When you enter the world of building science — whether through building a house, becoming a home energy rater/building analyst, or just hanging out in cyberplaces like this — everyone talks about the importance of getting actual heating and cooling load calculations based on ACCA Manual J. A great number of HVAC contractors sell and install oversized equipment with air distribution systems that don’t work because these contractors base their choices on rules of thumb.
OK, but what if you hire a contractor or third-party designer to do Manual J load calculations and you’re not an expert and don’t want to be? Suddenly you’re faced with a bunch of seemingly indecipherable reports. How do you know if they’re accurate or not?
Fear not, dear reader. I’ve got some help for you today.
Don’t confuse load with capacity
I don’t think I can make this distinction often enough. Heating and cooling loads are not the same as the equipment capacity needed. I just did it in my last article, and now I’m doing it again. It’s that important. The first thing you need to know is that the term loads refers to how much heating and cooling the building needs and capacity refers to how much heating and cooling the equipment can supply. Here in the U.S., both are measured in British Thermal Units (BTU) per hour.
When you look at Manual J reports, you’ll see the loads. They’re shown separately for heating and cooling, and cooling is further divided into sensible and latent. When the contractor or designer picks a piece of equipment, they’ll have to go through a “derating” process to match the equipment’s performance specifications with the building’s loads. (For more on this, see “Manual J…