This FAQ is designed for energy raters. There is often a large range of acceptable tested whole-house mechanical ventilation system rates that show on the EnergyGauge ENERGY STAR General Requirements status page (accessible via the Calculate menu’s ENERGY STAR option). Why so large a range and why doesn't it agree with values on the mechanical ventilation page or my own calculations of ASHRAE 62-2?
First we start with the ASHRAE 62-2 2010 and ASHRAE 62-2 2013 equations. These include allowances for natural infiltration if there is sufficient air leakage to qualify. You can see these values on the EnergyGauge MVent input screen by clicking the Calculate 62-2 2010 and Calculate 62-2 2013 buttons (you must have entered a ventilation system to see these buttons). You will see values for Fan CFM and Total CFM for each of these.
EPA indicates the Rater must verify that key parameters of the installed system fall within specified tolerances relative to the design, as defined in Section 7 of the Rater Field Checklist.
Item 7.1 requires the Rater to measure the actual airflow of the system using ANSI / RESNET / ICC Standard 380, and then to compare this measured value with the design airflow documented in Item 2.3 of the HVAC Design Report. The two values must be within either 15 CFM or 15% of each other. EnergyGauge takes the lowest fan CFM design value (55.3 CFM in the example below) and will subtract from it either 15% or 15 CFM whichever value is larger. In this case the larger amount is 15 CFM, so the low range on the report is 55.3 CFM - 15 CFM = 40.3 CFM. For the upper end of the range EnergyGauge uses the total CFM calculation in case the designer chose to ignore infiltration from air leakage. In this example the higher Total CFM is for 62-2 2013 and is 128.8 CFM. Adding the larger of 15% or 15 CFM, yields 15% of 128.8 which is 19.3 CFM for a total of 128.8 + 19.3 = 148.1 CFM. Thus the range of acceptable mechanical ventilation rates on the ENERGY STAR General Requirements status page for this example will show 40.3 CFM to 148.1 CFM. Values outside that range will receive a warning, however if the rater has information from the designer as to why the design value was lower they may share it and it may still be accepted.