Overview

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.

Example: calculating baseline and proposed Btu demand

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:

• GPM_base is the baseline fixture flow rate. For showerheads, the current federal standard is 2.5 gpm.
• GPM_low is the proposed fixture flow rate. For this example we’ll use 2.0 gpm, a standard assumption for low flow fixtures.
• L_base and L_low are the length of time, in minutes, of the average shower. A Michigan field study of 135 homes found the average length to be 7.8 minutes.
• SPCD is showers per capita per day. The same Michigan field study found the average to be 0.6.
• 365.25 is simply days per year (average).
• EPG_gas is the energy per gallon of hot water supplied by natural gas. The TRM gives a lot of details on how to calculate this based on incoming water temperature, shower temperature, and water heater efficiency. In this case, because we’re going to plug this into Sketchbox which will separately account for water heater efficiency, we’re going to use a value of 0.0039, which is the energy per gallon of hot water demanded by natural gas.
• 100,000 converts therms to Btu

Putting this all together we can calculate approximate change in hot water demand, per person per year, of 333,000 Btu.

Sketchbox input values

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.