Careful Consideration of Loads and HVAC Systems Will Ensure Modern Comfort in Buildings of the Past

mechanical system retrofits

HVAC systems have a recommended life span of 20 to 25 years. Most building owners try and squeeze an additional five to 10 years on systems to defer upgrades for financial reasons. Lack of reliable comfort conditioning and obsolete parts force most owners to retrofit and upgrade systems. However, in the last decade, other factors are starting to add to the push for upgrades. These include improved energy efficiency, thermal comfort and air quality.

Understanding options for upgrades requires comprehension of available technologies but also an understanding of the existing building envelopes, which typically outlast systems in most buildings. Building vintage often provides insight into building envelope compositions. Buildings built prior to the 1970s had little or no insulation in walls and other opaque assemblies. Major advancements in fenestration—both frames and glazing—have doubled thermal performance over the last three decades. Further, most buildings have undergone lighting upgrades to capture the low-hanging fruit for energy savings sweetened by utility company incentives. These lighting improvements have changed the dynamic of how we heat and cool buildings. For example, office buildings designed in the 1980s and 1990s used 3 watts per square foot for lighting power and often used this lighting power to keep them warm in the winter. The new standard for lighting is 0.8 to 1.0 watt per square foot.

Load profile for a retrofit project.

Load profile for a retrofit project.

Load Calculations

The first step in an HVAC retrofit is to re-evaluate the heating and cooling loads. The knee-jerk reaction to replace systems in-kind for capacity can not only add to costs but also reduce
comfort by reducing the range over which a system can adjust under partial-load conditions. Some key aspects to consider while performing this load calculation include:

  • Determining accurate values for existing wall and glazing thermal properties. In some cases, this may require explorative demolition of wall assemblies and laboratory testing of glazing.
  • Determining infiltration through the building envelope. Old rules of thumb don’t always work. Modern technology now allows us to measure these in very affordable ways.
  • Establishing building use with the building owner for both type and schedule. Very often these have evolved over the age of a building. This also helps determine the plug loads in the building—something very different from when the building was first occupied.
  • Establishing lighting power density based on current or proposed lighting design. LED lighting has reduced these values by 200 to 300 percent!

An analysis of the loads will provide guidance on peak loads for heating and cooling. To improve efficiency and controllability, it is key that an hourly load profile for the year be reviewed. A profile with bins indicating number of hours at every 10 percent increment in load is a very helpful tool and generated by most load calculation software. A building with a peak load of 500 tons may be best served by a 200-ton and a 300-ton machine rather than two 250-ton machines.

Systems Evaluation

Once loads are finalized, system choices should be evaluated. Some key considerations for evaluating system choices include:

Load profile for a retrofit project.

Load profile for a retrofit project.

  • Building location and climate.
  • Building size and type.
  • Space for systems and floor-to-floor heights to accommodate distribution.
  • First cost and operating costs.
  • Expertise of building maintenance staff.
  • Life expectancy, controllability, air quality, acoustics, etc.

Systems can be broken down broadly into two categories—mixed-air systems and de-coupled systems. Mixed-air systems handle fresh air and thermal conditioning via the same air stream while de-coupled systems separate fresh air from thermal conditioning. Most traditional systems, like variable air volume (VAV), constant volume with reheat, etc., are mixed-air type while most modern systems, like variable refrigerant flow (VRF), radiant heating and cooling, chilled beams, etc., are de-coupled. Older buildings with no central air generally have de-coupled heating-only systems with steam or hot-water heat at the room level.

VAV systems have been the work horse of our industry for most non-residential applications greater than 50,000 square feet. Being mixed-air systems, they blend fresh air and return air in varying ratios, resulting in spaces that either have too much fresh air (not ideal for energy efficiency) or too little fresh air (not ideal for good air quality). Further, these systems use all ducted air supply to rooms, which requires the most space for equipment and duct distribution. VAV systems need ceiling spaces between 24 to 36 inches. These systems have been around for several decades and installers and operators have the most familiarity with them. They were the most efficient systems for most non-residential buildings from the 1970s to 2000.

Industry evolution and improvement to building codes along with the green- building movement have pushed the use of de-coupled systems. These systems process and deliver fresh air individually to each space ensuring that neither too much nor too little of it is being supplied. This fresh air system is commonly called a Dedicated Outside Air System (DOAS) and processes about 50 to 75 percent less air than a mixed-air system and hence requires substantially less equipment and distribution space. Ceiling space requirements vary between 12 to 24 inches. Heating and cooling for comfort is delivered to the space using refrigerant or hydronics (water), both of which carry significantly greater amounts of heating or cooling energy per unit volume compared to air, making de-coupled systems more efficient and needing significantly less space.


About the Author

Sachin Anand, P.E., LEED AP BD+C
Sachin Anand, P.E., LEED AP BD+C, principal at Chicago-based dbHMS, brings more than 15 years of diversified design, commissioning and management experience to the field of engineering. He believes the structure, site, lighting systems, HVAC systems, indoor environment and end use of a building should be viewed as an integrated system.

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