PERFORMANCE TANKLESS WATER HEATERS


1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60


Std Mid Low


Figure 2: Estimated Energy Factor (EF) and Average Delivered Temperature Non-condensing


Std Mid Condensing 1 Low


Estimated EF Avg. Delivered T (ºC)


55.0 54.0 53.0 52.0 51.0 50.0 49.0 48.0 47.0


Mid (active buffer tank)


Condensing with BT


Mid


(inactive buffer tank)


Condensing with BT


‘mid’ pattern testing both extremes, with the thermostat disabled and with it active all day. When active, the average delivered temperature is good, though at a significant energy cost. When inactive, which effectively doubles the internal pipe volume, the average delivered temperature suffers.


Conclusion The benefits of gas TWHs are known and documented – more efficient hot water generation than gas SWHs and the ability to sustain a given hot water demand. However, they are not without drawbacks, concerning the degradation of delivered efficiency with rated versus more realistic usage. It has been shown that using more realistic draw patterns versus the US standard has an impact on the performance of all water heater technologies. From this, TWHs may suffer a disproportionate impact, as they are best suited for steady state operation. To better inform stakeholders, and in support of developing enhanced simulation tools, this laboratory study built on prior research by characterising TWH performance through standard versus realistic daily use patterns and a battery of short term tests, consisting of unusual, if not extreme, usage patterns. Ultimately, how this impacts the end


Tankless water heater


user is still a subject of investigation. For example, the delays to delivering hot water are as much a function of the water heater itself as the piping layout. Additionally, while emphasis in this study was placed on delivered efficiency, the difference in annual operating cost between the


44 CIBSE Journal April 2013


non-condensing and condensing TWH in California is less than $10 (using the ‘mid’ pattern)9


. Thus the added cost of a higher


efficiency TWH – 0.92 EF versus 0.82 EF – may be hard to justify while natural gas is so inexpensive in the US. CJ


*the amount of hot water being drawn from the system throughout the day/week/year **the time (delays) between the time at which there is a controller asking for hot water to be produced, to the time that the gas ignites (fires) and the time that the hot water flows


References


1 Data from the Air-conditioning Heating and Refrigeration Institute (AHRI)


2 Grubb, D. ‘Installing On-Demand Water Heaters’. Journal of Light Construction, Volume 24, Number 5, February 2006. Hanley Wood, Washington, D.C.


3 Rinnai Corporation, http://www.rinnai.us, 2012.


4 ‘2008 Building Energy Efficiency Standards: Residential Alternative Calculation Method Approval Manual’, California Energy Commission, CEC-400- 008-002-CMF, 2008.


5 The hot water draw pattern used by the U.S. Dept. of Energy is six 10.7 US gallon draws, drawn at 3.0 US gallons per minute, each spaced an hour apart.


6 Lutz, J. and Melody, M. ‘Typical Hot Water Draw Patterns Based On Field Data’, Lawrence Berkeley National Laboratory - Environmental Energy Technologies Division, November 2012.


7 The full set of test results and analysis can be found in a technical paper published through the Proceedings of the 2013 ASHRAE Winter Meeting, held in Dallas, Texas


8 The standard EF is specific to the standard draw pattern, which its calculation includes adjustments for departures from specified outlet temperature, ambient conditions, and other variations.


9 Assuming $0.15/kWh and $0.97/therm


PAUL GLANVILLE, senior engineer, DOUGLAS KOSAR, institute engineer and JASON STAIR, engineer, work at the Gas Technology Institute in Chicago, IL, USA.


www.cibsejournal.com


Estimated EF (EF for Std)


Average Delivered Temperature (ºC)


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