500 BTUs per Person Per Hour?

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I heard a great presentation by Ron Auvil on VAV systems and it got me thinking…

Can you size a commercial system / perform a block load by the number of occupants?

Yes! 

No, just kidding that’s crazy talk. There is way more too it than that.

However, in a commercial environment, while the perimeter of the building is affected by heat loss/heat gain to the outdoors, the internal zones are “cooling only” zones with the primary load usually being PEOPLE.

This is where the 500 btus per hour comes in. On average a sedentary worker in a building will add 500 btus per hour to ALL areas of the building whether it is hot or cold outside. This creates an issue in the winter when the perimeter of a building requires heating and the center of the building requires cooling.

Now, keep in mind, a sleeping person generates heat more in the neighborhood of 260 btu/ hr so if it’s a REALLY boring job where workers dose off at their computers it may be less.

Add in the internal electrical loads from lights, computers and other equipment and you start to realize that EXTERNAL loads are only part of the equation, especially in large commercial buildings with many occupants. In fact, in a busy commercial space the internal loads generally far outweigh the heat entering from the outside (external load).

This is where the concept of thermal diversity comes in. On a cold day there may be a need for heat at the perimeter of the building to offset heat losses to the outside while still requiring cooling in the center of the building to offset the internal loads.

In a good commercial design you must have some method of dealing with the thermal diversity between internal and perimeter zones along with maintaining appropriate ventilation / outdoor air.

Food for thought.

— Bryan

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One comment

  1. Ratib Baker says:

    Very true now. But this is now a concern in today’s commercial buildings, when energy savings are pursued. While replacing incandescent with florescent light did not change the heat gain much, going to LEDs drops the heat gain a lot. Almost all the load is now people. Building managers are finding the reheat systems too small in the wintertime. A lot more cold complaints.

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