Can you trust your Load Calc Software?

Attention Wrightsoft Users!

  Have you ever completed a load calculation and it just seems like it was short of what you had expected?  Particularly when you are first starting out with a tool, contractors can make mistakes. After all, we tend to learn "the hard way", right?  In an effort to make software work, the companies tend to start with some default values that require adjustment.  For instance, most have the duct system in the conditioned space calculating zero gains or losses.  If the ducts were in the attic, this could be as much as a one ton load that is missed! Internal gains tend to start at zero, and the foreground for windows is something between crushed rock and green grass.  Most contractors find these values, over time and as they gain experience with the program.  One still overlooked setting in Wrightsoft, a feature that no other ACCA approved software employs, is the Rate Swing Multiplier (RSM).  This can cause you to lose as much as 11% of your sensible gains in an otherwise perfect load calculation, maybe more!

  This adjustment, the Rate Swing Multiplier (RSM), is used as a makeshift way of selecting cooling equipment by using AHRI data.  Those familiar with ACCA Manual S, Equipment Selection in the residential design procedure, and International/Local codes, know this makeshift procedure is not proper design and will likely contribute to over sized equipment.  What many users do not realize is this RSM de-rates your sensible gains on your Summary Report!  If you then use what you thought was the correct numbers, you may install a system that is too small, or set your fan speed too low due to the decreased sensible gains.

If you do not change the default Sensible Heat Ratio (SHR) from the .70 in your equipment selection, then the RSM remains at .89.  If you use suggested values from local utilities (SHR .85), carefully calculated to reduce selection of over sized equipment, this RSM raises to .93.  If you know how to calculate the SHR of the home, a fairly simple process, the RSM will fall anywhere between those values already discussed.  The example images do not reflect the same load calculation and are for RSM & SHR reference only.

  Great news!  You can just turn this sabotage of a design feature 'OFF', as you should!  If I was a building inspector, I would fail any report that showed up with a RSM other than 1.0.  To do so, go to 'Options' in top menu, then uncheck 'Adjust Load for AHRI Standard Rating'.  Easy as that, and now you will not be short changing yourself on those accurate, aggressive load calculations.  Of course, this alone will probably not change your equipment size, but combined with another small misstep and it very well might!