Ventilation

Overview

Until the advent of modern building codes, most buildings were designed to “breathe,” with air exchange between the interior and exterior. This strategy helped accommodate older technology around open combustion for heating as well as poor moisture control at the building envelope. As codes have prioritized energy efficiency, air leakage requirements have led to tighter buildings. While this reduces energy use associated with air infiltration, it also means that buildings require a dedicated source of fresh air to ensure adequate indoor air quality. The way this fresh air is introduced into the building has significant implications for energy consumption.

Technology and Design

Standard Ventilation Design

Natural Ventilation

Natural ventilation is an older design strategy where occupants are expected to open windows for fresh air as needed. As Colorado experiences more frequent outdoor air quality issues due to wildfire particulate matter, natural ventilation can be a health hazard to residents. Additionally, because the outside air is not heated or cooled, residents must choose between thermal comfort or air quality on extreme temperature days. Many jurisdictions and green building programs no longer allow natural ventilation in multifamily new construction.

Exhaust-only Ventilation

Exhaust-only ventilation is a common design strategy in Colorado affordable housing. This involves running a bathroom or kitchen exhaust fan to expel stale air and induce negative air pressure within the unit. The negative pressure draws outside air into the unit to provide fresh air. Exhaust systems are relatively inexpensive to both install and operate. However, there are drawbacks:

There is no ability to filter air that is being pulled into the unit, which can introduce harmful pollutants.
Fresh air is unconditioned; cold air is introduced in the winter and warm (sometimes humid) air is introduced in the summer. This reduces resident comfort and increases risk of mold and condensation.
Odors can infiltrate from neighboring units.

Improved Ventilation Design

Supply-only Ventilation

Supply-only ventilation involves bringing a direct supply of fresh air into the unit using mechanical equipment. Examples include:

An outside air damper in the packaged terminal heat pump (PTHP)/vertical terminal heat pump (VTHP)PTHP/VTHP that draws in fresh air when in heating or cooling mode.
Ducting outside air to the return of a fan coil or ducted mini-split
Central ventilation equipment supplying tempered fresh air to each apartment

Unlike exhaust-only, these systems all condition and filter outdoor air. Introducing fresh air at terminal equipment like PTHPs/VTHPs is less expensive upfront, but typically results in higher utility costs. On the other hand, central systems can have a higher construction cost, but also provide better air filtration, improved comfort, and lower operating costs.

One drawback of supply-only ventilation is air balance. Supply-only ventilation can positively pressurize units, resulting in odors migrating into lower pressure spaces like corridors.

Balanced Ventilation

Balanced ventilation involves a combination of supply and exhaust ventilation to create neutral pressure and consistent air changes within a unit. Unlike supply- or exhaust-only ventilation, which rely on leakage to achieve air exchange, balanced ventilation pairs the fresh air supply with mechanical exhaust to predictably circulate fresh air through the space. As new energy codes require a tighter building envelope, balanced ventilation becomes more essential to indoor air quality.

Best-in-class Ventilation Design

Balanced ventilation with energy recovery represents the highest performing option for ventilation and reduced utility costs. Energy recovery ventilation (ERV) pre-heats or pre-cools the fresh air supply using a heat exchanger connected to exhaust air. This results in all the benefits of balanced ventilation (air filtration, conditioned air, neutral air pressure) with the added benefit of reduced energy costs. Drawbacks include increased equipment costs as well as added complexity in design, installation, operation, and maintenance. In cold climates like Colorado, ventilation contributes significantly to heating loads, making energy recovery a powerful operating cost reduction strategy. This is especially true for all-electric buildings, where the electric resistance heat of ventilation air on very cold days can result in high demand charges for commercial meters.

Ventilation heat recovery comes in a few varieties, but the most common type is an ERV. ERVs can be part of large central systems or individual unit-level appliances, depending on building design and cost factors.

Central ERV Systems:

Higher heat recovery efficiency and energy savings
Superior air filtration and air distribution options within units
Additional ducting requirements
Ventilation operating cost paid by the owner

Individual ERV Systems:

Limited air filtration options and large number of filters to change
Less ducting required
More penetrations in the building envelope, and additional in-unit space requirements (typically dropped ceilings)
Design approach needs to accommodate cold air discharge when outside air temperatures are 10 degrees Fahrenheit and below
Ventilation operating costs paid by resident

Ventilation vs. Spot Exhaust

One common point of confusion is the difference between ventilation and spot exhaust. Ventilation provides dedicated fresh air into a unit and requires regular operation. Spot exhaust consists of a fan, typically located in a bathroom or kitchen, meant to exhaust moisture or other contaminants to the exterior on an as-needed basis. Spot exhaust operation is typically intermittent and controlled by a switch, whereas ventilation is continuous. The confusion arises from the fact that a single fan can provide both spot exhaust and ventilation. For example, a bath fan can be wired to operate continuously at a low level and then ramp up when the switch is activated. Many green building certifications require both spot exhaust and ventilation, so it is important to understand what is required for certification compliance.

Ventilation Rates

A common issue with new construction projects is over-ventilating some spaces. While it is important to ensure adequate fresh air supply, too much fresh air increases utility costs with minimal benefit. Ventilation rates are a function of occupant count and area. Developing a right-sized ventilation rate for each standard apartment type is best practice. Ventilation equipment that can operate at an appropriate range of airflows will allow the installing contractor to select settings that ensure the right amount of fresh air.

Retrofit

Summary

Upgrading ventilation systems offers building owners a tangible way to enhance resident comfort through improved air quality and better moisture and odor control. In properties with natural ventilation, however, incorporating dedicated mechanical systems can be challenging. Retrofit feasibility is limited by existing duct infrastructure, as installing new ductwork is often financially impractical. Nevertheless, there are practical solutions that can enhance indoor air quality without requiring extensive construction scope.

Retrofit Considerations

Central Ventilation

A central system retrofit is most feasible when existing ductwork and ventilation infrastructure are already in place. Typical configurations include roof-mounted exhaust fans connected to vertical risers, which may operate alongside natural ventilation or a makeup air unit (MAU), and in more recent construction, dedicated outdoor air systems (DOAS).

Retrofit Options for Central Ventilation Systems:

Roof-mounted Central Exhaust Fan
- Description: Replace constant-speed fan with an EC motor fan or a fan equipped with a VFD to enable variable-speed operation.
  - Additional Upgrades:
    - Add static pressure reset controls
    - Add demand-based controls (humidity or occupancy sensors)
  - Additional MAU Upgrades (if applicable and not already implemented):
    - Add cooling
    - Integrate MAU and exhaust fan controls to manage building pressure
    - Add VFD control for MAU supply fan
- Benefits:
  - Reduces over-ventilation (common in older systems)
  - Cuts fan energy significantly
  - Improves pressure stability
  - Improved tenant comfort
  - Minimal disruption to tenants
- Key Considerations:
  - Existing ductwork condition and sealing should be evaluated to ensure proper airflow control
  - Control strategies must be coordinated with makeup air systems to maintain appropriate building pressure
  - Electrical capacity and controls integration should be verified when installing VFDs or new fan motors
When aligned with envelope improvements that tighten the building shell, upgrading a roof-mounted central exhaust system can offer a moderate return on investment. Although it does not match the performance of a fully balanced DOAS with central ERV, it provides enhanced energy performance and resident comfort.
DOAS
- Description: Replace existing system with DOAS with integrated energy recovery (ERV) to pre-condition incoming outdoor air using exhaust air
  - Additional Upgrades:
    - Upgrade heating and cooling capacity to improve temperature control (gas heat, electric resistance heat, and heat pump options available)
- Benefits:
  - Reduces heating and cooling loads through energy recovery
  - Lowers operational costs
- Key Considerations:
  - Existing ductwork condition and sealing should be evaluated to ensure proper airflow control
  - Controls integration with existing heating and cooling systems is required to avoid operational conflicts and maintain proper building pressure
Retrofitting a DOAS with a central ERV represents a higher capital but high impact ventilation upgrade. When existing duct infrastructure is present and exhaust air is accessible, integrating energy recovery can substantially reduce operating costs, improve pressure stability, and align the building with long-term electrification and decarbonization goals.

In-unit Ventilation

In-unit ventilation strategies are often the most practical pathway for adding ventilation to existing projects, particularly where central duct infrastructure is limited or absent. By addressing ventilation at the unit level, owners can improve indoor air quality and meet code compliance while minimizing disruption to residents and avoiding costly distribution upgrades.

Retrofit Options for In-unit Ventilation Systems:

Continuous Exhaust Fans
- Description: Existing bath or kitchen fans that provide spot exhaust can be replaced with ENERGY STAR rated fans with controls to operate continuously or intermittently to meet minimum ventilation rates.
- Benefits:
  - Leverages existing ductwork
- Key Considerations:
  - Limited improvement to indoor air quality
  - Negative pressure pulls in air through envelope infiltration
Despite its limitations, continuous exhaust ventilation can improve baseline air exchange and moisture control using existing infrastructure, typically with minimal impact on operating costs.
Single-point ERVs
- Description: Where no bath or kitchen exhaust fan exists, a through-wall ERV can provide balanced ventilation with added heat recovery for additional energy savings.
- Benefits:
  - No additional ductwork required (ductless)
  - Fits in a standard 2x4 or 2x6 exterior wall.
- Key Considerations:
  - Requires new exterior wall penetrations
  - Less heat recovery capability than fully ducted in-unit ERVs
While single-point ERVs do not operate as efficiently as a fully ducted ERV, it is a cost effective retrofit solution that limits construction scope while still providing improved energy savings and indoor air quality compared to a natural ventilation or an exhaust-only system.
Fully Ducted ERVs
- Description: A fully ducted in-unit ERV is among the highest-performing options for in-unit balanced ventilation. It provides continuous fresh air and exhaust with energy recovery, improving both indoor air quality and energy efficiency.
- Benefits:
  - Balanced ventilation with energy recovery
  - Enhanced indoor air quality through higher performing filtration
  - In some cases, supply air may be introduced into existing ducted HVAC systems for even distribution throughout the apartment.
- Key Considerations:
  - Requires new exterior wall penetrations for intake and exhaust
  - Requires additional in-unit ductwork
  - Ceiling or bulkhead space needed
  - Higher first cost compared to exhaust-only or single-point systems
Unlike single-point ERVs, fully ducted units typically offer greater energy recovery capacity, resulting in improved comfort and lower heating and cooling costs. However, higher equipment costs, added duct routing, required envelope penetrations, and space constraints can make this approach cost-prohibitive in many retrofit scenarios.
Outdoor Air through Existing HVAC System
- Description: Ventilation can be added by introducing controlled outdoor air into the return side of an existing ducted HVAC system for filtration and distribution throughout the unit. In packaged systems such as PTACs and VTACs, factory dampers or designated openings can provide outdoor air through the supply stream.
- Benefits:
  - Leverages existing equipment and ductwork
- Key Considerations:
  - Increases heating and cooling loads
  - Extends run time
  - Reduces overall system efficiency
While this approach leverages existing equipment and ductwork, it increases heating and cooling loads, extends run time, and may reduce overall system efficiency if not carefully controlled. Pairing this strategy with energy recovery can significantly improve performance and help mitigate the added conditioning load.

Design Considerations and Constructability Takeaways

Ventilation retrofits must be evaluated not only for performance benefits, but also for constructability, cost, and impact on occupancy. The feasibility of any strategy is often dictated by existing infrastructure and the level of renovation planned.

When retrofit plans rely on existing ductwork, the condition of retained infrastructure must be carefully evaluated. Common issues that compromise system performance include:

Leakage due to poorly sealed ductwork
Corrosion or damaged dampers
Improper sizing relative to new airflow requirements

Depending on existing conditions, duct sealing or partial replacement may be required to ensure that upgraded equipment performs as intended.

Installing a centralized ventilation system in a building without existing duct infrastructure also faces significant design and constructability challenges. Early assessment is essential to evaluate the required ceiling, structural, or architectural modifications required. Typical barriers include:

Routing new ductwork through corridors
Creating vertical shafts
Identifying space for rooftop or interior equipment
Required electrical service or panel upgrades to accommodate added loads

In contrast, in-unit ventilation strategies typically present fewer constructability barriers, making them more feasible for occupied buildings. However, key considerations include:

Individual exhaust fans or single-point ERVs generally introduce modest electrical loads that existing unit panels can often accommodate, though capacity should still be verified.
Space constraints remain an important consideration, as new equipment may require additional wall or ceiling clearance.
Where new ductwork or envelope penetrations are required, constructability risks increase, particularly if hazardous materials such as asbestos are present or if façade modifications are necessary.
Phasing and resident coordination should also be considered, as unit-by-unit installations may extend construction timelines.

In practice, in-unit ventilation strategies often provide greater flexibility and more cost-effective implementation pathways, particularly where central duct infrastructure is limited or absent. Early evaluation of existing conditions and renovation scope is critical to selecting a strategy that is both technically viable and constructible within project constraints.

Operations and Maintenance

Most older affordable housing buildings lack mechanical ventilation, so adding these systems typically increases operational costs. Depending on the building's size, the energy consumption of central ventilation or individual fans can accumulate quickly. When possible, opting for a ventilation strategy with heat recovery can help reduce associated utility expenses.

Mechanical ventilation also introduces additional maintenance requirements. Central systems are relatively complex and require monitoring of controls and operational data to ensure proper function. These systems, along with many in-unit models, require periodic filter replacements. Individual exhaust fans may also need to be cleaned to prevent dust buildup from impacting operation. Replacement will also likely be required as fan motors fail or reach the end of their service life.

Implementation

Integrating mechanical ventilation into existing affordable housing can be a high-impact strategy for improving indoor air quality and supporting resident health. Because many properties lack dedicated ventilation systems, this should be a key consideration when assessing capital improvement needs.

However, retrofit feasibility varies significantly. Before selecting a retrofit strategy, project teams should evaluate:

Existing duct conditions and routing
Electrical capacity at both the building and unit level
Envelope penetrations and constructability constraints
Long-term maintenance capacity and operating budget

Careful upfront assessment helps ensure that ventilation improvements are both technically feasible and operationally sustainable.