powerful the furnace or the air conditioner, the better heating and cooling performance that will be achieved.

Oversizing a furnace will result in many different shortcomings. First of all, what is most important in having a uniform distribution of the warm air is the optimum runtime of the furnace. The same statement is also true for an air conditioning unit with respect the uniform distribution of the cool air. When going with a more powerful hvac unit, it will satisfy the temperature setting quicker than when it should and it will not run that optimum length of cycle for you to have a uniform temperature everywhere in your home and the existence of those hot and cold spots in the house is inevitable in that case. With an oversizing of the gas furnace, there also exists a great chance for the overheating of the furnace. This is mostly caused because in the case of oversizing of a furnace, the existing ducts in the home will not support such high volumes of air flow causing pressure build-up in the return air duct and therefore overheating. Also, do keep in mind that the maximum level of efficiency with the gas consumption of a gas furnace is achieved when the right size furnace is installed in your home. In fact, the right size furnace should almost continuously run during the coldest day of the year. Of course, it should be able to satisfy your desired temperature setpoint.

Aside from the non-uniform air distribution, oversizing an air conditioner will lead to uncomfortable and higher humidity levels in your home. As you know, an air conditioner is also meant to dehumidify the air. This is done through the indoor evaporator coil – installed inside the plenum (duct above the furnace) – that removes moisture and water vapour from the airflow passing through. When the air conditioner is oversized, it will stop running sooner than the optimum length of time it needs to run in order to also dehumidify the air.

Another misconception is also the belief that by going with a bigger size furnace or air conditioner, their will saving on fuel/electricity costs as a result of the units working less, which is certainly not the case. You always require the same amount of energy in order to heat or cool your house. The factor that determines this consumption is not the power rating; it is the efficiency of the unit. When the furnace or A/C is more powerful, it will use more power in a shorter period whereas a smaller size furnace or A/C would end up using the same energy but in the manner of less power in a longer period – which works out to have the same consumption levels. In fact, in the case of an oversized unit, some energy is wasted and the maximum efficiency of the unit would not be achieved.

Keep in mind that the following furnace output power estimates are aside from the furnace’s nominal power ratings. For example, a 95% rated efficiency furnace with a power rating of 80,000 BTU/hr has a power output of 0.95 x 80,000 BTU/hr = 76,000 BTU/hr.

Again, the following tables are meant to provide estimates and a general understanding and overview of this topic; however in rare occasions and depending on some of the above mentioned conditions in your home, such as having open to above areas, the following may not be very accurate in generalizing your heat gain/loss calculations. Contact us to find out which furnace and/or AC will be a better fit in your home.