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Thermal Environmental Indices
Copyright (c) 2013 Robert Bean, R.E.T., P.L.(Eng.) and content providers. All world rights reserved.
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Outside of the student and practitioner of indoor climate engineering, virtually all of those involved in the North America construction industry contract thermal comfort exclusively with air temperature when it by itself is the most inadequate ambassador to the HVAC system. Listed below are other indices of far more utility when representing conditions for thermal comfort.

Figure 1.  Environmental indices for evaluating thermal comfort in indoor spaces.

Though several of the indices can not be represented easily with "off-the-shelf" equipment, operative temperature is one that is within reach of all projects. In our opinion for thermal comfort control, the most suitable arrangement would be a radiant based system controlled by operative temperature in parallel with a dedicated outdoor air system of which humidity is just one element of the control strategy for conditioning the air. Operative temperature thermostats are commercially available or the value can be presented by a matrix of embedded sensors for measuring surface temperatures and integrated with the dry bulb temperature for operative values.

Incidentally, one of the persistence errors in literature when it comes to the graphical method of thermal comfort using the ASHRAE Standard 55 psychrometric chart is taking the x-axis as dry bulb temperature when in fact it is operative temperature.  There are circumstances where dry bulb temperature equals operative temperature but this (again in our opinion) only occurs in high performance buildings or what we would classify as a Type 4 structure and where the met rates, clo values, humidity and air speeds fall within the graphical comfort zone.


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