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Multifamily Passive House Ventilation Design Part 1: Unitized or Centralized HRV/ERV?

 

Project teams pursuing Passive House frequently ask, “Where do we locate the HRV/ERV?” The answer is complex when the Passive House concept is scaled to a multifamily program.  While there are two primary arrangements for HRV/ERV systems, the trade-off is dynamic and needs to be carefully considered as multifamily Passive House projects begin to scale. A low volume HRV/ERV unit ventilating an individual apartment is a unitized HRV/ERV. High volume HRV/ERV units ventilating multiple apartments and often servicing several floors, is referred to as centralized HRV/ERV.

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 HRV/ERV system cannot be located in a drop-ceiling due to low floor-to-floor height, it is placed in a dedicated mechanical closet. This closet is typically no smaller than 10 ft2 and includes the necessary ductwork connections to the HRV/ERV unit. The alternative solution is to increase the floor-to-floor height to accommodate the HRV/ERV unit and horizontal duct runs in the ceiling. Centralized HRV/ERV systems, however, allow short horizontal duct runs but require floor space to accommodate vertical shafts. With supply and exhaust ducts coupled together the required floor area is about 8-12 ft2. As a result, centralized HRV/ERV systems may actually require more floor area than a unitized system.

Example: In the case of Cornell Tech, vertical supply and exhaust duct work for the centralized HRV/ERV system required 222.5 ft2 per floor, or 13 ft2 per apartment (see image 1 below). Unitized HRV/ERV mechanical closets would have required an estimated 170 ft2 per floor, or 10 ft2 per unit (image 2 on right).

Comparison images HRV/ERV

Image 1 & 2:  These images compare the amount of floor area required for centralized and unitized HRV/ERV systems. Image 1 on the left, shows the 12ft2 floor area required for vertical shafts servicing the centralized ERV at Cornell Tech. Image 2 on the right is hypothetical, showing the typical location and 10ft2 floor area required for a unitized HRV/ERV mechanical closet.

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Recovering from Heat Recovery Woes

IECC Image

The International Energy Conservation Code (IECC) has a number of requirements involving energy recovery on ventilation systems. Requirements vary based on climate zone, building type and size, equipment capacity, and equipment operating hours. As a result, many new construction projects must now incorporate energy recovery considerations into their design.

An energy recovery unit (ERU) equipped with a heat wheel can be a great way to satisfy these energy recovery requirements. The ERU can be a roof-mounted air handling unit, or can be an air handling unit located inside a mechanical room with outdoor air and exhaust streams ducted in. The heat wheel is positioned so that half of the wheel sits in the exhaust air duct and the other half sits in the outdoor air intake duct. During cold weather, the wheel spins, transferring heat from the exhaust stream to the outdoor air intake stream. During hot weather, the wheel transfers heat from the outdoor air intake stream to the exhaust stream. In both cases the heat exchange enables the building to take advantage of the more comfortable conditions of the exhaust air, while still allowing fresh air to enter the building. During extreme weather conditions, heat wheels can save energy on space conditioning while still allowing for healthy indoor air quality.

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When the Rubber Meets the Road

 

As the Passive House standard continues to make waves across New York City and the U.S., an entirely new design process has evolved to respond to the challenges of higher insulation levels, balanced mechanical ventilation, and perhaps the most difficult hurdle – an air tightness level that most would think is impossible. For the recently certified Cornell Tech building on Roosevelt Island, the tallest Passive House in the world, a several year-long coordinated effort was required to achieve such a feat. So what is the requirement, how is it measured, and what are the strategies and considerations required to achieve it?

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Transformers: Problems in Disguise

Sometimes a significant source of energy inefficiency in a building can be hiding in a place difficult to detect. In some buildings, a single transformer can have a substantial impact on electrical consumption.

Image of currents flowing through a transformer

click to enlarge

Some Background

Transformers are responsible for stepping the incoming voltage to a building up or down depending on the design, intended use, or connected equipment.  A standard electrical socket in a US home or office will deliver 110-120 volts AC. Some appliances require 240 V instead. Large mechanical equipment, such as the air handling units, distribution pumps and chillers found in commercial or multifamily buildings may require 460 V. In buildings where the incoming voltage from the utility does not match the voltage required by connected equipment, a transformer is used to deliver the necessary voltage.  The voltage entering the transformer is called the primary voltage and the voltage delivered by the transformer to the facility’s equipment is called the secondary voltage.

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What We’re Looking Forward to at Building Energy Boston ’15

Building Energy BostonBoston has been making national headlines a lot this winter, mainly due to the #snowpocalypse that they’ve been enduring.

Never fear, the first sign of spring is about to appear in Beantown! NESEA’s Building Energy Conference is just around the corner on March 3-5, bringing experts from around the country to share their knowledge about new trends and innovative solutions within the realm of building science and renewable energy.

Road? Where we're going, we don't need roads.

Boston digs out in time for Building Energy 2015

We’re sure it will rejuvenate and reinvigorate Bostonians and all Northeasterners alike. Note: Northeasterners like the people, not northeasters like the storm, just to be clear.

As with most of the Building Energy Conferences, there will be many speakers from SWA there to lead workshops and sessions about improving the efficiency of buildings and their systems (You can read more about that here). Today though, we want to talk about the other presenters and topics that we’re excited to see!

Here are a few of our recommendations that we can’t wait to check out:

  • We have been preaching about addressing building resiliency and energy efficiency, want to hear it from another trusted voice? Attend Alex Wilson’s session Putting Attention Where it is Needed Most – Building Resiliency In Multifamily Affordable Housing.  March 4, 11am – 12:30 pm
  • Codes are raising the bar, owners are seeing the benefits of building more efficient housing and more owners are addressing energy use at the time of capital upgrades and refi, all great news. The next wave we will see is the increase in multifamily Passive House construction and renovation using techniques where possible. Want to know how to incorporate passive house to your next project, you can talk to SWA’s Lois Arena, and you can also hear it from Katrin Klingenberg at her session The Building Science of multifamily Passive House. March 4, 4pm – 5:30 pm
  • Water makes up a sizable portion of utility bills, we recommend attending Reinventing the Water Grid Part 1: Science, Behavior and Dollars. Water reduction strategies and monitoring can save money and reduce operations and maintenance costs through leak detection. We will be attending this session to reinforce what we are recommending and to see if there are new applicable techniques or recommendations we can incorporate into our projects. March 5, 10:30am – 12pm

To attend Building Energy 2015 in Boston, register here. We look forward to seeing everyone there!