Heating and cooling sizing factor in energyplus

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We should certainly avoid oversizing equipment, both for the sake of cost and for embodied energy/material efficiency reasons.

However the oversizing is required since the weather files we use don't represent the harshest winters/hottest summers. It's reasonable to expect extreme weather events, especially given expected climate change. Without oversized plant there is a risk that the occupants will be left uncomfortable, and that should be one of the prime drivers when designing a system.