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+CSustainable Buildings & Operations Project
Manager *
*Sustainable Solutions Corporation*
Designers and modelers run into this all the time ? putting Appendix G aside for now, there is a real impact to the operating cost of the building, and also the site/source energy use, and also the construction cost of the building.
The developer may be thinking about other factors though, such as how the energy will be metered to the units when the facility is occupied, or they may prefer for the units to have their own water heater instead of a central system, if natural gas was used to do this where to run natural gas lines, where to put the flues, etc. and construction cost for that would likely be higher.
Anyway, you?ll have to run the numbers in the simulation to see whether the net impact is large or not relative to your other energy efficiency measures, and identify any trade-offs that need to be made to offset the energy costs. The potential is there for it to be a step backwards for Appendix G performance.
There are a few variables that can make the impact larger or smaller in the design of the DHW system as well. You mentioned high-efficiency water heater units. Hopefully the facility is also using low-flow fixtures to reduce the total DHW quantity consumed, which should reduce the impact for using electricity as the fuel in the proposed case.
David
David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP
Grumman/Butkus Associates
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
[cid:image002.jpg at 01D2521D.BA6D4C20]
I agree with David that in-unit electric heaters are often specified due
to low first cost, and to shift the burden of paying for water heating to
tenants. This also allows lowering the rent (not necessarily by the
sufficient margin), making apartments more marketable. Interestingly, the
Federal Trade Commission requires EnergyGuide labels for a wide range of
appliance (e.g. to show that a given TV may cost $15 a year in electricity
bills), but there is no such requirement for apartments, even though
difference in annual utility costs paid by renters can be $1,000+. So there
is no easy way for tenants to do apples to apples comparison of options
when shopping for an apartment, such as Annual$=12*Montly_Rent +
Annual_Utility_Costs. I think it is safe to assume that a label like that
will not be developed on the federal level in the next four years,
but there were some initial discussions about it in NY, and there is a tool
used in CA for low income tenants called California Utility Allowance
Calculator (CUAC). The technical side does not seem too complicated - it
could be an asset-style label showing anticipated energy cost paid by
tenants, including typical appliances/lighting, HVAC and DHW.
The incentive program in NJ allows either 90.1 2013 Appendix G w/addendum
,
path, and has no
BM package
or ASHRAE bEQ As-Designed
rules against fuel switching. With App G path, a central gas storage water
heater is modeled in the baseline for multifamily projects irrespective of
the DHW fuel in the proposed design (Table G3.1, #11 (a) and Table
G3.1.1-2).
I agree with Fred that DHW distribution losses do not seem to be covered in
App G, and that there should be some related savings with in-unit heaters
versus central system. One challenge with using exceptional calculations
for that is to come up with a design for the central baseline distribution
? e.g. pipe length, sizes, location, etc. It seems that the intent of App G
was to prescribe DHW heating fuel and system efficiency, and that G3.1.3.6
should be expanded to include DHW. There seems to be no logical reason for
treating space conditioning piping losses differently compared to DHW
piping losses.
I am not convinced that distribution losses in *new* buildings meeting 90.1
, solar DHW
2013 are significant given insulation and control requirements in Section
7. Quoting the NREL?s study in Fred's email, ??the range of distribution
loss varied widely (5%?61% due to loop configuration and operational
differences?. So even with the exceptional calculations, I?d expect a
sizable penalty from electric resistance water heaters. As David pointed
out, this penalty may be reduced by specifying low flow fixtures, and
also drain
water heat recovery
pre-heat, etc. The program in NJ has incentives up to $1.7/SF, which is
hopefully enough to get developers interested in looking at measures that
affect tenant bills.
Maria
*Maria Karpman *LEED AP, BEMP, CEM
________________
Karpman Consulting
www.karpmanconsulting.net
Phone 860.430.1909
41C New London Turnpike
Glastonbury, CT 06033