If the person who ran the calculations has already gone through the derating procedure and specified the equipment, it may be accurate. That’s just a guess at what equipment capacity you’ll need. total capacity at 0.70 SHR,” as in the screenshot above. Depending on which reports you’re looking at, you may also see something like “Req. I mention this topic here because some Manual J reports can confuse you on this distinction, especially for cooling. (For more on this, see What a Load Calculation Does NOT Tell You.) 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. They’re shown separately for heating and cooling, and cooling is further divided into sensible and latent. When you look at Manual J reports, you’ll see the loads. Here in the US, both are measure in British Thermal Units (BTU) per hour. 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. I just did it in my last article, and now I’m doing it again. Heating and cooling loads are not the same as the equipment capacity needed. I don’t think I can make this distinction often enough. How do you know if they’re accurate or not? Fear not, dear reader. 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. 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. 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.
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