A load calculation is the process of determining the proper size of a furnace and/or air conditioner for a home.
In the early days of the industry, hvac companies made an educated guess to determine the heat gain and loss for the home. The guess was based almost exclusively on the square footage of the home and was not even close to being accurate. The main concern of the hvac company was to make sure the unit was big enough so they would not be sued. Therefore, they usually installed a unit that was much bigger than required.
Later, an organization called the Air Conditioning Contractors Of America (ACCA) was formed. They developed standards and a method to more accurately find the home's heat gain and loss. It involved many complex mathematical equations and required a lot of time to perform. Therefore, most contractors did not use the system.
With the age of computers, this process has become even more accurate and requires less time. The computer software takes into account every detail of the construction of the home and how it affects the heat gain and loss. The software is pretty expensive and therefore many contractors still guess and hope for the best.
One of the basic laws of thermodynamics is that heat moves from a warmer area to a colder area. When it is colder outside than inside the home, heat travels to the outside. The heat that travels to the outside is called heat loss. At the same time, heat is added to the home constantly by things such as appliances, the sun shining in a window, or people. If the heat being added by those sources is less than the heat escaping, then heat from another source must be added to maintain a desired temperature. The amount of heat that must be added is called the heat load.
When it is hotter outside than inside, the heat from the outside travels to the inside of the home. That heat plus the heat from the internal sources mentioned above is referred to as the homes heat gain. To maintain a given temperature, this heat that is gained must be removed. The amount that must be removed is called the cooling load.
A load calculation is the method used to find these heating and cooling loads.
An improperly sized air conditioner can cost you in several ways. The comfort level of your home will be adversely affected with an improperly sized system. If the system is too small, you will not be able to adequately remove the sensible heat. (You will not get the temperature of the home to the desired setpoint.) If the system is too large, the unit will not run long enough to adequately remove latent heat. (The home's air will have that sticky feeling because the humidity level of the air will remain too high.) If your furnace is too small, your home will be too cold during the coldest nights of the year. If the furnace is too large, you will probably have hot and cool spots throughout the home.
Another way an improperly sized system will cost you is in the equipment purchase price. In general, the larger the size of the unit (in tons or btu/hr) the more it will cost. Therefore, with an oversized unit, you will buy excess cooling and/or heating capacity that you will never use.
An oversized unit will also cost you more to operate. This occurs due to the system efficiency. The efficiency rating (SEER) is given for a unit at design conditions and steady state operation. Typically the air conditioning unit is about 10% less efficient from start up until steady state conditions exist. This steady state condition is not achieved until the unit has ran long enough to coat the indoor coil (evaporator) with condensation. This can typically take from five to fifteen minutes.
The basic load calculation, called a block load, is used only to find the required unit size. It looks at the home as a whole and gives a total load.
Several factors affect this such as:
A more advanced version of load calculation requires more information such as individual room measurements. This is called a room by room calculation and it not only gives you the equipment size but also designs the duct system based on the needs of each room.
It is important to know what proportion of conditioned air is required in each room.
**NOTE** In many cases, the required proportion will change between the heating and cooling seasons.
Once you know how much air is required in each room, dampers in the duct system can be adjusted to get the proper airflow through each register. This process is commonly called air balancing and it helps to eliminate large temperature differences from room to room in the home. It results in a higher level of comfort.
**HOT TIP** Proper airflow balancing can save you money on system operation.
A room by room load calculation is critical for homes with more than one level. If this is not done, the result can be a large temperature difference between floors in the home.
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Another type of calculation is called a home energy rating. This is used in the real estate industry to provide potential homebuyers a comparison of the relative efficiencies of different homes.