QuestionsCategory: QuestionsHVAC sensible and latent cooling numbers, calculator results versus equipment specs
Handyman Staff asked 2 months ago

Looking to size a new heat pump for my home, I've used two online calculators. They give pretty similar results.


For cooling, the former gives "total" load and "latent" load, while the latter gives "total" load and "sensible" load. I understand that "sensible" cooling removes the heat from the air without changing the moisture content, and "latent" cooling dehumidifies the air by causing condensation. The two calculators simply choose to display these numbers in different ways.

But I'm confused about evaluating a system's specs to determine if it can satisfy the requirements specified by the calculator. I've attached a page from the tech document for a system I'm considering.

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Look at the box containing the yellow-highlighted numbers. Note that for the constant indoor dry-bulb temperature of 75 degrees, as the air gets dryer (so the wet-bulb temperature drops from 71 to 59 degrees as the humidity drops from 87% to 58%), the total number (labeled "capacity") stays roughly the same, but the "S/T" numbers (sensible to total ratio) increases fromn 0.5 to 1.0.

So for the 71 wet-bulb column, the sensible number (capacity x S/T) is WAY too small (compared to the "sensible result" from the calculators). Similarly, if I look at the 59-degree wet-bulb column, the latent number (capacity x (1-S/T)) is way too small (in fact, it's zero).

Should I take all this to mean that I can use the 71-degree column for determing if the system's latent capacity is sufficient and the 59-degree column for determining if the system's sensible capacity is sufficient ? Put another way, if initially the air in the house is humid, the system dehumidifies the air, and then once the RH% has dropped into the 50s, it concentrates on actually cooling the air ?