Manual J Battle with Ed and Nathan

Nathan Rothenberg and Ed Janowiak join Bryan to talk about the good, the bad, and the uncomfortable of ACCA Manual J, D, and S.

Manual J load calculations exist to make the BTU inputs and outputs predictable. Then, Manual S comes in to assist with equipment selection to meet the load calculations and the customer's comfort needs. Manual D is a collection of mathematical formulas that exist to help you calculate your friction rate, which is important for comfort in terms of noise in the ducts (from excessive air velocity).

Ed believes that the best way to learn Manual J is from the physical manual; several instructors will teach the calculations straight out of the book, not on computer software. The difficulty of learning Manual J is one of the manual's shortcomings. Also, while Manual D is often required by code, Manual J is not often required, meaning that technicians can get away with poor designs. A common argument against Manual J is that comfort needs also tend to vary with each customer; therefore, standardized calculations and targets may not help individual customers meet their preferences.

The typical temperature and humidity targets are 75°F at 50% relative humidity. Under those conditions, the dew point is 55°F (meeting the 20°F delta T rule of thumb), meaning that the air should remain well above the dew point. When the air remains above the dew point, the risk of a moisture problem greatly decreases, even at the expense of comfort.

Nathan, Ed, and Bryan also discuss:

  • Bad square-footage rules of thumb
  • ACCA Manual T (register placement)
  • Temperature and humidity effects on comfort
  • Oversizing equipment
  • Single-stage vs. two-stage equipment
  • Ductwork in unconditioned spaces

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