An HVAC contractor can determine what size of air conditioning unit to install in your house, but will that size unit be the best unit for your home? There are many variables to consider when having a new unit installed in your home. These variables are brought together by a load calculation performed either by your contractor or a mechanical engineer. How do they do it though? What can make an engineer designate a larger home with a unit smaller than what is in another home with fewer square feet in some instances?
Let's use the three little pigs as an example (with some artistic license). The first little pig lives in a metal house that sits on a pier four inches off the ground. The second little pig lives in a wooden home that sits on a concrete slab which has been beloved into the ground. Finally, the third little pig lives in a house built with 10 inch thick mason blocks, also sitting on a slab. None of the homes are insulated. All of these houses have the same internal dimensions, but they will have different load calculations, which means, they may each have a different sized air conditioning unit.
A load calculation is a mathematical function put together to determine the amount of heat that needs to be displaced in order to cool or heat a home. The main reason for the differences in the load calculations is what is called the R-value of each home. R-Value is a measure of thermal resistance (the 'R' comes from the word, resistance) of a substance or an object. Although you may not be able to determine the R-value of your home, you can easily understand the concept by taking a look at each of the little pigs' houses.
The metal building the first little pig lives in is off the ground. This allows outside air to flow under the home as well as over and around it. Air flowing under a home decreases the R-Value of the home since it's another route for heat to enter or escape the home. Also, because this house is made of metal, it is a better conductor of heat than other materials, so that also decrees the R-value of the home. The lower the R-value of the home, the larger the unit needed to heat / cool it. This home has an extremely low R-value.
The wooden building the second little pig lives in is on a slab which was scattered in the ground. The slab keeps the floor of the home a relatively even temperature through the year, summer or winter. The wooden walls are built of one inch thick clapboard siding and less less heat conduction as well. Neverheless, without added insulation, this home conducts heat in or out fairly easily as well. The R-value in this home is higher than that of the metal home. It's possible that this home can use a smaller central air unit than the metal home.
Finally, we come to the brick home constructed on a concrete slab and built of 10 inch mason blocks the third little pig lives in. Although this home has a wooden roof, it is constructed of a thick wood with a higher R-value than that of the wooden house, and is shingled with a better quality shingle which is designed to reflect heat rather than absorb it. Since this house is also on a concrete slab, there is no outside airflow underneath it. The 10 inch walls work very well at insulating against heat conduction. These materials all work toward this home having a very high R-value. This high R-value causes the home to hold whatever temperature air inside for a longer period of time. This means a smaller air conditioning unit can be used in this home than in the previous two.
How a home is insulated plays a key role in how much heat must be displaced to keep the home cool or warm. To cause a central air system to work as efficiently as possible, better insulated walls, floors, ceilings, doors and windows are key. If these things are used, then a home is well on its way to becoming a 'green' home. The air conditioner will run less often, and it will run for just the right amount of time in order to cool the dwelling adequately and use the least amount of energy possible.
If you really want to know the answers for your house, call your local HVAC expert.