A COMPONENT common to all Passive House buildings is an Energy Recovery Ventilator (ERV). Here, the focus is on selecting ERV for lowest electrical consumption, to minimize building yearly energy demand and peak load.
While any ERV could be used to ventilate a Passive House, not all ERV have the same electrical efficiency ratings. In particular, ERVs used must have above average heat recovery efficiency (>75%) in order to minimize additional energy consumed for heating (and/or cooling) and they must ventilate and recover energy without using too much electricity.
A measure of ERV electrical efficiency is specific power, the sum of all of the power inputs required to operate an ERV (power to drive fan motors, energy recovery wheel motors and controls), divided by the volume of air moved over a period of time. Specific power is expressed as Watts per cubic foot of air per minute or just W/CFM. Specific power varies dramatically between ERV models and manufacturers and low specific power results from intentional product engineering.
The target maximum specific power for ERV used on Passive House Institute (PHI) projects is 0.765 W/CFM, and the typical range on Passive House projects is 0.30 to 0.76 W/CFM. Specifying lower specific power on ERV – or any air handling unit – will lower the owner’s building operating cost. When entering ERV data into Passive House Planning Package (PHPP), consultants can choose to enter PHI certificate data, PHI-certified manufacturer’s data, or manufacturer’s data not certified by PHI. Consider the impact of the specific power on electrical load for three ERV selections, discussed below and summarized in Table 1. Each of the ERV in the table satisfy the project requirements of 3450 CFM and minimum 75% heat recovery efficiency.
ERV 1 is a Swegon GOLD ERV, selected using the public domain PHI-certificate value for specific power. PHI also refers to specific power as electrical efficiency and electric power consumption. From the sample of a PHI certificate above, the electric power consumption is shown to equal 0,45 Wh/m3, which converts to 0.765 W/CFM. When starting a building project in Passive House Planning Package (PHPP), setting the ERV electrical efficiency field to 0.765 W/CFM is a good starting value, since it represents the worst-case power rating that would still be approved for a PHI-certified project. But it likely over-states the actual, as-installed performance.
ERV 2 is the same physical GOLD unit described as ERV 1, the only difference is that Swegon’s GOLD selection software was used to determine the actual specific power at 3450 CFM. At this operating point, the specific power is actually 0.63 W/CFM, representing a 17% reduction in power consumption over the certificate value. Project verification will benefit by entering this certified as-operated value into Passive House Planning Package (PHPP), because it is more accurate and because it impacts project electrical demand and project effective heat recovery efficiency.
ERV 3, by comparison, was selected using the Value Engineering (VE) approach of selecting the smallest GOLD unit meeting the airflow and energy recovery requirements of the project, regardless of Passive House requirements for ERV electrical efficiency. Swegon’s GOLD selection software indicates a specific power of 1.03 W/CFM for ERV 3, representing a 66% increase in electrical demand relative to the PHI-certified ERV 1.
The resulting nominal electrical load at selection conditions is derived by multiplying the airflow by the specific power. The actual electrical consumption requires knowledge of the climate, how the ERV operates over the course of the year through the different operating conditions experienced. For instance, the ERV will spend a few hours every year at full flow (3450 CFM), but occupancy conditions may allow for the unit to turn down the airflow if many of the occupants are gone. This sort of control is called Demand Control Ventilation (DCV), and it should be specified so that the ERV can spend fewer hours at full flow, resulting in even more operating energy savings. Swegon provides this sort of energy use analysis to their clients on request. PHPP provides fields for entering hours spent at various flowrates.
ERV consume significant electricity. Specifiers of all types of buildings can save the most energy by requiring low Specific Power. Low energy consumption doesn’t happen automatically by specifying the smallest unit, as seen in the comparison above. Rather, the greatest ERV operating efficiency must be intentionally specified. Swegon’s PHI-certified GOLD ERV offer specifiers the benefit of units pre-qualified to operate at the lowest electrical demand and consumption.
Contact Swegon today for assistance selecting a GOLD unit that consumes the least electricity!