This is Part 6 of a blog series describing construction of the Orchards at Orenco project in Oregon. The first installment was titled The Largest Passivhaus Building in the U.S.

While an ultra-high performance enclosure lies at the heart of the Passive House concept, the mechanical systems constitute the “yang” to the enclosure “yin.” The HVAC system, water heating, lighting, appliances, and conveying systems all contribute to the building’s high level of performance.

Because the Orchards at Orenco project is a large multiunit residential building, these systems and components are in many ways quite different from those one would see in single-family residences designed to meet the Passive House standard. Part 1 in this blog series introduced the basic pieces of the mechanical system design at Orchards. This post will provide more information about the design and discuss some of the challenges encountered during construction and commissioning of the system.

A hybrid approach: HVAC “pods”

After reviewing the respective pros and cons of centralized vs. unitized mechanical systems, the team ultimately decided to utilize a hybrid approach, optimizing the mechanical design to capture the best qualities of those two different approaches.

While the mechanical design uses a centralized water heating system, a decentralized “pod” system is used to provide ventilation air as well as space heating and cooling. The project has three mechanical penthouses (see Image #2, below), each of which houses a large energy-recovery ventilator (ERV) with a heat pump condenser coil in the the supply air duct to temper the ventilation air. Each pod serves a zone of 10 to 24 apartments and adjacent common areas (see Image #3, below).

The “pod” approach optimizes cost and performance while also reducing future maintenance. Regular maintenance of 57 individual ERVs would have been required if a fully unitized approach were used. Going with the pod approach allowed the use of smaller ERVs as well as shorter duct runs than would have been required with a centralized approach. Ducts were smaller, reducing system costs while also reducing distribution losses.

The mechanical penthouses are built on top of the roof and accessed through large ceiling hatches at the third-floor corridors. The walls and roof of each penthouse have the same level of insulation and airtightness as the main walls and roof of the building, so the penthouses are functionally within the Passive House enclosure.

Ventilation

Each penthouse has a different size ERV, rated at 1,500 to 3,500 cfm, based on the flow rate and volume of the building zone served (see Image #4, below).

The ERVs are manufactured by the Loren Cook Company. Initially the team had specified Zehnder and UltimateAir units since they were the only available units identified that met the ventilation efficiency requirement of the Passive House standard. Midway through design, however, the team identified the Cook units as potential candidates. Several months of back and forth with the PHIUS technical committee resulted in acceptance of the Cook units for our project.

These units have a heat-recovery efficiency that ranges from 73% to 76%. The Cook units were considerably less expensive that the UltimateAir units which had been previously specified, and the savings allowed the owner to afford other valuable features on the project, such as the “green screen” I will discuss later.

Distribution of the ventilation air from the ERVs is via 4-inch hard ducts running directly to and from each apartment. These duct “home runs” are collected into main trunk lines running above the ceiling at the third floor corridor (see Image #5, below). By keeping the duct penetrations no larger than 4 inches in diameter, this approach eliminates the need for fire-smoke dampers at each apartment (see Image #6, below).

A continuous 50 cfm of supply air is delivered to each bedroom, with continuous exhaust from each bathroom and kitchen area. To filter cooking pollutants, recirculating hoods are provided above the kitchen ranges. Continuous air flow regulators installed within the ducts help to ensure balanced continuous airflow into and out of each apartment.

Heating and cooling

At each penthouse, a Samsung AM024 VRF heat pump coil is placed inline on the supply air distribution line. This air-source heat pump has a COP of 4.31 and provides approximately 80% of the heating for the building, as well as some cooling during the summer, though it does not meet the entire cooling load. The heat pump is in parallel with the ERV so that it does not create static pressure in the ventilation system.

About 80% of the heating load is met with the ventilation air. Electric-resistance cove heaters in the apartments are designed to meet the remaining heating load (see Image #7, below). These heaters are expected to operate only on the coldest of days, or in some apartments not at all. The team has metered eight of these heaters to provide verification of our assumptions.

The Pacific Northwest summer is relatively cool and dry compared to much of North America, and air conditioning is usually not provided in new multifamily buildings in Oregon and Washington. Though the HVAC system at Orchards does not provide full air conditioning, the ventilation air does provide some degree of cooling.

The 50 cfm requirement for supply air at the bedrooms is intended to provide additional airflow to temper the apartments during the summer months. The system includes heat-recovery bypass with an economizer to assist with night-flush cooling. The residents are instructed to close their windows in the morning, keep them closed during the day, and open them again in the evening. This helps minimize heat flow to the interior during the day during warmer periods and further assists with night-flush ventilation. Upon initial occupancy of the building, this was not well understood, but after a few hot spells, nearly all of the residents have learned the protocol.

It should be noted here that two key elements of the basic building design, the balconies and the eyebrows, serve an essential shading function to keep the apartments cooler during the warmer months (see Image #8, below). Low-solar-heat-gain glazing is used at the east- and west-facing windows (LoE 366/180, argon-filled units, SHGC = 0.25) to further control against overheating.

Water heating

Two central boilers (Bradford White eF Series boilers with 100-gallon tanks and 98.5% thermal efficiency) provide hot water to the building. These are housed in a first-floor mechanical room near the center of the building.

Over the course of multiple design iterations, the team determined that a temperature maintenance system with heat trace tape was likely to be more energy-efficient than a recirculation pump. Hot water piping to the apartments is insulated to minimize distribution heat losses (see Image #9, below). High efficiency plumbing fixtures are used at the apartments to minimize the water heating load and further improve system efficiency.

Other active systems and components

High-efficiency lighting is used throughout the building to minimize the electrical load. Pinned fluorescent lighting was designed to meet a target of 0.4 Watts/square foot at the apartments. LED lighting is provided in the common areas, with lighting controls used at the corridors and stairs to further reduce loads.

Upper tier Energy Star appliances were specified for the apartments (see Image #10, below). Appliance selections were made to achieve the best balance of cost and performance as well as accessibility requirements (some of which can significantly limit the number of models to choose from).

Only two stairs are needed in the building to meet code egress requirements (one near each end); however, the Orchards design includes a third stairway, adjacent to the main lobby. This richly detailed stairway is intended to encourage residents to access their apartments on foot rather than using the elevator, enhancing resident health and interaction while reducing energy use even further.

A MLR traction elevator is used in lieu of a hydraulic model (the type of elevator that is typically used at a building of this size). This choice, combined with the provision of the “attractive” stairway, is expected to reduce elevator-related electricity usage by 57%.

Green screen

The building has an information screen mounted prominently in the main lobby (see Image #11). This screen provides news that is important to the residents, while also indicating the energy use of the overall building and the electrical usage in each apartment (see Image #12). Intended as a tool for educating the residents about their energy usage, the green screen also fosters a fair degree of social interaction.

A forthcoming blog will discuss the key lessons learned at the Orchards at Orenco project, and will report measured performance data from the first year of operation of the building.

Mike Steffen is a builder, architect, and educator committed to making better buildings. He is vice president and general manager of Walsh Construction Company in Portland, Oregon.