Gas or Electric (System 5 or System 6)

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Hi All,

A 50,000 sf building we are looking at has electric DX cooling and electric heat VAV terminal boxes in the zones. The only gas use in this system is for a small pre-heat coil on the RTU and there is also an energy recovery ventilator. In developing the Baseline model, should we use System 5 (gas boiler) or go with System 6 (all electric)?

With gas, I know the default is to use Sys 5 but in this case the pre-heat is a very small part of the system so the "predominant" heating energy use is electric. The Baseline will still be modeled with the same gas pre-heater per Appendix G, either way.

Many thanks for your thoughts.

Jeff Ross-Bain, PE, LEED Fellow

Build Green. Save Time. Save Money.

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Not sure which version you are looking at, but I believe you will need to
go with a Appendix G System 5, sorry. That is officially considered a
Fossil/Electric Hybrid system, not a preheat coil. See this ASHRAE
interpretation for 90.1-2007:

If you are looking at newer versions, there might be a more recent ASHRAE
interpretation - worth investigating, but unlikely to have a different
outcome based on their previous interpretation.


*Elizabeth Gillmor PE, BEMP, LC, LEED AP*

*e n e r g e t i c s **consulting engineers, llc* | 303.619.0091

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