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Hey Ed, What Is the Significant Difference Between a Heat Gain and a Heat Loss in Manual J®?

In this edition of "Hey Ed," Ed talks about the difference between heat gain and heat loss in Manual J®.

Hey Ed, what is the significant difference between a heat gain and a heat loss in Manual J®? When we see the term "significant" used, I’m going to say the big difference is that when we do a heat loss, we don’t take into account internal loads that actually help us heat the house, such as the heat coming from the blower, lights, people, appliances. When we do a heat gain calculation, we take all of those sources of heat into account, and it can make a difference in those calculations. It’s a big deal for air conditioning because we want to be honest and aggressive with those calcs and be as close to the heat gain as possible with our math. When it comes to the heat loss of a structure, we’re not nearly as critical when it comes to the accuracy of that calculation, because being able to maintain 70 degrees on a design day, if we don’t take into account those small amounts of free heat, it’s really not a big deal. Again, on the cooling side, it’s a huge deal because if we have too much capacity because we exaggerated or didn’t account for it, then either we have a piece of equipment that is not going to be able to keep up or worse - if it’s too big, it has a hard time dehumidifying at part load conditions. Part load conditions are the bane of my existence. I want systems to do what they’re supposed to; it’s easy for them to do what they’re supposed to when they run. When they run, water comes out of the white pipe, right? But when they don’t run, water comes out of the white pipe, and that’s when the houses start to feel uncomfortable due to elevated humidity. And that’s the way I see it.

Ed Janowiak is the Manager of HVAC Design Education at ACCA.

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