When Sizing HVAC Systems, Focus on Proper Margin for Safety

  July 2, 2009

When the HVAC system is being designed, one important decision is how big equipment needs to be. Peak demand on very cold and very hot days is one important consideration. And flexibility for change in use of a space may be another element in the decision.

Of course, there’s a price to be paid for extra capacity, and it’s not just the higher first cost of larger units. Operating expenses are likely to rise for equipment operating at part load. What’s more, larger units may very well be noisier. Properly sized units, by contrast, will likely run better, be more energy efficient and have fewer maintenance problems.

Clearly it’s important not to oversize units. But there’s evidence that does happen in some cases. One possible source of trouble is industry rules of thumb for determining occupancy when occupancy isn’t known. Those rules of thumb may lead to an overestimation of loads. Another potential problem is building in too large a margin for so-called design days, when temperatures are at their highest and lowest, so that designers have a cushion — just in case.

For facility executives, the way to avoid oversizing is to work closely with the design team to ensure that the system can handle the loads it will be facing, with an appropriate margin for safety — but not an excessive margin.


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