[Bldg-sim] [External] Multi-Family DHW

Fri Dec 9 12:11:21 PST 2016

Interesting! First, Sunayana, are you sure you should be using 2013 App G/PRM? Even if the local energy code specifies ASHRAE 2013, LEED v4 uses ASHRAE 90.1-2010. And if utility programs are involved, there is usually some restriction on fuel switching. But that’s just bureaucratic detail.

For the purposes of actually determining operating costs of realistic alternative systems, it’s not clear the standard (or most BEM software) is addressing this adequately. In larger multi-unit buildings, it is probably not practical to duplicate your proposed electric DHW system (in-unit, tank or instant), using gas, because all these heaters would require multiple flues, etc.  So a REAL alternative is a central DHW heater with recirculation.  That recirculation typically has considerable loop losses, which the PRM seems to have directed us to ignore. However if you read the text carefully, this restriction (G3.1.3.6) targets circulating fluids used for space conditioning.
(“G3.1.3.6 Piping Losses (Systems 1, 5, 7, 8, and 11). Piping losses shall not be modeled in either the proposed or baseline building designs for hot-water, chilled-water, or steam piping.”)

However, the DHW language seems ambiguous on this point. In any case, central DHW loop losses are sometimes close to the actual DHW use energy. So the real cost penalty of gas vs. electric may not be as large as it appears from a simple ratio of utility cost/Btu. California’s T24 compliance software, though based on E+, has developed a much more rigorous approach to DHW calculations that at least considers piping losses in multi-family residential applications, along with researched draw profiles. The program passes off the DHW energy calculations to CSE (California Simulation Engine) for multi-family residential DHW.  Perhaps more accessible, a recent paper “Strategy Guideline: Proper Water Heater Selection” (http://www.nrel.gov/docs/fy15osti/63880.pdf) includes a broken link to MultiFamily_Central_DHW_EvaluationTool_v1.0.xlsx. You might try contacting the authors or Bldg America to see if you could get a copy. Output illustrated in the report is pasted below.

If the project really can’t take the energy cost penalty caused by electric vs. gas DHW, and if the space heating uses HW boilers, there are systems that combine the DHW and HW piping; essentially the HW coils are supplied with potable water. This reduces the cost of piping and boilers in the building compared with separate DHW and HW. It does seem to penalize the efficiency of the heating side a bit, as the “HW” return temperature is often not as low as it would be for HW only.

A related item for all to consider is that new gas tank type condensing DHW heaters are being rated at efficiencies of up to 99%. However, this is at a test condition which is rarely duplicated in reality (large flows of 60F inlet water). In any recirculating DHW system, the inlet T is usually much warmer, and in other cases, the draws are smaller and intermittent. Particularly the former reduces efficiency. That effect is considered in eQ and E+ models of HEATING condensing boilers, but not in the eQ DHW heater representation. So just plopping 1.01 into the “HIR” field of an eQ DHW heater is not realistic. (I’m not sure if E+ could be set up to represent this correctly or not.)
I think there is some fodder here for LEED CIRs, ASHRAE Interpretations, proposed changes to the PRM, or direct questions to a code official or utility program manager. In addition to the never-ending battle to have simulation software that provides reasonable representations of building components and operation!

Good Luck,
Fred
Fred Porter, BEMP, LEED© AP
Principal Engineer
Sustainability Services
NORESCO
www.noresco.com<http://www.noresco.com/>

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From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Sunayana Jain via Bldg-sim
Sent: Friday, December 09, 2016 10:36 AM
To: Angelica Walsh via Bldg-sim
Subject: [External] [Bldg-sim] Multi-Family DHW

All,

We are working on an energy model of a high-rise multi-family building in the New Jersey climate zone. This project will be using high efficiency in-unit electric water heaters (0.95 EF) for DHW requirements.

As per the latest code (ASHRAE 90.1-2013 Appendix G), baseline DHW system for a multi-family building has to be a gas storage water heater. As I understand, there will be a significant energy cost penalty for using electric fuel versus natural gas in the design for DHW.

Has anyone encountered similar situation?

Thanks,

Sunayana Jain CEM, LEED AP BD+C
Sustainable Buildings & Operations Project Manager
Sustainable Solutions Corporation

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