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!

http://excessair.blogspot.com/2014/02/can-you-trust-your-load-calc-...

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Comment by Eric Kjelshus on March 17, 2014 at 12:42pm

Solar gain is a very very big thing.  This may the biggest and important to show on loads.   Next big thing is duct work is it sealed? 

Comment by David Williams on March 17, 2014 at 8:52am

A four year (hourly) monitoring and verification project has proven the need for passive solar gain calculations in heat dominant climates.  We have found up to 45% discrepancy between yearly energy use measured vs. modeled.  Wile the heat system sizing is remains relatively constant, from program to program, (Solar gain is not always available!!) The annual modeled energy use neglects the energy savings derived from good passive solar design.  When building designers are unable to model these significant annual energy savings, they are not included.  We use HOT2000 to help identify the passive solar contributions.  As with any software, garbage in = garbage out.  Take the time to calculate whole wall R-values and add those options to your drop down menus.

Comment by Jim Peck on March 1, 2014 at 10:18am

Agreeing with Ted here,

"Models are tools for modelers like hammers are tools for carpenters"

Just cause you own a hammer doesn't mean you can set a nail without a dimple :)

Comment by Eric Kjelshus on February 24, 2014 at 9:11am

Your out a very important note, Wrightsoft encourages equipment selection to be done via Manual S and WS is the only software that gathers Manual S data directly from the manufacturers.   When I started AC 40 yr ago the coils were very small and very cold, the max was 8 eer with mostly 5-6 eer out there.   Gramps would say above 35' air temp so coil did not ice  up.  then all were just .7 SHR  now with coils as big as the air handler  one l look at saterday was .94 SHR  thats not a cold "A"coil.   To take RH% out of a room we need a very cold coil,  59' will not do it.  it might cool the room but not take the water out the air like that very cold coil.

Comment by tedkidd on February 24, 2014 at 7:51am

Getting equipment to avoid cycling in reasonably tight structures seems to net surprising energy savings.  The way to reduce cycling is aggressively downsize equipment.  

The dry climate guys find Manual J oversizes equipment by between 25 and 35%, this leads to bad design decisions.  I've found the same to be true.  

Models are tools for modelers like hammers are tools for carpenters.  Modelers who fall too much in love with their http://bit.ly/sphericalcow models without building intuition based upon actual results of their work are likely to be pretty ignorant of what good work looks like.  Relying too heavily on tools can be a cop out, responsibility avoidance.  Tools don't tell a good carpenter what to do.  

Want to see what really happens in houses, sell your clients some Ecobees: http://bit.ly/4ecobeethermostats  We need to stop claiming we do "great work" without any tracking, without doing measurement, verification, or getting occupant feedback.  

Comment by Bob Blanchette on February 23, 2014 at 4:29pm

Even when load calculations are done customers typically end up with oversized equipment. How often have you seen equipment run "wide open" when design conditions are exceeded by 5 degrees? Most "can't keep up" complaints are a result of equipment/ducts not delivering rated capacity.

Comment by Curt Kinder on February 20, 2014 at 11:23am

I remain quite pleased with Elitesoft. No RSM, SHR defaults to 0.75, and ducts default to attic, typical in my area and certainly is responsible for approx 1 ton out of every 4.

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