When you're installing a new air conditioner, it's important to have your HVAC installation contractor perform a Manual J load calculation. This calculation has been developed by the Air Conditioner Contractors of America, and it allows you to find out how much home your heat gains during the summer and how much it's losing during the winter.
Knowing your heat gain allows you to install an air conditioner that's powerful enough to keep your home comfortable and also ensures that you're not installing an air conditioner that's extremely overpowered for your home. To learn more about the Manual J load calculation and why it's important to have an HVAC installation contractor size your air conditioner correctly, read on.
What Factors Does a Manual J Load Calculation Consider?
In order to understand why the Manual J load calculation is important, you'll first need to know all of the factors that go into it. All of these are measured by an HVAC contractor and entered into a computer program that tells them your home's heat gain during the very hottest days of summer.
The number of windows you have, their size, and which direction they're facing are part of the load calculation. Windows tend to be poorly insulated, so they're a major source of solar heat gain for your home.
Your local climate is taken into consideration as well. It's entered into the load calculation based on historical temperature and humidity data for your area. An HVAC contractor will also measure the insulation in your walls and in your attic to determine how much of an effect that outside air temperatures have on the interior of your home.
Sources of heat gain inside the home are taken into account as well. The most important part of this calculation is the number of occupants who reside in your home — people can produce quite a bit of heat on their own. In addition, it also accounts for the number of lights you have and how much heat they generate while they're on.
Finally, the square footage of your home and the height of your ceiling are entered into the calculation. This determines the volume of the space you're trying to keep comfortable — larger homes will lose more heat during the winter and gain more during the summer due to their size.
Why Is a Manual J Calculation Important to Find the Right Air Conditioner?
Most buying guides for air conditioners recommend sizing your air conditioner based solely on the square footage of your home. Air conditioners are rated in British Thermal Units (BTUs) per hour, and the general rule they provide is that you'll need 12,000 BTUs for every 450 square feet in your home.
Unfortunately, this number is likely to be inaccurate. As you can see from the numerous factors involved in the Manual J calculation, basing your air conditioner size solely on square footage isn't sufficient for selecting an air conditioner that's properly sized.
In particular, homes with a few energy efficiency upgrades often need a less powerful air conditioner than the square footage rule suggests. These upgrades will be taken into account during the Manual J calculation. If you simply go by square footage, you will often end up with an air conditioner that's much too powerful for your home.
What's Wrong About Installing an Over-Sized Air Conditioner?
The primary problem with purchasing an overpowered air conditioner is that it will turn off and on frequently. This is a phenomenon that's called short cycling, and it wastes quite a bit of energy — air conditioners draw an enormous amount of power when they first start up and engage the compressor.
In addition to the energy waste, a short-cycling air conditioner won't dehumidify your home very well. Air conditioners pull water vapor out of the air in your home when condensation forms on the air conditioner's evaporator coils. When an air conditioner turns on and off frequently, there isn't enough time for condensation to form on the coils. As a result, your home will remain uncomfortably humid.
To prevent an over-sized air conditioner from wasting energy and making your home uncomfortable, always choose an HVAC installation contractor who performs a thorough Manual J calculation. Not every contractor provides this service. Sizing your air conditioner properly will save you money by reducing your energy usage and lowering the cost of installation.