ASHRAE 90.1-2013 App G Modeling- DHW System Efficiency

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When modeling 90.1-2013 Appendix G Baseline buildings on projects with relatively large DHW plants (>1,000,000 BTUH), what do you believe would be an appropriate performance curve for the equipment? Specifically, if the baseline is required to have a 90% Et boiler/heater, that seems to fall in-between what I'd consider to be condensing or non-condensing boilers/heaters.

Under 90.1-2010 App G modeling we typically would use standard non-condensing curves for the 80% efficient baseline DHW heater, and if high-efficiency condensing units (92% or greater efficiency) were specified in the proposed, we would apply an appropriate curve for a condensing boiler, which obviously offers significant savings for low incoming water temps, such as with a DHW heater.

Obviously in 2013, much of that credit is gone because we compare to a 90% efficient baseline, but it seems like there might still be some reasonable performance-curve savings to be had if you compare condensing to non-condensing technology. We looked in the AHRI database, and there is very little equipment that is rated at 90% efficiency (basically just a few 50 gal single-family-scale tank heaters). For commercial equipment, there is essentially a performance gap between the "near-condensing" boilers (top out around 88% efficient) and the condensing boilers (start at about 92% efficient). So there isn't a "real-world" piece of equipment to look at to establish a performance curve.

What have you all been modeling for this scenario?

Example- Multifamily High Rise Project with > 1,000,000 BTUH of DHW boilers (gas fired).

Baseline details:
Equipment per G3.1.1-2, efficiency per 7.4.2:
[cid:image005.png at 01D480D7.C61A18B0]

Table G3.1.1-2 dictates baseline equipment type would be gas-storage-heaters:
[cid:image006.png at 01D480D7.C61A18B0]

7.4.2 sends you to table 7.8 for efficiency requirements:
[cid:image008.png at 01D480D7.C61A18B0]

Table 7.8 would nominally require 80% Et equipment, which would fairly obviously be a standard non-condensing heater, but footnote f of the table sends you to 7.5.3
[cid:image010.png at 01D480D7.C61A18B0]
[cid:image017.png at 01D480D7.C61A18B0]
7.5.3 requires 90% Et equipment:
[cid:image018.png at 01D480D7.C61A18B0]


Nathan Miller, PE, LEED AP BD+C - Mechanical Engineer/Senior Energy Analyst
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