When an air conditioner is described as 3 tons, we realize that we are not describing the weight of the unit. It is a measurement that describes how much heat the unit can remove from a living space in one hour. A 3 ton AC unit will be able to remove 36000 BTUs from a living space in one hour. A BTU which is also known as a British Thermal Unit is the energy that is required to increase the temperature of one pound of water one degree Fahrenheit. However, for the majority of people, 3 tons will mean 6,000 lbs.
In a bygone era, ice was harvested from rivers and other bodies of water and used for cooling purposes, prior to the advent of air conditioning units. A ton of ice will take about 286,000 BTUs to completely melt. Using 24 hours as the reference of time, if the ice melts in 24 hours, it will absorb heat at 286,000 BTUs per 24 hours or 11,917 BTUs per hour. This number was rounded up to 12,000 BTU/hr. Therefore, to this day, one ton of cooling capacity is equal to 12,000 BTU/hr.
Now, we will provide you with some information as to why your 3 ton AC unit will rarely have a 3 ton capacity. We are not going to describe the numerous reason that have to do with faulty installation, design, or poor maintenance. We are going to describe why your 3 ton AC unit may not have the capacity you think it has, even when it is installed, designed and maintained well.
Actual vs. Nominal Capacity
When we describe AC capacity, we are normally talking about the nominal value. An AC unit that is 3 tons will have a nominal capacity of approximately 36000 BTU/hr, however its actual rating when using AHRI specified operating conditions will very rarely be the same value as nominal capacity. For instance, if an AC unit is called a three ton unit or 36000 BTU/hr it will have an actual capacity that will equal 2.8 tons or 34000 BTU/hr.
Actual Conditions Of Operation As Opposed To AHRI's Indoor Conditions Of Operation
AHRI ratings are performed for an indoor temperature of 80 degrees Fahrenheit for its dry bulb test and 67 degrees Fahrenheit for its wet bulb test. However, the temperature most homes will operate at will be closer to 75 degrees Fahrenheit for the dry bulb test and the relative humidity will normally be about 50%.
We can examine this temperature difference and try to determine how it will effect the cooling capacity of an AC unit. If cooler air is brought into the air conditioner than it was rated for, it will take more energy to cool it further than it would take to cool warmer air. Since AHRI's rating assumes the temperature of the air that is brought into the AC unit is 80 degrees instead of the normal operating conditions that will often have 75 degree air intake, the cooling capacity of your unit will be overstated by AHRI. To put it differently it takes more energy to remove heat from lower temperature air than from air that is at a higher temperature.
Since it is more difficult to further cool air that is at 75 degrees F than 80 degree F air, this means that your 2.8 ton AC unit will not be 2.8 tons under actual operating conditions. In addition, humidity levels must also be taken into consideration. The net take home message is that the AHRI rating of your 3 ton AC unit will be less than the AHRI figure, unless you maintain your thermostat at a temperature higher than 80 degrees.
AHRI Outdoor Conditions Of Operation Vs. Actual Conditions Of Operation
AHRI makes use of 95 degrees Fahrenheit as the outdoor testing temperature. If the outdoor temperature that you are operating in is different, you capacity will again be different from the AHRI rating. If your outdoor temperature is less than 95 degrees Fahrenheit it will make it easier on your unit, but if you live in a very hot desert climate, like Las Vegas, with temperatures frequently in excess of 100 degrees Fahrenheit your 3 ton AC unit will have less capacity. This occurs, since it is more difficult to disperse heat into hotter air than cooler air.
Further Reasons For Rating Variation
The reasons for rating variations that we have listed are not related to installation, design, or maintenance issues. There are numerous factors that relate to these issues that will also have an affect upon capacity. This may include the following:
- Poorly designed and constructed duct work
- Condensing coil air flow is poor
- Refrigerant is not charged properly.
- Filters and coils are dirty
These are all reasons why you may need a larger air conditioner than you initially thought you would need.
Take Home Message
The thing you need to remember is that you must know a bit about your operating conditions, outdoor conditions, and your installation in order to assess the size air conditioner that you will need. For the reasons we described above, there is a good chance that you will need more capacity than you originally anticipated.
Taking all these issues into consideration will help you in choosing an appropriately sized air conditioner for your particular needs. However, it is also not beneficial to have too much cooling capacity, or an oversized AC unit. This type of unit will certainly be more costly, but operationally, it will not work as well. This is because an oversized unit will tend to cycle on and off more frequently.
The temperature in the living space will certainly be reduced to whatever you have set your thermostat at, but the air in your living space will have a higher humidity than you would likely desire. Since the unit will cycle on and off more frequently, because of its excess cooling capacity, less moisture will be removed from the air, resulting in a higher level of humidity.
