Manual J Load Calculation

If you call companies for pricing on a new system, and they don’t come to your home and do some inspecting before giving you a price, do not go with that contractor!

HVAC contractors use software to perform load calculation about a home being heated and cooled, and this is called a Manual “J” Load Calculation. The software accepts many detailed parameters about the home (ducting, window types, windows shades, outside shade, etc), and then sizes equipment precisely based on the details entered. When talking to an HVAC contractor about a new heating and air conditioning for your home, it is a good idea to ask them whether they will do a Manual J Load calculation.

Avoid “Rule of Thumb”

Again, an HVAC contractor who wants to do sizing based solely on rule of thumb, or the square footage of your home, should be scrutinized. Sizing equipment too large or too small affects comfort and cost. We recommend you include other HVAC contractors in your search. Remember, you get what you pay for. Our own HVAC contractor locator tool is a great place to start.

The Design Temperature

HVAC equipment has what is called by most as a design temperature. A design temperature basically means that there is a limit to which equipment can heat or cool a certain space. An improperly sized air conditioner might do fine in early summer, but there could be an outdoor temperature reached that buries the ability of the unit. If your HVAC contractor does a manual J load calculation and your home computes out to needing a 4-ton air conditioner, there are parameters he or she had to enter to receive that sizing calculation. For example, your HVAC contractor has to specify the outdoor temperature maximum, or design temperature when performing the manual J load calculation. If they use 90^Farenheit as the design temperature, the new unit will be fine as long as you don’t exceed 90° outside. If the summer heat hits 100°, your house is going to become warm. Many other things affect your heating and air conditioning as well including types of windows and shades, insulation, ducting design and leakage, properly charged equipment, and much more.

Bigger Isn’t Always Better

If you purchase a new air conditioner, and obtain one that is too large for your home, common sense would say that it will be a better unit with more power right? The reality is that an air conditioner that is too large for your home can actually cost more to run than one that is properly. sized with the correct parameters entered when doing the manual J load calculation. Other problems can surface too like less humidity removal and shorter equipment life. Not only could you be spending more on the new equipment, but more each month on staying cool!

Likewise, if proper load calculations are not done, you could end up with a new air conditioning system that can’t keep up with the heat in the middle of a hot summer day. An air conditioning system for a home in Seattle may differ in size than an air conditioning system for the same size of home located in Houston.

So what is the answer to obtaining the right design and sizing? A Manual J calculation. This will keep you from spending too much on utility bills, provide better comfort and also better system life. However when performing a Manual J, your HVAC contractor needs to do it precisely. If they start adjusting details “to be safe” and do not properly regard facts about your home, the calculation will not be accurate.

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Manual J Load Calculation

Los Angeles, CA 90232