As I had brought up years ago, right here on Excess Air, ACCA's Manual S was in dyer need of an update. Written in 1995, Manual S Residential Equipment Selection was passed by with huge technical and efficiency strides within the HVAC industry. Think about it: ECM Motors were in their infancy, condensing furnaces were still prone to failures, and variable refrigerant flow was far from mainstream in the U.S. All addressed in the updated Manual S! I have to say, their committee did a thorough job addressing updates to a process that is essential to system design and a code requirement in most of our country! I highly recommend HVAC Sales Staff, System Designers, and Building Analysts purchase a copy of the new text, but in the meantime here is a teaser:

1. Multi-Stage Central A/C: can be sized between .90 and 1.20 of the total building load. This additional 5% of capacity vs. 1995 becomes increasingly important as homes get tighter and Sensible Heat Ratios rise. Higher airflow to handle the increase in sensible loads lends to higher capacities.

2. Variable Capacity Central A/C: can be sized between .90 and 1.30 of the total load. See #1 for sensible capacity reasons, but variable capacity equipment is great for handling high latent capacities when operating on part load days as well.

3. Cold Climate Heat-Pumps: can be sized to the total cooling building load + 15,000 btu/hr maximum. This goes for single-stage, two-stage, and variable capacity equipment. For mini-split heat-pumps, this 15K is above the total block load.

4. Heat Pump Supplemental Electric Heaters: can be no larger than 5KW if the heat load is  <15,000 btu/hr.

5. Fossil Fuel Burning Furnaces and Boilers: can be sized between 1.0 and 1.40 of the total heat load. Ok, this isn't a change, just thought it was important to know! If installing a furnace that needs a larger blower for cooling, maximum is now 2.0.

Loaded with up to date examples, the new Manual S is far from a dust collector. A great reference manual, but lets just hope it is not for another two decades like it's predecessor...but don't take my word for it!