The latest residential CBECC release included in EnergyPro 7.1 has been having problems with Existing+Addition calculations and will generate a CSE Error 2 message in some cases. This FAQ will cover the topic and should be read, although we hope the CEC will fix this one soon.
The CEC has acknowledge this bug, and provides direction as follows:
You may be encountering a bug in CBECC in the standard design generated that occurs when the existing zone contains new windows. You can tell you have this error if the error message goes away by changing the status of any new windows in the existing zone to altered.
It may be that not all of the windows need to have the status changed, so start by changing them all to altered and then change them back to new one at a time. It’s also possible that small changes to the window areas, like limiting the areas to the nearest whole square foot, will also solve the problem.