[Bldg-sim] Modeling Electric/Fossil Fuel Hybrid Boiler under ASHRAE 90.1-1999

[Donal O'Connor]

Hey,
 
I am looking for help with regards to modeling a HVAC system under ASHRAE 90.1 
1999. I am trying to determine which baseline HVAC system type to choose, as the 
proposed model has a fossil fuel/electric hybrid boiler. Unlike 90.1 2007, where 
the Baseline HVAC System Types include a fossil/electric hybrid boiler system, 
it seems far more unclear under 1999. In Section 11.4.3 part k - (2) (page 59), 
it says that
 
"For systems with mixed fuel heating sources, the system or systems that 
use the secondary heating source type (the one with the smallest total installed 
capacity for the spaces served by the system) shall be modeled identically in 
the budget building design and the primary heating source type shall be 
used in Figure 11.4.3 to determine budget system type."
 
However in the building that I am modeling, it has a 850 kW electric boiler 
(lead boiler) with three further fossil fuel boilers with an installed capacity 
of 3000 kW for backup. So my understanding of the code above is; even though the 
electric boiler is the lead boiler in the proposed case, due to the fact that it 
has a smaller installed capacity than the fossil fuel boilers, it should be 
considered secondary, and thus the fossil fuel boilers will become the primary 
heat source and so I should be using a fossil fuel boiler to heat my baseline 
building. Does this seem correct? Also, I am unclear as to how I would be able 
to model the electric boiler "identically in 
the budget building design" as this is being 
used to supply all of the heat to the heating loop in the proposed case.
 
Any help and ideas would be greatly appreciated.
 
Thanks
 
Donal 		 	   		  
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