Trusted South Florida Licensed Plumbing, Air Conditioning & Electrical Experts Since 1983!
EMERGENCY HOTLINE 1-800-475-1504
Hours of Operation: Monday – Saturday 8am – 7pm

What is a Manuel J Calculation?

air conditioner calculations - manuel j calculation
Reading Time: 3 minutes

Air conditioning as a science began with Willis Carrier in 1902 to help air condition the printing plant in which he worked. In 1931, H. H. Schultz and J. Q. Sherman come up with a staple that we still use today – the window mounted air conditioning unit. In 1939 the Packard becomes the coolest car in town literally by being the first car to have air conditioning. The bad news on that was if you got too cold you had to pull over, shut the engine off and disconnect the belt that ran the compressor. In the 1950’s, with the American economy booming after the end of the wars, over one million window air conditioners were sold. It was in the early 1970’s that whole home residential air conditioners began to take over. The point I’m making here is that when it comes to how to properly cool a domicile from a whole home stand point, we have only been doing it for about fifty years. And my oh my, how things have changed in those fifty odd years!

Let’s begin with how air conditioners are sized. In the early days, depending on where you lived, there were rules of thumb when it came to air conditioning. These were mostly based on just the square footage of the home. Florida was one ton of air conditioning for every 450-500 square feet. At one point there was even a cardboard cutout that had the shapes of four different sized homes. You took the cutout and held it up to the house you were looking at from the curb and whatever size cutout fit the house, that was the size air conditioner you used! As you can imagine there were a lot of homes that were not sized properly, had hot and cold rooms and in general, problems galore.

Today, as we have done with most everything, computers have taken all the guess work out of how much air conditioning a specific structure will require. There are several software programs out there, however, they all run the same two calculations, Manuel J Load and Manuel D Load. We are going to start with the Manuel J.

Manuel J requires that every aspect of the home be measured to begin. How big is each room, how high are the ceilings, how many windows are there and what size are the windows? Next on the list of questions is what type of windows are they and what is the insulating value of that window. We also look at what direction each window is facing. Obviously, windows with eastern exposure get more heat in the morning than others in the home. We are also going to need to know how tight the building is along with what type of construction it is. How much insulation is in the attic, how tight is the duct system (retro-fitting only), what is the type and color of the roof and back to the windows, how wide is the overhang and how far is it from the top of the window to the overhang. Last, but not least, with the latest code changes we need to compensate for the amount of fresh air being brought into our building. All this data gets plugged into a software platform and boom, we know exactly how many BTU’s or tons of air we need. Below is a screenshot from the software I use, and yes, I actually do know what all those little buttons do. I told you things have changed!

Manuel J Calculation

On to Manuel D. Manuel D builds off all the information that our Manuel J has given us. The J load tells us how much air we need and the D load then tells us how to split that air up into each of the rooms. Air volume is measured in CFM (cubic feet per minute) and of course the larger the room the more CFM’s we are going to need.

My pro tip of the week is if you are thinking of purchasing a new air conditioner for your home and the contractor does not run this type of calculation before installing your new system do not use that contractor. Many years ago, air conditioning was a lot of guessing and hoping for the best. Today we don’t guess, we test.

Until we meet again my friends, I was behind a clown going into 7-11 and he held the door for me, it was such a nice jester.

House Whisperer out!!


art plumbing, ac and electric

Schedule a Service Call

For Immediate Emergency Assistance Call 1-800-475-1504

Schedule Service