I am trying to correctly determine the baseline system for my LEED energy model. I have a 7-story hotel building so it will either be System 1- PTAC or System 2 –PTHP based on ASHRAE 90.1-2007. The major difference between these is using gas vs. electric for heating. The proposed design will use VRVs. The most accurate comparison using just this information would be System 2-PTHP since the VRVs use electric. However, there are instances where we will use gas such as for domestic hot water and a small lobby portion of the building which will have a separate RTU. These instances are not indicative of the majority of the building, but does the baseline system need to be System 1-PTAC because of this use of gas?
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Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
January 13, 2012 - 12:38 pm
The interpretation ruling below was posted on 11/1/11. If your project was registered before that date I would suggest it is appropriate to model a system 2 in the guest rooms and a system 3 for the lobby area. Make sure to explain this in your narrative. If not yet registered by that date then it looks like the whole building should be a system 1.
Ruling
Clarification is requested regarding when a building heat source in Table G3.1.1A should be identified as "Fossil/Electric Hybrid" versus "Electric".
The ASHRAE 90.1-2007 User's Manual states that a fossil/electric hybrid source "refers to a system with any combination of fossil and electric heat, and the baseline system for this is a fossil fuel system". Therefore, the heating source for the proposed building would be considered "Fossil Fuel" or "Fossil/Electric Hybrid" if the building uses any fossil fuel source for space heating (including backup heating or preheating), and the baseline building heat source would be fossil fuel.
Exception: ASHRAE 90.1 Section G3.1.1 Exception (a) stipulates additional system type(s) for non-predominant conditions (i.e. residential/non-residential or heating source) if those conditions apply to more than 20,000 square feet of conditioned floor area.
EXAMPLES OF BASELINE HEATING SOURCE DETERMINATION:
The Baseline heat source from Table G3.1.1A for the following Proposed Case system types would be fossil fuel since the proposed system design includes a combination of fossil and electric heat:
1. Variable air volume system with gas furnace preheat and electric reheat 2. Packaged terminal heat pumps with outside air tempered by fossil fuel furnace 3. Water source heat pumps with fossil fuel boiler 4. Ground source heat pumps with backup fossil fuel boiler 5. 90,000 square feet is conditioned by a variable air volume system with electric reheat, and 10,000 square feet is conditioned with fossil fuel furnaces
The following buildings would be modeled with an additional system type with a different Baseline heating source in accordance with Section G3.1.1 Exception (a):
1. 90,000 square feet is conditioned by a variable air volume system with electric reheat, and 20,000 square feet is conditioned with Packaged DX systems with fossil fuel furnaces. In this case, the 90,000 square feet of area would be modeled with an electric heat source in the Baseline Case (System Type #6 - Packaged VAV with Electric PFP Boxes), and the 20,000 square feet of area would be modeled with a fossil fuel heat source in the Baseline Case (System Type #3 - Packaged Single Zone AC with fossil fuel furnace).
2. 50,000 square feet is conditioned by water source heat pumps with a fossil fuel boiler, and 25,000 square feet is conditioned by electric heat pumps. In this case, the 50,000 square feet of area would be modeled with a fossil fuel heat source in the Baseline Case (System Type #5 - Packaged VAV with hot water reheat), and the 25,000 square feet of area would be modeled with an electric heat source in the Baseline Case (System Type #4 - Packaged Single Zone Heat Pump).