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Physiology of Human Comfort – Our Approach

Coming to Terms


“I’m too hot”, says an occupant of a conditioned space. What does “hot” mean to the complainant?

If two glass beakers of pure water are set before you, one beaker measuring 5 liters, the second 1 liter, both precisely 75.0 degrees F, which, I ask you is “hotter”? In this context, “hot” does not describe temperature, but a sense of personal comfort that is the consequence of several factors. By no means do I suggest that high temperature will not cause an uncomfortable feeling of being “hot”, but I do wish to impart a multi-dimensional, multi-variable perspective on comfort control.


Another occupant of the structure complains his space is “humid”. Is everyone feeling alike? Is it just one person? Whatever you do, don’t argue the point. If a general feeling of discomfort, there is a mechanical remedy. We can do nothing about hormones and illnesses.

For a moment, let’s look at “humidity”. For this exercise, we run over to the nearest Walmart to purchase a sponge and a bucket for this next experiment.

The sponge will represent air and temperature will be represented by the compression of the sponge.

Step 1: fill that new bucket with cold tap water.

Step 2: plunge your new sponge deeply in the water and work it to remove all air.

Step 3: place your hand under the sponge and carefully lift it from the water without compressing it.

Question: At this point, what is the relative humidity of the sponge (representing air)? Note: some water ran out of the sponge as you lifted it from the bucket.

If you answered 100%, you are very close. In reality, it would be around 90%, due to runoff.

Step 4: carefully holding still that saturated sponge, squeeze until the sponge becomes half its previous size.

Question: What is the relative humidity of the sponge, now? If you answered 50%, you’re incorrect. That’s right, water flooded out and now relative humidity is 100%.

Relative humidity is a function of absolute humidity and dry bulb temperature.

Explanation: Air, represented by the sponge, becomes denser as temperature lowers. Principally, oxygen molecules get closer together as air temperature lowers. As distance between air molecules lessens, there is less space available for H2O, forcing water to condense into a liquid state. When a given volume of air cools to its dew point, moisture precipitates and relative humidity of the air becomes (for discussion purposes) 100%.

Conversely, adding heat to the same air volume causes air molecules apart, thereby creating space into which the air can absorb moisture.


One dictionary defines enthalpy as “a thermodynamic property of a system equal to the sum of its internal energy and the product of its pressure and volume”. Simply stated, enthalpy is the total heat, sensible and latent, energy in a given volume of air.

We use thermometers to measure the sensible and we use hydrometers (relative humidity indicators) to express the latent, or hidden, heat. Actual computation takes into account pressure, but for our purposes of simplification, we ignore barometric pressure.

Enthalpy is the factor that better expresses comfortable indoor climate than either temperature or relative humidity alone. Referring to the ASHRAE psychrometric chart below, note that a dry bulb temperature of 75OF intersecting to a relative humidity of 50%, indicates an enthalpy of approximately 27.5 Btu/lb. dry air.

Figure 1 ASHRAE Psychrometric Chart - Click on image for larger size

Physiology Brief (very brief)

One can identify specialized corporal zones where comfort may be more specifically addressed. The following is the Berkeley segmented thermal manikin with 16 segments.

Contrary to conventional thinking, blood temperature varies depending how each of these 14 zones (more can be identified) responds to external temperature.

Figure 2 Berkeley Segmented Thermal Manikin

Figure 2 Berkeley Segmented Thermal Manikin[i]


  • Skin temperature is typically 10 degrees cooler than core temperature.
  • Skin releases heat from the body two ways:
    • Heat as infrared energy radiates outward from the body
    • Perspiration cools by evaporation (more on this later)
    • The vascular system and your sinuses serve as heat exchangers.
      • Blood flows in parallel paths. Wherever you find an artery, there is a corresponding vein returning blood to the cardio-pulmonary processing station.
      • As blood moves to the skin, it exchanges heat with the environment.
      • Air breathed through the nose undergoes a conservation of energy exchange by means of blood filled membranes and bony parts. This process tempers air entering the lungs to prevent too hot or too cold air from entering the bronchia.

Getting Germaine

A host of factors affect personal comfort.

  • Temperature
  • Relative Humidity
  • Air Filtration
  • Air Volume (air exchanges per hour)
  • Air Velocity
  • Indoor Air Quality (IAQ)
    • Airborne pollutants like dust, dander, perfumes, human odors
    • Carbon Dioxide Concentration
    • Toxic Gasses
    • Human relations among co-workers and supervisory personnel

Localization of Comfort Control

Traditionally, temperature control, comfort control, has been viewed as controlling the entirety of a building, but doing so doesn’t always yield even comfort and heating and cooling unoccupied spaces has a distinct cost associated with it.

In our never ending quest to lower building operating costs, designers now focus on localized control of thermal comfort. This relatively new concept brings focus on individuals within the structure, instead of focusing on the building.

Figure 1 ASHRAE Psychrometric Chart – Click on image for larger size

Our Approach

Historically, HVAC mechanical design and associated control systems focus on the equipment, not the occupants. Our approach for over 20 years is to focus on occupants and work our way back to mechanical systems.  For example, a traditional method to control the hot deck of a multi-zone air handling unit has been the outdoor air temperature reset: as outdoor temperature riseses, the temperature contro system proportionally lowers the hot deck temperature set point, without regard to the needs of occupants.

Rockwall Controls Company, Inc. has developed several algorithms now widely deployed throughout Texas. iLoad is an adaptive software that manages air handling units by means of a body of knowledge around the occupants served by a particular HVAC mechanical system.

Is our method logical? Is it logical to equally apply costly heat to a vacant space as to a space fully occupied? The answer is obvious.

Typical Applications

  • Variable-Volume Variable-Temperature systems
  • Multi-zone Air Handling Units
  • Underfloor air distribution systems


Occupant comfort depends on a range of variables, not just temperture.

  • Temperature
  • Relative humidity
  • Air velocity (air motion)
  • Barometric pressure
  • Volume of conditioned space (confined spaces require more intense solutions than large spaces)
  • Stratification of conditioned air

How can we help you?

Rockwall Controls Company, Inc.

Call anytime (972) 771-3514 to discover how Rockwall can make life a little more comfortable and a little less costly.

[i] A model of human physiology and comfort for assessing complex thermal environments, Charlie Huizenga , Zhang Hui, Edward Arens, Center for Environmental Design Research, University of California, Berkeley, CA 94720-1839, USA