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The challenge here is determining how to translate changes in fixture flow rates (typically specified in gpm or gallons per minute) to the units used in Sketchbox for hot water demand: BTU per hour per person.
A simple way to do this is to utilize the TRM (Technical Reference Manual) for the state where the project is being built. As an example, here’s a link to the Illinois TRM, where the low flow plumbing savings are calculated in volume 3, sections 5.4.4 and 5.4.5.
Here’s an example for low flow showerheads in a multifamily building with natural gas hot water. Here’s the relevant equation, adapted for this example:
Btu savings = (GPM_base * L_base – GPM_low * L_low) * SPCD * 365.25 * EPG_gas * 100,000
Here’s what all those terms mean and some assumptions we can use:
Putting this all together we can calculate approximate change in hot water demand, per person per year, of 333,000 Btu.
Next we have to translate this into the appropriate units for input into Sketchbox. First, divide by the hours of DHW usage per day. You can find this by adding up the 24 load fractions from the DHW schedule – the total is 6.7 hours for the default residential schedule.
Using this value, we find that replacing all showerheads in the building with low flow plumbing would result in a change in hot water demand of about 136 Btu per hour per person.
To enter this as a measure, create a measure with the “Domestic Hot Water Demand” parameter, and set a custom value, subtracting 136 from the baseline.