Phius Certification Manager Al Mitchell dives into the new and improved CERV calculator to aid in Phius Certification.
Hello my passive building explorers!
I am back to inform you about another calculator of the mechanical variety. We at Phius get great satisfaction in seeing project teams succeed, and one of the key ways we can support your success is developing new calculators and design tools.
I am not a believer that modeling is the end all, be all of the building science world (just ask your clients if they would be comfortable living in the little box inside your computer), but I do understand that the more accurate the model is, the better informed the design decision you can make. One area WUFI Passive falls a little short on is mechanical systems, hence the plethora of Excel-based tools we offer to supplement your modeling in that area. As Dora the Explorer might say if she were a building scientist, “Calculator, Calculator, Calculator!”
One of the pieces of mechanical equipment that requires additional calculator support is the combined space conditioning and ventilation units, and this blog focuses on the new and improved CERV calculator, supporting the CERV2 device from BuildEquinox (pictured below).
For those not familiar with the CERV2, it is the improved version of the original CERV, utilizing a heat pump driven series of heat exchangers internally to move thermal energy between air streams. It runs in both ventilation and recirculation mode, providing space conditioning as well as ventilation and dehumidification.
The unit is capable of providing enough ventilation air for a house or dwelling unit in a multifamily project, as well as 2200 Btu/h of space cooling and 4700 Btu/h of heating in recirculation mode. Using controls for CO2, PM 2.5, etc., the CERV will do a demand control ventilation strategy, and oscillate between modes as needed to ensure adequate indoor air quality in the home. When in ventilation mode, the heat pump in the CERV is capable of providing up to 122% of the sensible heat recovery.
Previous guidance from Phius for modeling the CERV in WUFI involved two devices, a mechanical ventilation device with a sensible recovery conservatively set at 75% sensible heat recovery, and a heat pump device using the manufacturer's performance data for efficiency and capacity. However, this does not account for the extra energy utilized for heat exchange compared to a passive core ERV in ventilation mode and does not account for the switching between space conditioning and ventilation modes. For this, we have built a new Excel calculator that will aid in more accurate simulation of the CERV2 unit.
The calculator uses hourly weather data to simulate the performance of the unit for each hourly time step in a TMY weather file. Using WUFI, two different design temperatures are used to estimate the heating load at every hour. Then, using a capacity curve fit, the CERV heating capacity is estimated and the fraction of coverage is determined. The COP of the heating and cooling at each hour are determined using a curve fit, and the COPs at each hour are averaged throughout the year to have a simplified input into WUFI.
The final inputs are a Heat Pump device, not a Heat Pump Rated Monthly COP device, as the calculator’s output COP is already averaged for the year. Then a Mechanical Ventilation device with 100% sensible recovery is input to model the ventilation portion of the device. Since the CERV is running demand control ventilation, the average airflow input into the model should be the Phius required minimums: 20 cfm exhausted from bathrooms, 25 cfm exhausted from the kitchen, and 15 cfm of supply per person.
For prescriptive path projects, the calculator is not required, and with over 100% sensible energy recovery, the CERV2 would meet the requirements for prescriptive path in all climates.
Additionally, Phius has worked with BuildEquinox to develop better commissioning guidance for the CERV2 and a testing protocol for raters and verifiers is available directly from them.
As the calculator requires custom climate data to work properly, please reach out to Phius to request the calculator, and provide the climate location used in your WUFI model so you can get the correct weather location for your project.
