This article was originally published in the July/August 1994 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.



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Home Energy Magazine Online July/August 1994



Evaluating Ventilation in Multifamily Buildings


by Vicky Hayes and Ian Shapiro-Baruch

Vicky Hayes is housing director at GRASP in Philadelphia, Pennsylvania, and Ian Shapiro-Baruch is president of Taitem Engineering in Ithaca, New York.

The technologies used to ventilate multifamily buildings are not all that complex, yet problems with indoor air quality and high energy use are all too common.

While we in the energy conservation business have long been aware of the need for fresh air in a dwelling, we are just beginning to realize how vital it is to assess the quantity and quality of that air. In single family buildings, blower doors help us quantify air leakage; we have found a wide variation of leakage rates depending on builder, building type and maintenance levels. In multifamily buildings, however, leakage rates per occupied unit appear to be much lower. Since lower leakage levels can cause indoor air quality problems and lead to building system problems, the need for fresh air is more critical for multifamily buildings. However, since ventilation systems use energy and cause energy usage through the necessary heating or cooling of the incoming fresh air, a calculated balance must be struck to ensure that buildings are not over-ventilated. Mechanical ventilation in multifamily buildings varies: some buildings provide centralized air supply and/or air exhaust to the apartments or hallways, while others provide small fans in kitchens and bathrooms, some under tenant control and some under building staff control.

The New York State Energy Research Development Authority recently funded a study to assess the types, effectiveness, and energy efficiency of ventilation systems in multifamily units in New York State. In addition, a procedure and software were developed to audit these systems. Taitem Engineering performed the study using Syracuse Energy Service Company and the New York City Weatherization Coalition as subcontractors.1 We collected detailed information and measurements for ten sites and collected information characterizing ventilation systems on another fifty sites. We made certain predictable and many surprising discoveries.

Ventilation and Multifamily Buildings

ASHRAE mandates that if buildings fall below certain criteria for measuring indoor air quality, they must be mechanically ventilated. While the requirements for either intermittent high ventilation or continuous low ventilation of kitchens and baths appear appropriate, the important provision that ventilation for the rest of the building (0.35 air changes per hour) can rely on infiltration is questionable for multifamily buildings due to their lower infiltration rates. This was confirmed by air quality problems we observed in many buildings in this project.

These problems are endemic to multifamily buildings because most multifamily units share at least one common wall with another occupied dwelling. This cuts down the number of available portals (leakage areas) for fresh exterior air coming directly into the unit. Research recently conducted by Larry Palmiter at Ecotope has shown that air from the outside may move through several units before exiting the building.2 More people live in multifamily buildings than in single family buildings, which affects indoor air quality as well.

Building Types:

Three buildings we studied in New York City had a similar structure: three- to five-story tenement buildings with a single stairwell, generally of masonry construction with a flat roof. Two were high-rise, thirteen and fifteen stories, with interior entrances into corridors (see Table 1).

In upstate New York, the buildings surveyed were low-rise (from 2 to 4 stories), all-electric buildings participating in a demand-side management project. Some were garden-style or townhouse units with exterior entrances, and some were apartments with interior entrances from hallways.


In the New York City buildings surveyed, if bathrooms or kitchens did not have operable windows, they were given a central exhaust fan that ran much of the day. However, most of these buildings had bathrooms and kitchens with operable windows, in which case there was no mechanical exhaust present (82% of the buildings). Corridors in the tenement (walk-up) buildings were not mechanically provided with supply air, but did have ventilation openings (by code) at the top of the stairwell, usually in a skylight. Supply air was provided mechanically in only 4% of the New York City buildings surveyed, primarily in larger buildings where the fresh air is provided from a single fan into the corridor on each floor.

Most upstate buildings surveyed (96%) had some form of mechanical exhaust that usually consisted of tenant-controlled small exhaust fans in bathrooms and/or kitchens. Central exhaust, usually drawing from multiple bathrooms/kitchens, was present in 26% of buildings. Supply (fresh) air was provided mechanically in 36% of buildings, usually into corridors, which, although common practice, violates state code.

Fan Operations

In the buildings we inspected, one of every four fans was non-functioning or permanently turned off, usually because of mechanical failures, noise complaints, or comfort complaints (especially supply fans). The predominance of occupant-controlled bathroom and kitchen fans in upstate New York buildings led us to question how long tenants actually ran such fans. The findings show that such fans are minimally used: 28 of 32 tenants in units with kitchen exhaust fans used them for less than one hour per day, and the four remaining tenants reported not using them at all. Forty-five of 55 tenants in units with bathroom exhaust fans used them for less than one hour per day; nine of the remaining tenants reported not using the fan at all; and one tenant said the bathroom fan did not work.

We measured airflow on a number of fans in the buildings with a balometer (flow hood) and compared the airflow with design values, either taken from original blueprints or from fan nameplate data. Actual airflows ranged from 6%-254% of rated airflow, averaging 68% (almost one-third less than required). The culprit was usually dirty registers, many of which were heavily caked with dirt. We found some dirty filters, but they were not the primary cause of airflow obstruction, because most systems did not have filters. We also found unusual airflow obstructions--including tape or paper placed over registers, insulation in the ductwork, and even a bee's nest!

Fan power was also measured on several fans. Gross fan and system power-to-airflow ratios ranged from .19 to 1.25 watts per CFM, averaging .63 watts per CFM. This caused a wide disparity of power-to-airflow ratios (the inverse of efficiency): a variation of up to 600% from the most efficient system.

Comfort versus Supply Air

Tenant surveys revealed that mechanical ventilation was associated with complaints about drafts, confirming the separate finding that exhaust airflow is often greater than supply airflow, and that the shortfall comes from leakage into the building, often through dwelling units. Of eight buildings with supply fans, three had serious comfort complaints in the corridors, two were shut off, and two were severely under-ventilated. Clearly there is a technical challenge to providing comfortable supply air into a space when the outdoor temperature varies seasonally, and the heating load of the space depends on several factors and varies widely. Without changing how supply air is provided, the majority of multifamily buildings will sacrifice either comfort or sufficient supply-air ventilation.

This problem is not unique to multifamily buildings but is widespread. In large commercial buildings, it is usually addressed through complex controls. However, such strategies are limited to large buildings where the complexity and cost can be justified.

Exhaust ventilation is also not without dilemmas. When exhaust fans are tenant-controlled, it is difficult for a building designer to know how to size the supply air fan to balance the exhaust airflow. On the other hand, if exhaust fans are centrally controlled, much ventilation is unnecessarily provided, exhausting air even when tenants are not home. Results from the tenant survey indicate that the ventilation resulting from tenant control of exhaust fans is only sufficient to control temporary odors in the kitchen or bath. It is clearly not being used to draw in fresh air for the building as a whole or to address other sources of poor indoor air quality.

Indoor Air Quality

At the time of the initial audits and during follow-up visits, auditors identified pertinent ventilation-interactive issues such as health and safety, comfort, indoor air quality and building condition. This was done by interviewing building management, building facilities personnel, and tenants. Emphasis was placed on qualitative (descriptive and anecdotal) evidence, as well as on strategies for quantifying these issues at in the future.

Probably the most striking example of indoor air quality problems was found in a 15-story building in Queens. The 15 residential floors generally had adequate ventilation (both central exhaust and supply), but the basement served as a workshop for sorting, cleaning, and shipping used clothes. Trucks frequently backed up to a loading dock and we found severe evidence of particulates from truck exhaust throughout the basement, including traces found on exhaust registers in the residential section of the building. Other sources contributing to poor indoor air quality in the building were pets, cooking, bathroom use, cleaning fluids, smoke from other apartments, chemical pollutants, car exhaust, garbage room fumes, nearby factories, pigeons, and diapers. These findings emphasize the need for energy auditors to look out for anything that requires special ventilation or other measures, such as source reduction to mitigate.

There was a high correlation between the amount of ventilation and the level of air-quality complaints. Two tenants in different apartments in a well-ventilated Staten Island high-rise reported no problems when asked about possible odors. Tenants in an unventilated Brooklyn tenement complained about a wide variety of air quality problems, including pet odors, smoke, and stuffiness. Tenants in a Manhattan tenement where central exhaust fans were not being used complained about smoke and cooking odors, and then reported significant improvements after two of four central fans were turned on by the landlord.

Energy Conservation and Ventilation

While the sample size was small, New York City buildings used more energy for mechanical ventilation, on average (12%) than upstate buildings (5%), although the building with the highest mechanical ventilation energy use was upstate (20.6%). Buildings with mechanical ventilation--those which had only tenant-controlled bathroom and/or kitchen fans--used very little energy for mechanical ventilation, averaging only 0.9% of total building energy use. (This calculation assumes each tenant ran each fan for 1/2 hour per day which is representative of responses provided by tenants in the survey.) Buildings with central ventilation fans, which were usually run for more hours per day than the tenant-controlled fans, predictably used more mechanical ventilation energy, averaging 13% of total energy use.

We analyzed and compared three options for conserving mechanical ventilation energy with a generic air sealing (reduction of natural ventilation) conservation option. Each option was examined in isolation, using the existing installation and energy use as a baseline. Each was evaluated according to simple payback and applicability. Could the option be realistically applied given the existing mechanical system and ductwork?

Option 1: Heat Recovery

Heat recovery would be a viable option in three of the five New York City buildings, but would not be applicable in any of the upstate buildings. Heat recovery is still relatively unknown to the residential building sector and may also require engineering design, which adds to the cost.

The three candidate buildings in New York City had similar structures: three- to five-story tenement buildings with a single stairwell and rooftop exhaust fans each serving several apartments. The stairwell was an ideal location to provide makeup air, as there was only one of these per building (single location to provide air, and so less ductwork), and because the stairwell was easily accessible for routing fresh air ductwork from the roof where a heat recovery ventilator could be located. However, paybacks for heat recovery in these three buildings averaged nearly 18 years.

While two of the upstate buildings had central-supply air provided into the corridors, all buildings relied on tenant-controlled exhaust fans. The widely distributed exhaust ducts in these buildings would prohibit their use in the central exhaust duct required for heat recovery, and so would require additional duct installation, making this option prohibitively expensive.

Option 2: Timer Controls

Overall whole building timer controls for ventilation systems were not an option in the exhaust systems in the upstate buildings, because of the tenant controlled bathroom and kitchen fans. One timer was already in use on a central supply air fan in one building, and another central fan was turned on and off daily by building staff.

Timers would be appropriate for four of the five New York City buildings, because code requires ventilation 18 hours per day for bathrooms without windows. Most ventilation fans ran 24 hours per day, a mechanical timer could be installed and immediately save 25% of the thermal and motor energy required. Timers were also found to be appropriate for several central kitchen exhaust fans, which were ventilating above the required six air changes per hour. Timers are a simple and low-cost energy conservation measure, consistently providing an attractively low payback (average of 0.4 years).

Option 3: High Efficiency Motors/Fans

The economic viability of fan/motor replacement depends strongly on how long the motors run and the cost of electricity. In upstate New York, where electricity costs around 9cents/kWh, motor changes are not attractive if they run less than an hour each day. Payback is longer than the useful life of the motors. However, in New York City, with electricity around 15cents/kWh, and with continuous kitchen and 18-hour bathroom exhaust, high-efficiency motors are a more attractive option. Paybacks in New York City for high-efficiency motors averaged 5.4 years.

Option 4: Air Sealing

Air sealing showed higher potential savings than mechanical ventilation in upstate buildings (more than 11%) and lower potential in New York City buildings (0.5%). These results should be taken within the context that estimating air leakage in multifamily buildings is difficult with current audit techniques. The results may partially reflect the method used to estimate existing natural ventilation (air leakage). In the New York City buildings, we predicted air leakage with the EA-QUIP computer program, which uses an analytical method based on the auditor's description of windows, doors, and other apertures in the building (see Confessions of an Addicted Auditor, HE May/June '94 p.29). For upstate buildings, we derived air leakage estimates from a combination of blower door tests, building computer models, and auditor's experience. Air sealing should only be considered after a thorough investigation to determine the building's natural air change rate. This cannot be done solely on the basis of a blower door on units within the building, since part of the air leakage measured will be leakage from adjacent units. If the building's true air leakage cannot be determined, air sealing should not be considered as an energy conservation measure. We continually found prohibitively low air leakage in the buildings participating in upstate New York.


Standards for Fan Performance

The wide disparity of fan efficiency points to the potential for energy savings with more efficient fans and motors, and appropriate duct sizing, as well. One way to achieve higher efficiencies would be through a minimum fan/system efficiency requirement mandated by energy conservation codes, similar to the minimum efficiency requirements for heating and air conditioning equipment. Any minimum efficiency regulation would need to account for the role of ductwork in a ventilation system, and so might be appropriately applied to the final installed system (minimum CFM/watt), regardless of system static air pressure. This would provide a level playing field for manufacturers and ventilation installation contractors, as well as consumer protection from systems that consume inordinately high levels of energy.


Codes should provide for dampers in the ventilation system that disallow leakage through the ventilation system due to the stack effect when the system is not operating. It is unlikely that automatic dampers (motor controlled) are justified for the small kitchen and exhaust fans. Our observations indicated that simple backdraft dampers provide ample air leakage control prevention if oriented vertically. The vertical orientation allows the weight of the damper itself to prevent air leakage due to the stack effect. Spring loading of horizontally oriented dampers can accomplish the same thing, but such dampers can also prevent flow altogether, even when the fan is operating.


More information about fan power consumption, at design and off-design conditions should be provided. Some catalogs provide information on brake horsepower, but more important to energy auditors and system designers is power consumption over the whole performance curve. Providing nominal motor efficiency and power draw (watts) on the motor nameplate would also help. In addition, improved heater controls for supply air systems are needed, particularly duct heaters and cabinet heaters (rooftop supply air heaters are less common in multifamily buildings). At a minimum, controls should allow for constant supply air temperature, with setpoint which may be varied.


While architects and engineers clearly have significant control over the ultimate combination of comfort, indoor air quality, and energy consumption ventilation systems provide, it is important that they understand the shortcomings of existing systems: Current supply air strategies almost always fail. Providing temperature control of corridors independent of the supply air heater and providing temperature control of supply air that avoids cold blow into corridors can help avoid this problem. Other design-phase options would include the use of heat recovery equipment, particularly for constant-ventilation designs and/or high fuel cost areas and building designs.

Owners and Maintenance Staff

Owners and staff can contribute significantly to building air quality and reduced energy use of ventilation equipment with a few simple preventative measures: semi-annual cleaning of registers/grilles, filters and fans, and inspecting fans for operation and airflow obstructions.

Energy Auditors

Energy auditors should focus on several key issues when examining a ventilation system:

  • Check to see if the ventilation system is working at all, (25% of the fans in this study were not)
  • check to see if the system design makes sense and does the job as effectively as possible
  • Watch out for those systems that are over-ventilating the building
  • Be aware of indoor air quality problems associated with poor or improperly designed ventilation systems
  • Keep the energy conservation options listed above in mind when exploring those measures which can reduce energy usage, but be aware of possible installation and cost problems associated with each
  • Bear in mind the impact that improper maintenance of the ventilation system has both on the building and the energy usage of the system


Further Research

Much of the uncertainty surrounding ventilation codes and strategies could be eliminated if an accurate method of quantifying natural air change rates was available. Other areas for ventilation research include optimal ways to provide supply air; optimal ventilation system design for new construction; strategies to control ventilation on the basis of demand (need); and determining the effectiveness of backdraft dampers in preventing stack effect airflow out of exhaust systems.

The technologies used in multifamily building ventilation are not complex, yet our observations indicate that there are many shortcomings, relating to both indoor air quality and to energy use. These shortcomings can be overcome without developing new technology. However, the number of people involved in providing proper ventilation--from design engineers to installation contractors to code officials to building staff and tenants themselves--adds a level of complexity that often compromises the ventilation system. Equipment selection, system design, and commissioning after installation appear to be the keys to overcoming the barriers to healthy, energy-efficient ventilation in new construction. For auditors assessing an existing system, however, options begin to narrow. While technology is available to increase ventilation efficiency, care must be taken to ensure selected measures are cost-effective and appropriate. Most important is to ensure the ventilation system is adequate for its task and that residents enjoy good indoor air quality, which is what ventilation systems are installed to provide.


1. This article is based on the report, Evaluation of Ventilation in Multifamily Dwellings (Report 93-5), published by the New York State Energy Research and Development Authority, Two Rockefeller Plaza, Albany, New York. Tel:(518)465-6251, ext. 272.

2. See Measured Airflows in a Multifamily Building, ASTM Symposium on the Airflow Performance of Building Envelopes, Components, and Systems, October 1993, Dallas, Texas.


Table 1. Characteristics of Buildings Used in Multifamily Ventilation Project


Cost of mechanical ventilation as Heat a percentage Number of Central Exhaust Supply Supply Supply Individually Space recovery of total Site apartments Stories halls local central local central heat heat fuel metered fuel ventilation energy use _______________________________________________________________________________________________________________________________________________________ UPSTATE Drumlin Heights 18 2 N Y N N N N - Y E N 3.1 Salina Schools 38 3 Y Y Y N Y Y E Y E N 3.3 Clarendon Heights 250 3 N Y N N N N - Y E N 0.4 Sunset Woods 40 4 Y Y N N Y Y E Y E N 22.9 Country Gardens 83 2 Y Y N N N N - Y E N 0.7 _______________________________________________________________________________________________________________________________________________________ NEW YORK CITY Goodwill Terrace 208 15 Y N Y N Y Y O N N N 5.5 48th Street 15 5 Y N Y N N N - N N N 14.5 47th Street 15 5 Y N Y N N N - N N N 17.1 Putnam Avenue 6 3 Y N N N N N - N N N 2.9 Holland Avenue 120 13 Y N Y N Y Y O N N N 19.9 E - electric O - oil G - natural gas local - local ventilation, i.e., kitchen and bath _______________________________________________________________________________________________________________________________________________________

Figure 1. Typical multifamily ventilation system.


Figure 2. Average potential savings for each measure.


Figure 3. Potential payback for each measure.



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Ventilating Attics to Minimize Icings at Eaves (Tobiasson, Buska, and Greatorex)
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Infiltration: Just ACH50 Divided by 20? (Meier)
Introduction to Blower Doors (Keefe)
The Best Boiler and Water Heating Retrofits (Lobenstein and Hewett)
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Energy Education: A Kilowatt Is a Terrible Thing to Waste (Firari)
Energy Savings Rise High in Multifamily Buildings (Diamond)
The Key to Persistence (Nolden)
Making Low-income housing Affordable: The Northgate Retrofits (Patullo)
Profiles of Multifamily Weatherization Projects: A Tale of Five Cities (Kinney, Wilson, and MacDonald)
Using Fuel Bills for a Targeted Investment (Padian)

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