How to Choose the Best HVAC Office Design

By Paul Kenney | September 15, 2021 | HVAC, Architecture, Contractors | Design | Engineering | Owners | Sustainability

Introduction

For too many years our AEC industry has relied on past experience to continue to design and install systems for offices that are grossly less efficient than current HVAC systems that are available at similar or less of the cost. In this blog, we will focus on HVAC office design for multi-story buildings and compare some common systems that are designed and installed. The most popularly used three we will focus on are Self-Contained Water-Cooled Unitary Devices (SWUDs) with Cooling Tower, Chilled Water Air Handling Units (AHUs) with Air-Cooled Chiller, and Water-Cooled VRF/VRV with Cooling Tower. I hope this blog will help you make the most efficient decisions for your office designs, your client’s money, and Mother Earth.

What are the office HVAC systems being compared?

Self-Contained Water-Cooled Unitary Devices (SWUDs) with Cooling Tower

                                     Figure 1                                                                                        Figure 2

This system will have a SWUD on each floor (typically in a central mechanical room) with large medium pressure duct to Variable Air-Volume (VAV) Boxes and Power-Induction Unit (PIU) Boxes. The VAV boxes are typically used for cooling only zones and the PIU boxes have electric strip heat and are used for cooling and heating zones at the exterior of the building. The SWUDs have large, high pressure supply fans to provide air to the entire floor and packaged refrigerant cycle with compressor(s) in the same box that rejects heat to the condenser water. That water is pumped up to the cooling tower where it is rejected to the outdoors. The cooling tower, SWUDs, PIUs, VAVs, pumps, etc. are typically controlled by a central Direct Digital Control (DDC) system.

    Pros

    • SWUDs with Cooling Tower are a common design that contractors are familiar with installing.
    • Mechanical Contractors typically get the future office fit-up work because they are familiar with the controls installed in the base building.
    • Medium pressure ductwork with VAVs/PIUs allows for future fit-up flexibility since you can add VAVs/PIUs for tenant needs.
    • Water-cooled cooling is efficient.

    Cons

    • SWUDs are loud.
    • An electric strip heat is the least efficient type of heating.
    • Fan energy for a medium pressure loop gets expensive.
    • The medium pressure ductwork is large which can lead to lower ceilings and aesthetic challenges for architects/engineers to work around.
    • Cooling towers, pumps, piping, SWUDs, controls, and most of the system are installed in the base building. This drives up the base building cost for owners.
    • A central plant with cooling towers, pumps, water loop, etc. is more complicated and requires more maintenance.
    • Cooling tower evaporation requires make-up water which should be considered based on location.
      Chilled Water Air Handling Units (AHUs) with Air-Cooled Chiller

        Figure 3

        This system is similar to SWUDs, but you have a Chilled-Water AHU on each floor instead of SWUDs. The AHUs have large, high pressure supply fan(s) and use chilled water for cooling. The AHUs do not have compressor(s) like SWUDs but have the same ductwork, VAVs, and PIUs out in the office space. With this system, the chilled water is cooled by a chiller which uses the refrigeration cycle and compressor(s). Larger office buildings should use water-cooled chiller(s) with cooling tower(s) to improve cooling efficiency.

        Pros

        • Chilled water AHUs have reliable and accurate control of supply air temperature.
        • AHUs do not have compressors, so they are quieter than SWUDs but are still noisy due to the airflow in and out of mechanical rooms.
        • Mechanical Contractors typically get the future office fit-up work because they are familiar with the controls installed in the base building.
        • Air-cooled chillers are a reliable and simple closed water system that requires less maintenance than cooling towers.

        Cons

        • AHUs are loud.
        • An electric strip heat is the least efficient type of heating
        • Fan energy for a medium pressure loop gets expensive.
        • Air-cooled cooling is less efficient than water-cooled.
        • The medium pressure ductwork is large which can lead to lower ceilings and aesthetic challenges for architects/engineers to work around.
        • Chillers, pumps, piping, AHUs, controls, and most of the system is installed in the base building. This drives up the base building cost for owners.
          Water-Cooled VRF/VRV with Cooling Tower

          Figure 4

          This system has water-cooled VRF/VRV heat recovery units in the main mechanical room on each floor. The heat recovery units use the refrigerant cycle with variable speed compressors that transfers the heat from the refrigerant to the condenser water that is piped to the cooling tower(s). Unlike SWUDs and AHUs that move cold air to cool the space, the VRF/VRV systems move refrigerant to small indoor units (like PIU/VAV boxes) more efficiently to control zone temperatures in the office space. Most manufacturers have a large family of indoor units that can be installed in the ceiling, above ceilings and ducted, on walls, etc.

          Pros

          • Water-cooled VRF/VRV’s is energy efficient.
          • Water-cooled VRF/VRV’s are quieter than SWUDs and Chilled Water AHUs.
          • There are more options for indoor units versus other systems.
          • Smaller ductwork is needed.
          • Controls are included.
          • Water-cooled VRF/VRV heat recovery units can be brought up elevators and installed later during tenant fit-ups, which significantly lowers the base building cost.
          • Tenant fit-ups can be designed precisely to tenant needs.

          Cons

          • Indoor units have coils that require condensate drains.
          • Second and third generation tenants may require modifications to the system.
          • Mechanical contractors need to have experience installing VRF/VRV systems, but this is becoming less of an issue as more VRF/VRV systems are being installed in the industry.
          • A central plant with cooling towers, pumps, water loop, etc. is more complicated and requires more maintenance.
          • Cooling tower evaporation requires make-up water which should be considered based on location.

            How to select the best system for each building

            Get with your owner/architect/engineers/contractors early to evaluate system options for each project. This should be done early on so the architect/engineers can design the building to accommodate the selected HVAC system. It is also a good idea to get contractors involved for some pricing feedback. Figure 5 represents a score of 1 to 10 (10 being best) on a few key features the owner may desire in his building. Using a graph and discussing options with the team can help identify the project’s end goals.

            Figure 5

            At Proficient, we have developed spreadsheets that take simple inputs and provide instant feedback to the owner detailing estimated upfront cost and energy savings to help the owner make educated decisions on the best system for his\her building. Below is an example of inputs and results from a Proficient spreadsheet.

            Figure 6

            As seen in Figure 6, the above Atlanta, GA building and most others that we have evaluated, the VRF/VRV system is favorable for the owner. The heating efficiency and fan energy play a big part in why VRF/VRV saves so much energy. The VRF/VRV systems have drastically reduced cost by being manufactured on US soil. There are multiple manufacturers to keep competition high and prices low. We reached out to some mechanical contractors to get pricing back and the VRV/VRF is about the same cost as the SWUD system. Owners can also take advantage of saving a significant cost on the base building and install the VRF/VRV systems as they work out leases with tenants down the road.

              Closing

              It is time to stop installing the same basic office HVAC systems from decades ago! You have now been enlightened to become a progressive owner, architect, or engineer. There is no reason why SWUDs and/or CHW AHUs should continue to be installed on most of our U.S. office projects, yet VRF/VRV system are still the least installed system for office design in the U.S.

              If you don’t feel comfortable with what your owner, architect or engineer is proposing for your next office building HVAC system, give us a call or get a second opinion. Email me at pk@proficientengineering.com with some basic building information (location, SF per floor, # of floors), and I can provide some quick feedback quickly free of charge.