What Manual J Calculation Means for Indianapolis Homeowners

By: Brian Schutt  |  October 31, 2016

Ever heard the term “manual J calculation” and wondered what it means? Read on for all you need to know.

When we visit your home to perform an assessment, we prepare a comprehensive report. It helps us get to know your house in order to best determine the right-sized comfort system for your home—whether that be a furnace, air conditioner, heat pump, boiler, or humidifier.

Picking the right size unit takes careful consideration, and that’s why we’re so detailed in our report. Too small of an HVAC system does not allow your home to reach the desired temperature on the hottest and coldest days. And too large of a system costs more, breaks down more often, leads to large temperature swings, makes too much noise, and fails to properly dehumidify.

But we acknowledge that reviewing such a detailed HVAC report can sometimes be like trying to decipher a medical journal. We tend to speak our own language. And so here’s our attempt at more simply explaining our estimates in terms anyone can understand.

First up, the term “Manual J” and what it means for you as an Indianapolis homeowner.

What Does “Manual J Calculation” Mean in Your HVAC Assessment?

When Homesense technicians determine the right-sized HVAC system to place in your home, we rely on the Manual J method, which examines several factors:

  • Local weather
  • Size of your home (total volume, not just square feet)
  • Size of your windows
  • Orientation of your windows (direction they face)
  • Amount of air leaking in or out of your home
  • Levels of insulation in the attic and walls
  • Number of people and appliances.

In short, Manual J is an equation. We input the aforementioned variables and the answer we receive comes out in the total number of BTUs of heat lost during the winter and gained during the summer. It shows the maximum number of BTUs you need to keep your home at the desired set point on the hottest and coldest days of the year. We then find an HVAC system to match the BTUs needed for your home.

What is a BTU?

Before we continue explaining Manual J, let’s pause for a moment on the term BTU. All furnaces, air conditioners, heat pumps, and boilers are measured in British Thermal Units (BTUs) per hour. Air conditioners and heat pumps are typically converted to tons. And each ton equals 12,000 BTUs/per hour, as that’s how many BTUs it takes to melt one ton of ice.

The number of BTUs/per hour can be found on the boiler plate of furnaces and boilers, as well as encoded in the model number of the system. For example, 59MN7A060V1714 is a 60,000 BTU furnace, whereas a 59MN7A100V2120 is a 100,000 BTU furnace.

Does Manual J Calculation Always Work in Calculating Best HVAC System?

We find that the Manual J method works great in new construction homes, or when all factors are known. However, for older homes—which includes nearly all of Broad Ripple and the Meridian-Kessler areas—some of the most important factors might be difficult to discover. In these cases, our HVAC professionals will input as many known factors as possible, and then use averages or statistical information about similar homes to fill in the gaps. Our experienced team HVAC professionals can then tell if the output doesn’t match our expectations and search to discover the cause of the discrepancy.

We also engage homeowners and/or current renters to learn more about how the home currently feels in the summer and winter months and compare this anecdotal data against the existing system’s performance. This process does not replace the Manual J method in determining the right-size HVAC system, but simply gives us more information to compare against the numbers.

To ensure you have the right HVAC system for your home, give us a call at 317-203-8149. We’ll be happy to make sure you’re spending as wisely as possible.


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