Specialty Buildings

Dewberry project profile: Marriott Headquarters

Marriott Headquarters, Bethesda, MD
By Dewberry August 13, 2019
The primary ductwork upstream of terminal devices was smaller for DOAS sized at 0.24 to 0.30 cubic feet per minute per square foot versus LTVAV sized at 0.75 to 0.80 cubic feet per minute per square foot.

Engineering firm: Dewberry

2019 MEP Giants rank: 36

Project: Marriott Headquarters

Location: Bethesda, MD, U.S.

Building type: Hotel/motel/resort, office building, parking garage/service station

Project type: New construction

Engineering services: Automation, controls; commissioning, retro-commissioning; electrical, power; energy, sustainability; fire, life safety; HVAC, mechanical; lighting; plumbing, piping; energy modeling

Project timeline: March 2017 to November 2021

MEP/FP budget: $1,380,000

Challenges

The biggest challenge was achieving the high corporate standards for quality, energy efficiency, sustainability, resilience and life cycle set by the ownership and developer team balanced with competitive first cost budgets. Dewberry worked closely with the client stakeholders to meet or exceed expectations. A highlight of the design process was a comparative analysis between two high efficiency heating, ventilation and air conditioning (HVAC) systems for the 730,000-square-foot, 21-story office tower, which included direct outside air system (DOAS) versus a low temperature variable air volume (LTVAV) system. The DOAS was considered on the basis of superior energy efficiency, reduction in equipment room area and reduction in plenum height. The LTVAV system was on the table due to first cost advantages and energy efficiency. The results of the analysis and decisions regarding the HVAC system differed from expectations and conventional wisdom.

The primary ductwork upstream of terminal devices was smaller for DOAS sized at 0.24 to 0.30 cubic feet per minute per square foot versus LTVAV sized at 0.75 to 0.80 cubic feet per minute per square foot.

Solutions

The HVAC system analysis compared the total equipment area for each system. DOAS eliminated the typical floor air handling units (AHU), but increased the size of outside air (OA) and toilet exhaust (TE) duct risers and did not eliminate the HVAC piping risers. For the typical floor cores, the DOAS system saved approximately 1,800 square feet for levels two-21. However, for the penthouse level, the DOAS system utilized approximately 2,500 square feet more area than the LTVAV system due to the increased capacity of the DOAS heat recovery outside air AHU’s. The analysis concluded that the LTVAV system would use approximately 700 square feet less total area than DOAS.

Both HVAC systems use low-height, 11-inch deep, series-type, fan-powered terminal devices with high-efficiency electronically commutated motors (ECM), primary air inlet VAV valves and return air openings to the plenum. Primary ductwork was limited to a total depth of 11 inches for both systems to minimize impact on plenum height. The DOAS system adds the following to each terminal device: chilled water (CHW) sensible only cooling coil, CHW control valve, CHW supply and return piping, drip pan, filter and expanded service clearances. Both systems use the same type of heating, either hot water or electric. Both systems use low-temperature primary air upstream of the terminal devices at 46- to 48-degrees Fahrenheit. Both system use heat recovery outside air AHU’s sized at 0.24 to 0.30 cubic feet per minute per square foot for DOAS versus 0.14 to 0.17 cubic feet per minute per square foot for LTVAV. Both systems use pre-conditioned pre-filtered outside air. Downstream of the terminal devices supply air temperatures differ from 56- to 59-degrees Fahrenheit for DOAS versus 52- to 53-degrees Fahrenheit for LTVAV. At 73-degrees Fahrenheit room temperature, DOAS requires 24 to 42 percent more supply air from the terminal devices compared to LTVAV. Increased terminal device air flow with increased pressure losses for filters and coils translated to higher energy consumption for DOAS.

Additional differences between the two HVAC systems include:

  1. CHW control valves: 40 for LTVAV inside AHU rooms; 600+ for DOAS above the tenant space
  2. Filters: 40 for LTVAV inside AHU rooms; 600+ for DOAS above the tenant space
  3. Isolation and balancing valves: 80 for LTVAV inside AHU rooms; 1,200+ for DOAS above tenant space
  4. CHW piping above tenant space: No piping for LTVAV; 18,000+ linear feet for DOAS

In summary, the analysis indicated the DOAS system, when compared to a high efficiency LTVAV system, had a higher first cost, increased equipment space, increased maintenance and life cycle costs, increased ceiling plenum congestion, less tenant build-out flexibility and increased risk of piping leaks and condensation. Based on manufacturer’s terminal device fan energy data and energy modeling analysis, the DOAS system had higher annual energy consumption and operating costs. Therefore, a LTVAV system was selected for the project with a projected energy cost improvement of approximately 28 percent to 32 percent above ASHRAE baseline.


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