This is Part 2 of a two-part series. Here is a link to the first article in the series: “Ventilation for Passive House Multifamily Projects, Part 1.”
In climates with significant heating or cooling seasons, Passive House projects must have a balanced heat-recovery or energy-recovery ventilation system. These systems use a heat exchanger to transfer heat and moisture between the outgoing and incoming air streams.
A heat-recovery ventilator (HRV) transfers heat from the outgoing exhaust air stream to the incoming fresh air stream during the winter (or vice-versa during the summer). An energy-recovery ventilator (ERV) transfers heat and moisture from the exhaust air stream to the fresh air stream in winter (or vice-versa during the summer). The operation of recovery ventilators reduces the energy required to heat and cool, and in the process decreases the building’s carbon footprint.
What the industry has learned from the development of airtight buildings and programs such as Passive House and R2000 is that indoor relative humidity must be controlled; in some seasons, this can be achieved through continuous ventilation. Deciding between an HRV and an ERV gets more complex when the Passive House concept is scaled from a single-family home to a multifamily program.
The extremely airtight building envelope required of a Passive House combined with high internal moisture gains from an occupant-dense multifamily program (coming from occupants, kitchens, and bathrooms) forces additional moisture management considerations during mechanical ventilation design.
Maintaining acceptable interior relative humidity in both the heating and cooling season is paramount for building durability and occupant comfort. It’s appropriate that Passive House professionals claim this simple motto: “Build Tight, Ventilate Right!”
Comparing summer and winter operation
In New York City (Climate Zone 4A), where the multifamily Passive House market is rapidly growing, there is a significant heating season and a demanding cooling season with high humidity. With this seasonal variation there are four primary operating scenarios for an HRV or ERV that need to be considered during design.
Summer Condition – HRV
An HRV operating in the summer (hot-humid exterior air and cool-dry interior air) introduces additional moisture to the building through ventilation. Heat is transferred from the incoming outside air stream to the exhaust air stream leaving the building. This cools the supply air, but exterior moisture is not removed from the incoming air. The building’s dehumidification load increases as a consequence of additional moisture from the outdoor air.
Winter Condition – HRV
An HRV operating in the winter (cold-dry exterior air and warm-moist interior air) exhausts the moisture generated by building occupants. Heat is transferred between the two air streams at the recovery core, but moisture in the exhaust air is not transferred to the supply. As a result, controlling interior relative humidity in the winter can be less challenging with an HRV.
Summer Condition – ERV
An ERV operating in the summer (hot-humid exterior air and cool-dry interior air) reduces the amount of moisture in the outside air that is delivered to the interior. Heat and moisture are transferred to the exhaust air stream, reducing both the cooling and dehumidification loads associated with ventilation.
Winter Condition – ERV
An ERV operating in the winter (cold-dry exterior air and warm-moist interior air) transfers both heat and humidity to the supply air at the recovery core. As a result, controlling interior moisture levels can be more challenging with winter operation of an ERV.
In summer, outdoor humidity is a factor
Project teams should evaluate these situations and identify the highest risk scenario in relation to their climate and building program. The overarching Passive House design intent is to reduce both the heating and cooling demand, decrease equipment size and annual energy consumption, while maintaining occupant comfort and building durability. Being mindful of the Passive House Design intent will help guide this conversation.
Let’s consider the primary operating conditions, starting with summer operation. Good Passive House design should result in decreased cooling loads and thus, smaller cooling equipment capacity.
An HRV introduces additional moist air to the conditioned building in the summer. This may be problematic if there is a need for dehumidification but there is no need for sensible cooling because the temperature in the space is already low. Since dehumidification will only happen when the cooling system is running (unless supplemental dehumidifiers are installed), the occupants might experience prolonged periods of high humidity and discomfort.
Generally speaking, residential occupants are comfortable with higher summer setpoint temperatures only if the indoor relative humidity is kept between 40% and 60%. If the latent load cannot be met and the relative humidity increases beyond 60%, the majority of occupants will no longer be comfortable.
As such, ERV operation in the summer may be desirable when you consider that an ERV will aid in removing moisture from the incoming air and help maintain a lower dehumidification load. This aligns with the Passive House design intent to maintain occupant comfort and reduce annual energy consumption.
In winter, indoor condensation is a risk
Now let’s consider the winter operation. When evaluating the risk to building durability, winter building operation poses the highest condensation potential. During cold periods, heat is conducted through the building envelope. This can result in cold interior surfaces ideal for condensation especially at the least efficient components, such as windows and doors.
Condensation risk is increased by moisture generated in an occupant-dense multifamily building; the higher the interior relative humidity, the higher the surface temperature where condensation can form.
Even with high-performance window components, the risk of condensation on windows and doors may be present when the interior relative humidity is high and surface temperatures are near the dew point. Mitigating the risk of interior condensation must be considered during the selection of an HRV or ERV.
As a worst case, let’s assume an internal setpoint temperature of 68°F and window frame U-value of 0.275 Btu/hr·ft2·F (1.56 W/M2.K). With an exterior ambient temperature of 14°F (-10°C), frame surface condensation would occur at 60% indoor relative humidity. With an exterior ambient temperature of 4°F (-15°C), frame surface condensation would occur at 50% indoor relative humidity.
While these relative humidity levels may seem high, they can be easily realized in a new building with a dense population. It is not uncommon in our multifamily projects to have up to six people living in less than 1,000 square feet. The amount of moisture generated by occupant perspiration, cooking, and showers in addition to construction moisture in a new building can easily drive interior RH to these levels.
Even intermittent window condensation can be problematic from the perspective of mold growth and building durability. Condensation should be avoided in all buildings but should never occur in a Passive House where the design intent is focused on durability.
Controlling indoor humidity during winter
Remember that an ERV operating during the winter transfers some of the moisture generated inside the building back to the incoming supply air. An HRV operating under the same winter conditions exhausts internally generated moisture, helping to control indoor relative humidity and condensation risk.
This seems like a cut-and-dried argument for the exclusive use of HRVs, right? It is not that simple. The amount of moisture that is re-circulated by an ERV can be decreased with a centralized system through various control strategies and because the moist air from one apartment will be mixed with a much greater volume of air headed back to the ERV. This is in contrast to unitized ERVs, where the majority of the internal moisture gains would be returned back to the supply air of each apartment.
If we assume that all apartments are not experiencing high humidity levels at all times, the shear mixing of these streams will reduce the amount of moisture that can be returned to any one apartment. (For more on HRV/ERV system arrangements, see the first article in this series: Ventilation in a Passive House Multifamily, Part 1).
During periods when most apartments are likely to see increased humidity, such as the early morning and evening, moisture transfer of a central ERV can be controlled with partial recovery core bypass or by controlling the speed of the enthalpy wheel. This acts to reduce the latent moisture transfer efficiency from return to outdoor air streams.
As a consequence, the sensible heat transfer efficiency is also reduced temporarily. Our analyses show that supply air relative humidity can be reduced 10 to 15 percentage points with moisture recovery control. This additional functionality makes central ERVs a viable option for multifamily Passive House in winter.
The data shown below represents a multifamily Passive House building operating a central ERV in a New York City winter. The analysis assumes an equal mix of low, medium, and high humidity generation scenarios, demonstrating possible apartment types in a multifamily building.
The interior relative humidity peaks in the morning and evening at 55%. With the addition of moisture recovery control and a decreased enthalpy wheel speed, the peak relative humidity is decreased by 10 to 15 percentage points, to 45%.
There is no prescriptive path for HRV or ERV selection. However, centralized ERVs can be operated to control supply air moisture content in both winter and summer. This makes ERVs an attractive option for multifamily Passive House buildings in New York City.
Thomas Moore is a certified Passive House consultant and a building systems analyst with Steven Winter Associates, Inc.
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