500 BTUs per Person Per Hour?

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, I'm just kidding. That's crazy talk. There is way more to it than that.

However, in a commercial environment, while the building's perimeter 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 that a sleeping person generates heat more in the neighborhood of 260 BTUs/hr, so if it's a REALLY boring job where workers doze off at their computers, it may be less.

Add in the internal electrical loads from lights, computers, and other equipment. 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 building's perimeter 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 and maintaining appropriate ventilation/outdoor air.

Food for thought.

—Bryan

Related Tech Tips

Nomenclature and How to Use It
Nomenclature on HVAC/R equipment is a sequence of numbers and letters a manufacturer uses to speak directly to the technician. Lots of initial upfront information is handed to the technician by the manufacturer the moment the technician reads the nomenclature in the model and serial numbers. So, how do we make sense of these seemingly […]
Read more
How to Compare Heat Sources For Cost Effectiveness
This article is based on Ross Trethewey’s energy comparison sheet for fuels at select efficiencies and costs. All tables and their respective data belong to him. Ross Trethewey is the founder and lead engineer of TE2 Engineering and has served as the Home Technology Expert on “This Old House.” You can find a PDF file […]
Read more
Making a Flare - Quick Tips
First off, we need to clarify that very few unitary manufacturers use flares anymore. You will most often find flares on ductless and VRF/VRV systems and in refrigeration. A flare uses a flared female cone that's formed into tubing (usually copper). That cone is then pressed onto a male cone (usually brass) by a threaded […]
Read more

One response to “500 BTUs per Person Per Hour?”

  1. 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.

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from