In developed and rapidly industrialising countries, the average person spends about 90% of his or her life indoors, and much of that time is spent at work in commercial office buildings. While comfort is clearly important during all that time spent indoors, the traditional air temperature-centric design approach used in buildings for decades is inefficient.
There are many other variables, such as radiant temperature and air speed, driving a person’s comfort. By focusing on every variable, a design strategy can become more targeted and focus the building’s energy where it makes the biggest difference in occupant comfort – all the while using less energy. In this post, we will describe the variables that define thermal comfort and see how this can help with building design.
The Six Variables of Thermal Comfort
To shift the focus from the traditional comfort metric of temperature, the conversation must evolve from space temperature to include all of the variables that affect a person’s comfort. Instead of trying to cater to these variables with a large, central heating and cooling system, I am sure many readers of this blog will know of technologies that allow for fine-tuning of individual people’s personal comfort. These approaches can be as simple as a desk fan and as complex as a radiant ceiling fan.
Extensive research by ASHRAE has found six variables that may predict a person’s thermal comfort.
People intuitively know these factors affect their comfort, but have only recently been translated into metrics building designers can use. The ASHRAE 55 comfort standard uses a formula to translate these six variables into a single output, called predictive mean vote (PMV). In short, the PMV predicts what percentage of people will be comfortable in a given condition.
This formula was developed using principles of heat balance and experimental data collected in a controlled climate chamber under steady state conditions. The PMV scale stretches from plus 3 to minus 3 with zero as the neutral point. A relationship has also been developed with Predicted Percentage (of people) Dissatisfied (PPD). At a PMV of + or – 3, 100% of any group is dissatisfied. At zero, at least 5% of a group is still dissatisfied.
From the perspective of an architect or a contractor, PMV provides a clear, standardised specification for a building. When designing to all six variables, however, there are no set ranges for each that can be specified. Instead, the acceptable range for each variable varies with the state of the other five variables. To help designers, the Center for the Built Environment (University of California, Berkeley) took this complex formula and translated it into an interactive, graphical tool.
By varying the inputs for each of the variables, a user can find not only the predictive PMV, but also where those conditions fall within the range of comfortable conditions. For instance and especially relevant to this blog, when weighing a trade-off between additional cooling and extensive ceiling fans, this tool can visually demonstrate the significant impact of air speed on an occupant’s comfort.
A result from the aforementioned CBE tool would be displayed on a psychrometric chart (see above). Psychrometrics is used to help select the proper air conditioning equipment and determine the environmental conditions that affect human thermal comfort. The tool also helps to understand a building’s regional climatic context, and better address human occupancy and use and structural considerations.
The chart uses three main categories: temperature, moisture, and relative humidity, to inform the design of energy efficient and properly sized HVAC systems. The versatility of the chart lies in the fact that knowing any two of these properties fixes a point on the chart from which all the other properties can be determined.
Maintaining thermal comfort for building occupants is one of the most important goals of HVAC design engineers. If we can understand the variables of thermal comfort in the context of our regional climate, and the mechanisms by which they operate in relation to human physiology, then we can design buildings that provide comfort in more rich and economical ways than a standard HVAC solution.
Let us take a HVAC design example especially relevant to Asian climates.
There are different ways to remove moisture from the air in a building: cooling below the air’s dewpoint, desiccants that adsorb water (as seen in our post on energy wheels), or compression to condense water out of the air. But, to properly design a HVAC system in humid climates, it is important to consider both the sensible and latent heat of a given building.
Sensible heat refers to the temperature load of a given space.
Latent heat is basically the moisture load of the building.
It is the energy given up or taken up by the air as water changes phase, such as vapor condensing into liquid. It considers the moisture content, which engineers often forget about when designing and sizing HVAC systems. The latent load is crucial because it is a significant load and also accounts for the outdoor moisture at peak and part load conditions as well as moisture from the people in the room (Indoor and outdoor moisture loads). If latent load is not considered it may lead to increased moisture in the space that could create higher relative humidity, discomfort and building deterioration. Combining the latent and sensible heat gives the total heat load.
There are two main sources of building heat: internal and external.
Internal sources include people, lights, appliances, equipment; external sources include solar load, conduction, ventilation, and infiltration. Temperature increases from cold to hot along the x-axis of the psychrometric chart.
The temperature measured in this case is the ‘dry bulb’ temperature, which is that of an air sample as determined by an ordinary thermometer. It is called “dry-bulb” since the sensing tip of the thermometer is dry and does not take the moisture content of the air into account.
Moisture in the air is an important consideration of air conditioning system design, and thus an integral element of the psychrometric chart.
Like temperature, there are also two main sources of moisture: internal and external. Internal includes evaporation, desorption, and people (breath, clothes); external includes ventilation, infiltration, and permeation. Internal moisture sources are as important to consider as outside moisture sources since each person emits 0.25 pounds of moisture per hour, therefore 100 people at a moderate activity level produce three gallons of water per hour. Moisture is measured by a humidity ratio measurement, which is found on the y-axis, with lines of constant humidity ratio running horizontally across the chart. The humidity ratio is the weight of water per unit of dry air. This is often expressed as grains of moisture per pound of dry air, with 7,000 grains of moisture per pound of water.
The ASHRAE user’s manual states that HVAC systems that have dehumidification must be designed to control relative humidity when analyzed for either of the following design conditions:
1. At the peak outdoor dewpoint design conditions and the concurrent (simultaneous) indoor design latent load, or,
2. At the lowest space sensible heat ratio expected to occur at the concurrent (simultaneous) outdoor condition.
Relative humidity provides an indication of how close the air is to its saturation point. It compares the amount of water in the air to the Figure 6: Mean daily average dew point temperature, October amount of water that the air could potentially hold at that temperature. This affects the perceived temperature of an environment, as well as mold growth and stability of building materials.
On the psychrometric chart, lines of constant relative humidity are represented by the curved lines sweeping up from the bottom left and to the top right of the chart. Wet-bulb temperature is determined by circulating air past a wetted sensor tip, which is affected by air saturation.
In practice, this is the reading of a thermometer whose sensing bulb is covered with a wet sock evaporating into a rapid stream of the sample air. (Note: WBT will be the same as DBT when the air sample is saturated with water, since no water can evaporate in those conditions.) The line for 100 percent relative humidity, or saturation, is the upper, left boundary of the chart. This is also the dew point temperature.
Dew point temperature indicates the temperature at which water will begin to condense out of moist air. A rule of thumb is that the annual night time temperature for a given region is that area’s dew point temperature. See Figure 6 for a map of the daily average dew point temperature for October in the continental United States. High levels are represented by the red areas and range down through the spectrum to low levels in the purple and blue areas.