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Rejuvenating building exhaust air

Building exhaust air from commercial buildings has typically been thermally conditioned. Building exhaust air is typically dispensed to the outdoor air, requiring energy to condition replacement outdoor ventilation air. The general assumption is that building exhaust air is “contaminated”, says Prasad Natraj of AAF.

| | May 24, 2016 | 10:58 am
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Prasad Natraj General Manager, AAF International Air Filtration Systems

Prasad Natraj General Manager, AAF International Air Filtration Systems

Distinguishing the true Green strategies from the “green washing” that currently exists in the Green Building industry can be challenging. However, sometimes, a concept comes along that intuitively makes sense. This is the case with Building Exhaust Air Recycling Strategy (hereafter called the recycle strategy). This principle involves the recycling of building exhaust air by providing proven air cleaning technology for the removal of particulates and gaseous contaminants. Throwing away valuable conditioned air is not “Green” or cost effective when it can be filtered and reused.

The strategy has been implemented in several building types, including hotels, theatres, arenas and office buildings. Buildings that require exhaust air due to odour issues, including restrooms and locker rooms, are good candidate buildings for this solution.

Hotel buildings

Hotels are unique buildings with a rest room in each guest room and a high density of people within the building. This uniqueness results in significant quantities of outdoor ventilation air and exhaust air requirements. Providing outdoor ventilation air to the guest rooms can be a challenge in order to ensure the required air distribution in the room is achieved.

Perspective-may issue 2

Figures 1 and 3 illustrate two different ventilation configurations for a typical hotel guest room, which require different outdoor ventilation rates according to widely accepted ventilation standards. Both of these configurations can benefit from the recycle strategy by reducing the outdoor air ventilation rate to the guest rooms by recycling the exhaust air from the guest room toilet exhaust system.

How can exhaust air be recycled?

ASHRAE standard 62.1*, which has widely been accepted as code, classifies toilet exhaust air as Class 2 air, which is defined as “Air with moderate contaminant concentration, mild sensory-irritation intensity, or moderately offensive odours”. The standard has a provision, which allows for the reclassification of Class 2 air to Class 1 air, when air cleaning is provided. Class 1 air is defined as “Air with low contaminant concentration, low sensory-irritation intensity, and inoffensive odour”. Therefore, exhaust air from guest room toilets can be recycled, provided the appropriate air cleaning filtration system is installed to control particulate and gaseous contaminants.

Reduce carbon footprint

To illustrate the success of the recycle strategy in a hotel application, gas-phase and particulate filtration products were installed and an extensive environmental characterisation was performed for a large hotel. The hotel has been recycling approximately 80,000 cubic feet per minute (cfm) of guest room toilet exhaust for many years, which would otherwise have been conditioned for temperature and moisture removal.

Perspective-may issue 1

The energy cost savings associated with the recycle strategy has been estimated to be USD 121,000 annually¹. This equates to an annual energy savings of approximately 1.3 million kWh annually². In addition, there was a corresponding carbon footprint reduction of approximately 934 metric tons of CO2.

The Green Recycle solution

Figure 5 shows the configuration of the recycle strategy at this hotel, where guest room toilet exhaust air is being recycled back into the building where ventilation is provided via an atrium make-up air configuration (similar to Figure 3).

Gas-phase filtration reduces chemical concentrations by 40%

The team performed an environmental characterisation involving monitoring and sampling for indoor air contaminants within the exhaust air from the guestroom bathrooms (before filtration), in the air being recycled back into the hotel building (after filtration), and in the outdoor air. The environmental parameters measured included the following:

• Airborne Chemicals
– VOCs (EPA method TO-15)
– Aldehydes (EPA method TO-11A)
– Ammonia and Hydrogen Sulphide

• Bioaerosols
– Fungi (non-viable and viable)
– Bacteria (Coliforms with isolation of E.coli sp.)

Results of the sampling and monitoring were compared to industry guidelines, including ASHRAE Standard 62.1-2010 (Table B-3 “Concentrations of Interest for Selected Volatile Organic Compounds”), and the U.S. Environmental Protection Agency’s (EPA’s) Building Assessment and Survey Evaluation (BASE) study. Chemical and biological findings were generally found to be less than the levels recommended by cognizant authorities, including EPA, California Office of Environmental Health Hazard Assessment (OEHHA), and Agency for Toxic Substances and Disease Registry (ATSDR) and/or lower than what has been found in non-problem office buildings (using findings from BASE).

The gas-phase filtration system reduced the chemical concentrations by approximately 40%, ensuring a safe and odour-free reintroduction of exhaust air back into the hotel building as Class 1 air.


Recycling building exhaust air for energy savings and reduced greenhouse gases with the recycle strategy would be a good idea for a more sustainable growth.

Due to the prevalent weather conditions in Middle East, most of the occupied space is conditioned. It is, therefore, advisable to utilise this Green initiative to not only reduce outside air, the carbon footprint and equipment costs but also effectively bring down size to suit building requirements, from a design standpoint.


  1. Estimate presented at ASHARE Winter Conference 2009, Seminar 58 (Intermediate). Achieving Sustainability and Energy Reduction through the IAQ Procedure: Case Studies of Success; H.E. Barney Burroughs “Attaining Sustainability Using IAQP: Documentation of Successful Building Performance in 15 IAQP Sites in Atlanta”.
  2. Based on electricity rate of $0.08/kWh.
  3. Bryan Ligman, CIEC, Technical Market Manager, Gas-Phase Filtration. Brad Stanley, LEED AP, Global Technical Support Leader, Gas-Phase Filtration

Prasad Natraj is General Manager, AAF International Air Filtration Systems. He can be contacted at Pnatraj@aaf-ae.com

CPI Industry accepts no liability for the views or opinions expressed in this column, or for the consequences of any actions taken on the basis of the information provided here.

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