User manual

FAQ for CBECC-Res 2013
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6. Can you explain the 2-story ventilation change in more detail? Is this related to whole house fans?
Yes, it is related to whole house fans and their benefits in a 2-story home. The principal difference is
in the “stack effect,” driven by the opened window area and the height difference between the
open windows. In 1-story homes, a 2-foot height difference drives the stack effect of natural
ventilation, and in 2-story homes, a 10-foot height difference drives this stack effect. This difference
causes significant variance in the relative benefits of mechanical equipment that employ ventilative
cooling, such as whole house fans, because CBECC-Res was assuming that natural ventilation works
extremely well in 2-story homes, and only moderately well in 1-story homes.
While it may be realistic to assume this 10-foot height difference (meaning windows are opened
both upstairs and downstairs), realizing this large height difference is based on all bedroom doors
remaining open on both floors and an unobstructed airflow path between the downstairs and the
upstairs windows. Because the airflow model does not account for these obstructions, the 10-foot
height difference is not realistic. This difference between the CBECC-Res 1-story and 2-story
ventilative cooling impacts was only noticed recently. The effect was to discount the impact of
whole house fans (a prescriptive requirement in several climate zones).
In CBECC-Res v3 the change in window operations in 2-story homes is to assume that only the
windows on the second floor are open for cooling ventilation purposes. This will allow the
compliance software to implement the prescriptive baselines for each climate zone consistently for
both 1 and 2-story homes.
7. I am getting an error that seems to indicate a problem with the ducts in the attic. I previously had
a problem with a very small attic being unable to accommodate the default ducts in the attic. But
in this case the attic is over the entire house, so it should be big enough. Is there anything I can try
to correct this error?
First check your duct system. Make sure to select “use defaults” (as long as you are not doing a
verified duct design). Then go back to the envelope tab and delete the attic zone. The CBECC-Res
program will immediately recreate it. This may be enough to clear out the duct assumptions and
redefine them, which may be enough to allow the program to run.
8. Will I be able to take credit for changing an existing electric resistance heating system to a central
gas forced air unit (once verified existing conditions are available)?
No. There is no ability to show a change that involves different efficiency metrics (such as AFUE vs.
HSPF). Because the existing system is defined when using verified existing conditions, this situation
will create an error and cannot be simulated. Therefore, verified existing conditions cannot be used
for this particular change.
FAQS POSTED ON AUGUST 6, 2014
1. I thought I installed bug fix 2c, but I am not sure. How can I tell if I am using 2c?
Because it was a ruleset change only and there was no new executable file, it was not possible to
change the “about” screen. To verify that you installed version 2c, when you produce a CF1R
with the updated software, field 07 under General Information shows the Compliance Manager
version (595c).
2. I am still seeing HERS requirements for existing HVAC systems. Wasn’t 2b/2c supposed to fix this?
This is not fully fixed until Version 3 is approved. This was an unfortunate oversight. It is subtle but
the CF1R sections that specifically list the HERS requirements are correct—no HERS requirements