It seems to me that there is a large discrepancy in the baseline for gallons per minute versus gallons per cycle.
The LEED Online template states "When using the metering lavatory faucet, please convert all flow rates in gallons per minute (GPM) to gallons per cycle (GPC) based on duration from the product specifications. Provide a narrative or calculations to support the installed flow rate."
A .5 GPM aerator evaluated in gallons per minute is right at the baseline for public lavs. However, that very same aerator metered with an automatic sensor at a standard duration of 10 sec. uses water at a clip of .083 GPC (at 0.5GPM x 10sec/60min), which is only 33% of the metered faucet baseline.
This is a critical question for a health care TI project that we are pursuing as a LEED-CI. Due to concerns about bacterial growth, the state health regulations (OSHPD) prohibit any aerators except for laminar flow type, which bottom out at about a 1.5 GPM flow rate (i.e. there is no .5 GPM faucet we can legally use in this application). This is triple the LEED GPM baseline. However when metered with automatic controls on a 10 second cycle, it hits right at the .25 GPC baseline for metered faucets (at 1.5GPM x 10sec/60min). The faucet meter we've specified can be factory set for a cycle duration as low as 5 seconds (we have a manufacturer cut sheet to verify this); therefore the water use in GPC drops to 0.125.
It seems odd that an aerator which, evaluated in one compliance path, is three times over the baseline, but evaluated another way is half the baseline. And yet I can't see where either the math or the logic behind this is flawed. Obviously, this has a huge impact on our water calcs.
Does anyone out there have some insight that I might be missing, or is there truly that big a discrepancy between the two compliance paths?