Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
I would try to have EnergyPlus autosize the rated capacities and airflows of your separate heating and cooling heat pumps to meet load. You can probably find a way to do this in DesignBuilder or in the IDF it generates.
If you are hard-sizing your heat pump capacities and still getting high unmet hours, look at the rated airflow rate(s). I have noticed in past models that if I increase the rated heating or cooling capacity of my heat pump coils and do not increase the rated airflows through those same coil objects as well, the desired effect of increasing HVAC output to match load and reduce unmet hours is not produced.
Another thing to investigate are the performance curves of your heat pump coil objects. The performance curves dictate how your capacity and EIR/COP degrade or otherwise change with ambient conditions. The curve values (function of temperature and flow fraction) combined with low ambient outdoor dry bulb temperatures can produce instantaneous capacities and COPs that are lower than rated, hence causing unmet hours.
