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Ventilation for Passive House Multifamily Projects, Part 1

Weighing the pros and cons of unitized and central systems for large buildings

A building's worth of unitized ventilation equipment requires multiple penetrations of the air barrier, increasing the difficulty of hitting the Passive House requirement for airtightness. (Photo: Thomas Moore)

Project teams pursuing Passive House certification frequently ask, “Where do we locate the heat- or energy-recovery ventilator?” When the Passive House concept is scaled to a multifamily program, the answer is complex.

While there are two primary arrangements for HRV/ERV systems. As multifamily Passive House projects begin to scale, the trade-off between the two arrangements is dynamic and needs to be carefully considered.  A low-volume unit ventilating an individual apartment is called a “unitized HRV/ERV.” A high-volume unit ventilating multiple apartments, and often servicing several floors, is referred to as a “centralized HRV/ERV.”

Trade-offs

As Passive House consultants, we can attempt to address the system arrangement question with building science. However, in New York City, rentable floor space is very valuable, so considering the floor area trade-off is of particular interest to project teams.

When a unitized system cannot be located in a dropped ceiling due to low floor-to-floor height, it is placed in a dedicated mechanical closet. This closet is typically no smaller than 10 square feet and includes the necessary ductwork connections.

The alternative solution is to increase the floor-to-floor height to accommodate the unit and horizontal duct runs in the ceiling. While centralized HRV/ERV systems allow short horizontal duct runs, they require floor space to accommodate vertical shafts. With supply and exhaust ducts coupled together, the required floor area is between 8 and 12 square feet. As a result, a centralized HRV/ERV system may actually require more floor area than a unitized system.

In the case of of the Cornell Tech building in New York City, vertical supply and exhaust ductwork for the centralized HRV/ERV system required 222.5 square feet per floor, or 13 square feet per apartment (see image below). Unitized HRV/ERV mechanical closets would have required an estimated 170 square feet per floor, or 10 square feet per unit (on right).

In the case of the Cornell Tech tower in New York City, a centralized HRV/ERV system required 13 square feet per apartment. A unitized system would have required 10 square feet per unit.

Factoring in operating costs

When determining cost effectiveness, building operating cost is of equal importance to floor area. Placing unitized HRV/ERV systems in apartments allows owners to include ventilation electricity on the residential electric meter, so the tenant pays for ventilation. In contrast, a central HRV/ERV’s electricity cost is absorbed by the building owner.

Billing the tenants for ventilation may be attractive, but the trade-off is higher maintenance costs. All HRV/ERV units require routine filter changes, usually two or three times per year, as well as regular cleaning of exterior exhaust and intake vents. Building maintenance must be performed by the building management team. Multiply this by the number of apartments in the building for a unitized HRV/ERV system and the additional maintenance costs are likely to exceed any reduction to the owner’s utility bills.

For a multifamily building, regular filter changes and exterior vent cleaning quickly becomes impractical. With the first cost of the unitized and centralized system types being comparable, teams must consider costs related to floor area and maintenance.

Airtightness is a key consideration

Now let’s discuss building science. One of the biggest challenges for a project team pursuing Passive House certification is complying with the stringent building envelope air leakage rate of 0.6 air changes per hour (ach50). To meet this requirement, the air barrier system must be detailed carefully and installed continuously, so penetrations for the HRV/ERV exhaust and intake vents must be carefully incorporated into the air barrier system.

For a unitized HRV/ERV system, two penetrations per apartment are required: one for exhaust, one for intake. For a centralized system, only two to six penetrations may be required for the entire building.

The Cornell Tech building, for example, has 352 apartments. With a unitized ERV system, this would amount to 698 additional penetrations through the air barrier to accommodate the exterior exhaust and intake vents. With a centralized ERV system, only six penetrations through the air barrier were required to accommodate the three ERV units.

According to code, these intake and exhaust vents must be separated by a minimum of 10 feet and should be 2 feet from any window openings. Properly locating vents in a dense floor plan with a limited amount of exterior wall area per apartment is a challenge. With a unitized HRV/ERV system, any failure to correctly design or install the exhaust and intake vent detail impacts air leakage for the whole building.

The installation of these small penetrations can be challenging, and repetitive deficiencies have a large impact on an otherwise airtight building.

Passive House International requires a whole building air leakage rate of 0.6 ach50. Cornell Tech was tested with a whole building air leakage rate of 0.14 ach50. If we assume 4 cubic feet per minute of additional air leakage per apartment from two poorly detailed or installed HRV/ERV vent penetrations, the whole building air leakage rate increases to 0.19 ach50, an 8% increase in total air leakage.

This demonstrates the impact that multiple deficient penetrations have on whole building air leakage rates for a multifamily project, and a possible margin of certification failure for some Passive House projects.

Comparing condensation and other factors

HRV/ERVs have two ducts that penetrate the exterior walls. These ducts have cold surfaces that present condensation risks for each unit. Isolating the ducts between the HRV/ERV and the exterior walls requires careful installation of duct insulation and a vapor barrier. Simply put, the risk of additional air leakage, condensation, and poor installation scales with  the number of HRV/ERV units installed in a unitized system.

Centralized HRV/ERV systems require their fair share of penetrations and additional detailing. There may be fewer air barrier penetrations, but each floor slab will be penetrated multiple times, requiring large fire-rated shafts and duct sealing.

Vertical distribution for centralized HRV/ERV systems increases the length of duct runs from the HRV/ERV fan to the apartment registers. With long vertical duct runs, stack effect is increased, which makes balancing more challenging. Long duct runs also increase static pressure, which results in higher fan energy than for unitized systems. Any duct leakage in the system only exacerbates this problem.

Addressing duct leakage

Aerosealing ducts is critical to combat the stack effect, duct leakage, and to balance flow rates. This is an additional process where the ducts are sealed from the inside with a blown-in polymer.

Furthering the installation complexity of large, centralized HRV/ERV systems is the addition of control sequencing and dampering to balance flows. In contrast, unitized HRV/ERVs are easier to commission, as they may be controlled and balanced at the unit or the register. Considering systems control and commissioning is a key decision because maintaining balanced ventilation is critical in an airtight Passive House.

As the Passive House market grows and larger projects are constructed, project teams must consider HRV/ERV system arrangement. There is not a simple answer. However, the operating costs and long-term maintenance of centralized systems for a large multifamily building offer substantial benefits over a unitized system.

Each project is unique and examining the pros and cons of both systems is an effective exercise to evaluate this design challenge. The chart below sums up the pros and cons of each type of system; keep in mind that first costs are similar.

Unitized Pros Centralized Pros
Tenant pays for ventilation.

Maintains apartment comparmentalization.

No floor penetrations.

Lower fan power.

Lower maintenance costs.

Accessible for maintenance.

Minimal penetrations.

Reduced envelope leakage.

Insulated ductwork not required.

Reduced horizontal ductworks.

Supply air temperature and relative humidity easily controlled.

Unitized Cons Centralized Cons
Increased maintenance costs.

Access to apartments required for maintenance.

Two penetrations per apartment, with additional envelope air leakage at each penetration.

Difficult to properly insulate and air seal ductwork, resulting in a higher risk of condensation.

Increased floor-to-floor height for horizontal duct runs.

Reduced floor area.

Reduced floor area.

Owner pays for ventilation.

Numerous floor penetrations.

Fire-rated shafts and dampers.

Stack effect.

Aerosealing to reduce duct leakage.

More difficult to balance.

Higher fan power.

 

Thomas Moore is a certified Passive House consultant and a building systems analyst with Steven Winter Associates, Inc. The second part of this post will compare HRVs and ERVs.

3 Comments

  1. Jeff Stern | | #1

    Good summary of the pros and cons of each approach. In my limited experience trying to get a centralized system in a multi-family project, the need for fire / smoke dampers at each fire barrier penetration added significant costs and perhaps should be added to the discussion.

  2. Jeff Stern | | #2

    oops - I see it is in the list of cons already. my mistake.

  3. sethw | | #3

    Based on my experience, a prime developer concern is who pays the bills - AKA: unitized tenant-pay is the preference. Everything else is just the a/e/cm/gc's job to resolve.

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