PASSIVE HOUSE is one such high performance building concept, proving to result in buildings that are comfortable for occupants and energy efficient for owners. Passive House developers frequently face a dilemma, whether to apply the same old products in old, less efficient ways at lower first cost, or to permit newer products to be applied in new ways at lower life cycle cost.
Passive House Institute US (PHIUS) certifies systems and windows, and is developing an ERV certification program. Passive House Institute (PHI) certifies building components in order to “define quality standards, facilitate the availability of highly efficient products and promote their expansion, and to provide planners and building owners with reliable characteristic values for input into energy balancing tools.” In the building services category, PHI certifies heat and energy recovery ventilators. Swegon was the first to gain PHI-certification for an ERV in the “large ERV” category. Today, Swegon GOLD has been applied on dozens of certified Passive House projects around the world.
Passive House designers routinely accomplish their energy-savings goals by designing airtight and well-insulated buildings. PHIUS and PHI both require airtight Passive House buildings to be continuously ventilated to provide occupants with fresh air. Continuous ventilation is provided by energy recovery ventilators (ERV), like Swegon’s GOLD. Which GOLD provides the lowest life cycle cost for the project?
Imagine a Passive House project requiring 3450 cubic feet per minute (CFM) of ventilation air. Figure 1 compares two highly efficient GOLD energy recovery ventilators (ERV), both offering 3450 CFM.
The first unit is selected using the traditional lower-first-cost method, that is, selecting the smallest GOLD unit that will deliver the desired airflow. The traditionally held belief is that the smallest unit is best because it costs less to purchase and install, regardless of the cost to operate the unit year after year. The second, PHI-certified, GOLD unit is also selected to deliver the desired airflow, but with the additional goal of operating within the range of ERV energy consumption meeting Passive House certification criteria. The PHI-certified GOLD unit is larger and thus has a higher first cost, owing to its larger cabinet size and larger internal components. What should a developer consider then to justify application of the larger Passive House certified GOLD ERV as a lower life cycle cost component?
Certification Criteria – Pressure
ERV must meet several criteria in order to gain PHI certification. Here the focus is on pressure, and more specifically on PHI’s criteria for testing and certifying ERV with a maximum allowable external static pressure, the amount of pressure an ERV makes available to overcome downstream duct losses from duct friction, dampers, transitions, etc.
Pressure and ERV Energy Consumption
Electrically driven fans pressurize air in a building’s ventilation system. Fan pressure moves air from one place to another, usually through ducts. With an ERV, fan pressure draws air from outside in, then forces the air downstream through the ventilation system consisting of ducts, heat exchangers, filters and dampers and air outlets. ERV also use a second fan to exhaust air from the building, drawing air through another system of return air ducts and forcing it outdoors. Figure 2 shows a GOLD ERV during winter operation, for reference.
Ducts and other ventilation system components resist airflow in an amount that is calculated by engineers, and expressed as the external static pressure requirement, or ESP. This ESP serves as a basis for selecting ERVs, along with the amount of ventilation air required (CFM).
In a given building, a ventilation system with a lower ESP requirement consumes less ERV energy than buildings with higher ESP requirements. Shorter duct runs are one of several methods of lowering the ventilation system pressure requirements. Other methods include larger ducts and oversizing other ventilation system components like filters and air outlets.
For a given airflow, internal pressure drop in the ERV is reduced by enlarging the casing, allowing the ERV to move air with less resistance. Whereas manufacturers strive to develop and offer ERV that can be applied over a wide range of airflows and pressure drops, PHI limits the pressure at which ERV are tested and certified. For example, even though the example ERV from Table 1 may be designed to deliver 3450 CFM with 2 inches of external static pressure, PHI certification requires testing that particular ERV at an external static pressure of no more than 1.2 inches of water column (inWC). PHI certification does not mean that the unit selected cannot deliver more pressure; it only means that doing so would violate the PHI certification criteria. In this way, PHI indirectly promotes low pressure drop ventilation systems in Passive House projects.
Engineers are often tempted to select the lower first cost ERV at a higher available pressure for their ventilation system design. In Figure 3, however, we see the consequence of selecting the lower first cost ERV. For an ERV delivering 3450 CFM and for a given External Static Pressure requirement, the larger unit always consumes less energy. For example, if the building ventilation system requires the ERV to deliver 3450 CFM with available external static pressure of 1.0 inWC of, the PHI-certified unit will consume 0.58 W/CFM, while the lowest first cost unit will consume 0.91 W/CFM, or 57% more electricity.
A complete life cycle cost analysis will include other parameters like local utility rates, local climate, hours of operation and the ERV energy recovery efficiency, the subject of future articles. Before accepting the lowest first cost ERV on high performance building projects, compare the life cycle costs. Available ERV performance data and software tools make such a comparison easy and fast. A few minutes of evaluation time now could save the owner thousands of dollars per year for the life of the building.
In addition to certified flow rates by size, PHI also certifies ERV unit:
To see the actual certified data for Swegon GOLD, download here
A benefit of using a PH-certified energy recovery ventilator on passive house projects is that it eliminates the need for field certification and all the uncertainty that goes with the field certification process.
For Passive House ERV educational opportunities, visit Swegon Air Academy NA page