Exceptional calculation - hot water use reduction in hotel

Christine Robbins-Elrod

Principal

5G Studio Collaborative,llc.

LEEDuser Basic Member

15 thumbs up

October 10, 2011

We're working on a hotel that will have a reduction in hot water consumption throughout the hotel (particularly in the guestrooms, where 2.0 GPM showerheads and 1.0 GPM faucet aerators will be installed). We would like to include the predicted energy savings from the reduced hot water use in the energy model through an exceptional calculation, but no one on our team has used this method on a NC project before (we are currently working on a LEED for Homes Midrise project where the low-flow showerheads and faucet aerators are accounted for in the energy model). I found one LEED NC Interpretation which states, "energy savings for reduced hot water consumption can be counted in DHW energy calculation," but the interpretation does not specifically mention showerheads or faucet aerators. Has anyone successfully used this exceptional calculation for a NC 2009 project before? Also, would the hot water system need to be downsized in order to use the exceptional calculation, or can the system sizing remain as is since the hotel operator has approved the low-flow showerheads and faucet aerators and has no plans to change them in the future?

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David Posada

Integrated Design & LEED Specialist

SERA Architects

LEEDuser Expert

1361 thumbs up

Mon, 10/10/2011 - 19:06

Yes, we've had several projects use exceptional calculations for energy savings from lower flow fixtures, but not without some back-and-forth between our modelers and the EAc1 reviewers. ASHRAE 90.1-2007 makes this a little easier as the practice is allowed per Appendix G, Table G-1 Modeling Requirements, section 11. Service Hot-Water Systems, item i, Exception 1: "Service hot-water usage can be demonstrated to be reduced by documented water conservation measures that reduce the physical volume of service water required. Examples include low-flow shower heads. Such reduction shall be demonstrated by calculations." (Under ASHRAE 90.1-2004 this language was only added in Addendum A, so some projects following 90.1-2004 got questioned by reviewers; USGBC issued a memo on 4/26/2007 permitting the use of addenda.) In a nutshell, the calculations should estimate the reduction of hot water volume, not just the total water volume reduction, by excluding cold water use from fixtures. If possible, you may want to align any assumptions with historical data on hot water & energy usage from the local utility. The energy savings are thus demonstrated by reducing the total building process flow input (gpm) for service water heating. In that way you wouldn't have to change the system sizing. Hope that helps.

Marcus Sheffer

LEED Fellow

7group / Energy Opportunities

LEEDuser Expert

4454 thumbs up

Mon, 10/10/2011 - 19:20

No need to do an exceptional calculation. Make sure the calculations are consistent with WEc3. The reviewer will want to at least see the major assumptions which went into your calcuations including estimations of the percentage hot water versus cold water flow, the delta T of the DHW system, and the anticipated hot water temperatures at the fixtures. ASHRAE HVAC Applications, Chapter 50 is a good source of information. Most of the time you will be asked to show your calculations for this strategy but it should not be entered in the form as an exceptional calculation.