I am participating in a DOE Better Buildings  program in southwest Ohio that is in its infancy stages. During discussions while setting the technical standards of the program, the topic of proper sizing vs 2 stage equipment came up.  Many of the "seasoned HVAC contractors" involved in this conversation including the director of operations for the program adamantly were advocating for the standard to be" 2 stage trumps sizing".  Meaning if you cant find a 2 stage piece of equipment in the correct size as specified by your manual j then go to the next available size.  (Most 2 stage equipment is not available in 1/2 ton sizes.)   For reasons discussed in this and other threads, I was in favor of forgoing the 2 stage piece of equipment for the piece of equipment closer to the proper size.  I was hoping I could get some input from this thread on reasoning for or against up-sizing in this situation especially taking in to consideration our geographic location.

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Keep in mind that the MAJORITY of operation time is going to be at part-load conditions. More than likely, the larger unit running on its first-stage would be more appropriately sized, for more operating hours, that the "correctly" sized machine in a single-stage model (it would only be properly sized under the design conditions).

 

Manual-S states that the equipment selected must be able to satisfy both the sensible and latent loads. If you're wanting the benefit of a 2-stage machine (lower power consumption during part load conditions, ability to zone more effectively, etc.), you should just pick the smallest size that is available which also meets the design load. The first stage will handle the part-load conditions with the occasional help from the second stage. If an aggressive Manual-J is done, and the house effectively manages moisture, this should not be problematic.

 

If you use a program like Wrightsoft, you can look at the BIN data reports to see the number of estimated operating hours on first/second stages to help you make a decision like this.

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