Dear Ery,
Those !@#$%^ engineers are pretty opinionated and generally don’t want to talk about their load calcs, let alone defend them to an energy modeler! (n.b. I’m an engineer and sometimes represent that remark.)
Most of the time, I’m preparing an energy model of a completed design, so there’s not much reason to discuss the capacities unless the model shows undersizing. I think most engineers are committed to having plenty of insurance in the form of extra capacity and are unlikely to changes sizes downward. On a recent project for which I provided a commissioning review that included review of the load calculations (very unusual), I found that the engineer was including 5W / sq.ft. for lighting and plug loads … in a typical office. That seemed excessive by at least 100%, but I got no traction at all with a suggestion to reduce capacity. In his defense, the system used water source heat pumps, so there would have been dozens of changes, the equipment had already been ordered and some of the load differences would have fallen into the normal cracks between standard capacities.
That said, engineers routinely select equipment for a situation that will probably never occur … because it might occur and their client’s attorney will shoot them down if a problem arises. They may pick a DesignDay condition that’s excessive, 100% occupancy, lighting and plug loads and a substantial safety factor, too. After that process, if they find that their load is just above the standard capacity for one model, they pick the next larger model! That process will produce equipment which is definitely larger than E+ thinks is necessary almost every time.
The Building Performance Team
James V. Dirkes II, P.E.,
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