Many people seem to think HVAC design means you get a load calculation (Manual J in the ACCA protocols) so you know what size system to put in. Hey, that’s a great start. It’s way better than just using a rule of thumb or Manual E (for eyeball).
But there’s so much more to real HVAC design than simply finding out how much heating and cooling a building needs when it’s at design conditions. And we might as well start with the fact that my first statement is incorrect: The load calculation does not tell you what size system you need.
Load versus capacity
In an article I wrote last year, I went into detail about the difference between getting the load calculation results and sizing your heating and cooling system. You have to factor in the type of equipment you’re using, the efficiency of the equipment, the breakdown of the cooling loads into sensible and latent, and the difference between your design conditions and the conditions at which the equipment was rated. (Sensible load is related to changing the temperature; latent load is the part involved with removing moisture.)
But it really boils down to a difference between load and capacity. Heating and cooling loads are the amount of heating and cooling in BTU per hour (or Watts) that a building needs. Capacity is how much heating or cooling a piece of equipment can provide. Just remember that loads have to do with the building and capacity has to do with heating and cooling equipment.
Of course, there are three types of loads, but for design purposes, we use the design loads, which are based on design conditions. (The other two types are part load and extreme load.)
So, the load calculation is the…