Analyzing nontraditional HVAC systems

Incorporating set thermal comfort criteria provides many benefits while increasing the use of high-performance HVAC systems for energy reduction.


Learning Objectives:

  • Analyze thermal comfort criteria for improved user experience.
  • Consider partial-cooling systems.
  • Maximize value through zone cooling and heating.

Thermal comfort is an indoor environmental-quality feature that has proven to impact productivity. Variables that impact the air temperature at which one may feel comfortable are clothing, metabolic activity, indoor air temperature, humidity, radiant temperature, and air speed. In addition to these factors, user expectation can also shift if the space is naturally conditioned using operable windows with the availability of control. The variability of thermal experience may be different when naturally cooled using operable windows, which may be considered in conjunction with biophilia as an enhancing addition.

Thermal comfort standards

The 2013 edition of ASHRAE 55: Thermal Environmental Conditions for Human Occupancy specifies methods to determine indoor thermal environmental conditions that a significant number of occupants would find acceptable. The standard details thermal comfort for mechanically and naturally ventilated spaces, based on the following factors:

  • Clothing
  • Metabolic activity
  • Indoor air temperature
  • Humidity
  • Radiant temperature
  • Air speed.

The U.S. Green Building Council (USGBC) LEED and International Well Building Institute WELL Building standard refer to ASHRAE 55 for compliance. The WELL Building standard includes thermal comfort as a precondition for certification, with radiant thermal comfort and individual thermal comfort as optimizations for high levels of certification. The USGBC LEED guideline, in addition to ASHRAE 55, provides an option to use International Organization for Standardization (ISO) and European Committee for Standardization (CEN) standards. LEED includes one credit for meeting the thermal comfort requirements per ASHRAE 55 and providing individual controls for 50% of the individual occupant spaces.

Figure 1: Exterior shading was designed to reduce cooling load. The higher ceilings would not have been possible with a conventional variable air volume (VAV) system.User goals and priorities identified as enhanced user experience, health, and environment

Operative temperature is simply defined as the average between air temperature and the mean radiant temperature. This is because the temperature experienced or perceived by the user is based on the heat exchange with the immediate environment, through convective and radiant heat loss, or heat gain to our surroundings. The air temperature impacts the convective heat exchange and the mean radiant temperature of the surfaces around us impacts the radiant portion of the heat exchange. Operative temperature is how one may experience indoor temperature, even though a space is typically controlled or monitored solely using air temperature. This is often one of the main contributors to occupant thermal discomfort, even when the air temperature meets the typical 75°F during cooling conditions. ASHRAE 55 also establishes the acceptable operative temperature range for naturally conditioned spaces.

Heating and cooling design criteria

HVAC systems are generally designed for a predefined heating and cooling setpoint (typically between 70° and 75°F for heating and cooling simultaneously), based on 99.6% dry-bulb temperature for winter design conditions and 0.4% or 1% dry-bulb temperature for summer design conditions. In addition to this, there are numerous assumptions that have to be made for the cooling loads regarding infiltration, envelope, lighting, occupancy, and plug loads. Fifteen-percent oversizing for cooling and 25% oversizing for heating is also typically allowed, however these allowances are slowly being eliminated by codes.

For the end user, this does not provide much clarity on what to anticipate in terms of hours above the comfort threshold, especially with changing outdoor temperatures due to global warming. The design conditions also are typically based on air temperature, even though the radiant temperature, along with other factors previously mentioned, can change the perception of comfort to a large extent. Typically, a thermostat in a space is controlled by air temperature, yet what is experienced or sensed is operative temperature. Ongoing discussions should consider the following questions:

  • Should the industry define new mechanical-design criteria instead of continuing to use the ASHRAE setpoints as the design conditions?
  • Should there be conversations with clients to establish appropriate and acceptable thermal comfort criteria set for their building?
  • How often do buildings in reality use a lower cooling set point than 75°F, perhaps due to a perception of higher temperature in the space?

The goal of the criteria would be to create designs that can allow for variability and prevent oversizing to meet unforeseen conditions or extreme peak conditions. Oversized systems do not achieve thermal comfort for 100% of users and can lead to over-cooling of spaces for a portion of occupants, especially during shoulder seasons. Establishing set thermal comfort criteria will allow for enhanced mechanical design for varying outside conditions of spaces with operable windows, rather than focus alone on a fixed setpoint at peak design conditions. Operable windows can provide free economizer cooling, while the mechanical system provides the minimum ventilation required during conditions when windows may not be opened due to outside temperatures, noise, or odors.

Figure 2: This illustration shows predicted modeled savings from a dedicated outdoor air system (DOAS) plus chilled-beam system compared to a LEED baseline. The building includes a large percentage of plug loads energy consumption; this was kept consistenThe reason to discuss adaptable thermal comfort criteria is not only to limit overdesign, but also to provide solutions that can make it possible to integrate the use of operable windows with HVAC systems. An enhanced indoor/outdoor connection is often discussed during design, but due to air conditioning, it is often limited to large amounts of glazing with views to outside in place of a true connection. Other hurdles for operable windows include acoustics, air quality, and allergens. New true connections between the indoors and outdoors may be associated with health and wellness characteristics.

Currently, these don't carry additional credit within the WELL Building or the LEED guidelines, but they are included as a separate pathway to thermal comfort. These goals and objectives can provide truly integrated design opportunities between architects and mechanical engineers to maximize the value of the building envelope. The potential to allow natural conditioning when appropriate can impact the depth of spaces, further enhancing occupant daylight and views. The envelope is where the architectural and mechanical conversations need to start, which should include operable windows, glazing percentage and specification, and external shading to allow for more efficient mechanical systems.

In addition to maximizing user comfort and experience, using adaptive thermal comfort design criteria can potentially:

  • Decrease initial costs by reducing the need to overdesign
  • Reduce annual energy and utility costs
  • Increase the potential to use high-performance HVAC systems to meet energy-reduction goals and reduce global warming
  • Open up the possibility for systems that can meet net zero potential for some buildings.

Making a case for partial-cooling systems

The meaning of partial cooling can vary based on the intent, but simply stated, the design is not projected to meet the required 75°F setpoint during peak outdoor conditions based on 0.4% or 1% dry-bulb temperature for summer design conditions. Partial cooling is designed to meet set thermal comfort criteria, which allow a set number of hours above 75°, 80°, or even 85°F. The hours are determined using ASHRAE 55 or by discussions with the owner, often based on what might lead to complaints or what they are contractually required to provide.

There are several reasons to understand and embrace the concept of partial cooling as an alternative to full cooling.

  • Partial cooling can enhance the use of operable windows when outside air conditions are optimal, expanding the indoor-air band that is deemed comfortable.
  • Partial cooling can help quantify and optimize the building form; fine-tuning the envelope is key to integrating partial cooling.
  • Partial cooling can allow the building to "sail" through the shoulder months, but still provide some thermal stability by means of a right-sized mechanical ventilation and cooling system during peak conditions.
  • Using natural cooling can potentially allow ductwork to be sized for 100% outside air, which can provide the benefit of improved indoor air quality year-round, due to eliminating the mixing of return air.
  • In some cases, buildings are able to incorporate a geothermal heat pump system, because it is sized for heating conditions and provides partial cooling only. If this system was to be sized for full cooling, it would have to be oversized and would be beyond the project budget or even the required site area. Sizing for the peak cooling load can change the system type selected for the building due to budget or space needs.
  • When developing set thermal comfort criteria, partial cooling can be discussed to optimize the system type for thermal comfort throughout the year, annual energy and utility cost savings, and potentially better usability and control of operable windows.

Designing for annual climate conditions

The outside air conditions designed for are usual peak conditions. System choices, options, and recommendations would be different with a deeper understanding of the climate, and even more so if the goal is to set thermal comfort criteria. A better understanding of operating hours and temperature bins for a variety of ranges would provide different insight. Annual, hourly weather data is available through a number of different sources.

Spaces should be designed for thermal comfort criteria instead of a setpoint. For some locations, with the right architectural design, this would provide a way to eliminate cooling systems altogether and reduce infrastructure due to reduced number of hours when cooling is absolutely required.

Use of energy and thermal modeling can help engineers make envelope recommendations, such as shading and glazing specifications, as well as limit the size of the systems and/or incorporate more efficient systems like radiant ceilings, which are harder to model and size using conventional sizing tools. Advanced tools can help quantify the number of hours spaces may be above required temperature setpoints or if they meet the set thermal comfort criteria.

Mechanical engineers should play a larger role in helping architects define the envelope so the control of thermal comfort can be optimized. This includes glazing percentage, interior shades, etc. that are typically not discussed with mechanical engineers.

This approach could increase the viability of a larger number of deep green buildings, due to the decrease of the financial impact of oversizing systems that only serve a handful of hours in a year.

Adaptive thermal comfort is often discussed during the initial design phases. However, implementing adaptive thermal comfort requires a risk factor, and clients often shy away so as to minimize occupant complaints.

Adaptive thermal comfort

In the Pacific Northwest, these conversations require an understanding of the regional climate. Not having cooling is slowly becoming a rarity, despite the potential to achieve passive cooling through design. The following ideas need to be addressed by the industry or through conversations with the client or users of the spaces:

  • Is the connection to the outdoors being lost by the use of cooling instead of designing operable windows for use during desirable weather?
  • Are there benefits in changing the way access to the outdoors is designed to allow for some variability in the space conditions, rather than a monotonous constant temperature and environment?
  • How could providing controls to open the louvers to the outside when the weather meets desired conditions be integrated into the design?
  • How many hours in a year during which indoor air temperatures are above 75° or 78°F are acceptable to the client? Are ceiling fans acceptable?

An adaptive thermal comfort strategy requires integration and in-depth conversations between engineers and architects in determining the glazing percentage in the building based on orientation, shading, operable window free area, and glazing solar-heat gain values. The integrated design team needs to evaluate the envelope to optimize for daylighting as well as to reduce cooling loads by means of dedicated performance analysis. Additionally, thermal comfort modeling can highlight features that can be detrimental to thermal comfort in a space, like large glazing areas or glazing specifications that can lead to high mean radiant temperatures.

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