Rating System

LEED BD+C: New Construction, LEED BD+C: New Construction, LEED BD+C: Schools, LEED BD+C: Schools, LEED ID+C: Commercial Interiors, LEED BD+C: Core and Shell

Rating System Version

v2 - LEED 2.2, v3 - LEED 2009, v2 - Schools 2007, v3 - LEED 2009, v3 - LEED 2009, v3 - LEED 2009

Inquiry

Conventional vented domestic clothes dryers require approximately 200 cfm of exhaust when operating. In large multi-story residential buildings, the dryer exhaust is typically provided by dryer exhaust risers that vertically link multiple units with a constant or variable speed exhaust fan. Several exhaust risers may be used to meet the needs of all apartments in a building. Variable speed fans typically modulate based on static pressure in the exhaust riser and are limited no less than 25 percent of design flow. Constant speed fans assume some diversity and do not modulate. Either fan operates 24 hours per day. The dryer exhaust requires continuous makeup air that must be conditioned either at an outside air handling unit or as in additional infiltration load in individual residences. An alternative to conventional vented dryers are ventless condenser dryers. Condenser dryers still use heated air to evaporate water from the clothes, but use an air-to-air heat exchanger to condense water from the humid air rather than exhausting the air and replacing it with fresh air from the room. Heat from the dryer remains in the room and no external venting or makeup air is required. Vented dryers are the "standard practice" in large residential buildings. This is probably due to the fact that (1) vented dryers are the more familiar technology, (2) drying times are shorter with conventional dryers than with condenser dryers, and also because (3) vented dryers are less expensive than condenser dryers. Based on a 1998 study by James Kao of the National Institute of Standards and Technology (NIST) titled "Energy Test Results of a Conventional Clothes Dryer and a Condenser Clothes Dryer," condenser clothes dryers use between 5 and 30 percent more energy per pound of laundry than a conventional vented dryer (depending on the size of each load). The NIST study only accounts for the energy to operate the dryer. The study does not account for the additional effects on the HVAC system due to the outside air requirements. The net effect of using condenser dryers in lieu of conventional dryers is a reduction in overall energy use in the climate zone for the building we are studying (New York City climate). We propose the following as an exceptional calculation methodology to simulate the performance of condenser dryers over standard vented dryers: Baseline Building: 1. Model typical dryer energy patterns based on standard washing machine use patterns from EnergyStar. 2. Model the dryer such that none of the dryer energy results in heat gain in the space. 3. Model the Baseline Building with 50 cfm of air exhausted from each residential unit with a dryer. To do this, include dryer exhaust fan energy assuming that the fan runs at an average of 50 cfm, 24 hours per day, at the same static pressure as the other rooftop exhaust fans. Include 50 cfm of additional infiltration 24 hours per day for every residence with a dryer. Proposed Building: 1. Increase the dryer energy use by 20 percent based on a conservative rounding of the average results from Kao\'s study of dryer energy use. 2. Model the dryer such that all of the dryer energy results in heat gain in the space. 3. Model the proposed building without the dryer exhaust fans and without the additional 50 cfm of infiltration. Is this exceptional calculation method acceptable for LEED EAc1 credit?