If you are searching for a new HVAC system you may hear the term manual J, or at least you may see the term in your online research. However, surprisingly few contractors that come to your home to give you bid on your new HVAC system are actually performing a manual J.
What is a manual J?
To simplify, it is a 3 dimensional computer program, that comes up with the CORRECT size of your A/C and heating system. Be aware, a lot of contractors won’t do the little bit extra work or use scary words like math, computers, and warranties –yikes.. However, it is the ONLY way to ensure your system is sized correctly.
How does it work?
We input different the factors of your home into the program, these factors include; window sizes, window efficiency, insulation efficiency, roof and wall materials, the way the home faces, your climate area, the efficiency of your ductwork and the type of HVAC system you need. After this information is inputted in the computer program, it will tell us how big your air conditioner and heating system should be. In some cases it will also help us realize that a duct system might not be working well, or leaking, and that may need to be addressed as well.
Why does this matter?
An HVAC system (furnace, coil, condenser and ductwork) all work together to provide you with comfort, cost you utility bills, and determine the longevity of your system. The most common mistake contractors make when they don’t perform a manual J is to oversize your system. Why do they oversize it? Because you probably won’t call and complain until a couple years later when your warranty is up!
If you have a system that is too big you will be uncomfortable, paying too much in utility bills, and your system will not last as long. A properly sized air conditioner will run for long periods of time in the hot parts of the day and still keep your house cool and the humidity low. If the system is too large it will cool down the house really quickly and turn off, and then turn back on and cool off the house really quickly and repeat. In Austin, Texas humidity and heat battle every day, we must get the humidity out of the house to make us comfortable. Your system must run long enough to remove the humidity and only a properly sized system will run for long enough to accomplish this. Also turning on and off frequently as an oversized system will do, will ruin the system and your investment much more quickly. Sort of like highway miles vs. city miles, the mechanical components of the system are at peace when they are running, but experience more wear and tear on the startup and shut down of the unit. Especially in the spring and fall months where removing humidity is more important than cooling.
You will also be paying too much in utility bills if your system is not sized properly. If it is too large the frequent turning on and turning off of the system will cause it to constantly be running at its peak amperage. The startup of the unit has a large spike in the system electrical usage, as opposed to when it is running for several minutes the electrical usage levels out and becomes more efficient. It is a little bit counter intuitive that your system is more efficient when its running for a long time, but it is. Just like slamming on your gas on your Jaguar vs. cruising on the highway in your sweet Prius.
If a manual J is not done and a system is too small, the system will just not cool the house down to the temperature you would like it too be. This is the worst case for a contractor because you are sure to call back right away when it doesn’t cool you down to a reasonable temperature. That is why so many contractors oversize the systems and don’t do a manual J, they don’t want to go through the work of performing manual J and are scared of undersizing your system. Bad contractors just oversize your system due to a lack of knowledge and to cut down on call backs, until your warranty is up, and it’s too late to do anything about it.
Contractors are literally replacing systems that are 20 years old without checking that the size is correct. These older system may be cooling your house to your satisfaction, but they probably also are working 3 X’s HARDER than they should because the components are dirty, old, and costing you much more in utility bills. Because the system is old and dirty, it isn’t putting out the amount of cooling it is rated to be putting out. So while you may feel the size was correct of your old system, and tell your contractor to just put in the same size, you need to know, it isn’t going to be the same size. The newer system will be all shiny and clean and will deliver the correct amount of cooling it is rated at. Which may be way too much for your house because it isn’t slowed down by all the dirty and failing components.
If your contractor is not explaining this to you, find one that is up to date with the current requirements.