Baseline Systems- System 4 heated only Spaces
Currently modeling a baseline building that requires system 4 from Ashrae 90.1 2010. In the proposed model there are thermal zones that are heated only using constant volume electric unit heaters as the heat system. When running the Openstudio Baseline Measure, the measure keeps these unit heaters in the thermal zones, but changes the parameters of the system. When modeling this type of baseline system, would the heated only spaces contain the proposed system electric unit heaters, or would they be modeled with the required PTHP baseline systems with a thermostat setting that would require no cooling loads?
My assumption would be that the heated only spaces would be modeled with the Baseline PTHP systems that have a cooling thermostat setting high enough that does not allow the HP to provide a cooling load into the space. Any thoughts?
What is your thermal zone type? Curious if it's considered semi-heated or conditioned space? It sounds like sys 10? Systems 9 and 10 apply to "thermal zones designed with heating only systems in the proposed design, serving storage rooms, stairwells, vestibules, electrical/mechanical rooms, and restrooms not exhausting or transferring air from mechanically cooled thermal zones in the proposed design" from G3.1.1 exception (e). If it's none of those types mentioned, I think you do have to model PSZ-HP. As long as thermostats are the same for both models, I think it is acceptable approach.
All of the heated only spaces except for 2(restrooms) are above the heated space criteria for climate zone 4 in table 3.1. The 2 thermal zones that are below this criteria fall between the 15 btuh/sf (Heated space) and 3.4 btuh/sf (Semi heated). Does this mean that the two restroom thermal zones would be categorized as semi-heated because it falls between these values? If they are considered semi-heated, how would their systems be modeled as system 10? These restrooms are exhausted/ventilated by the RTU that supplies conditioned air to VAV terminals of other thermal zones on the floor.
First thing to know is that the OpenStudio Baseline Measure does not check the values from Table 3.1 to distinguish semi-heated from heated or cooled; if it a zone has a heating schedule that is set above 41F, it is assumed to be heated.
Second thing, right now the measure is not checking the whether or not the proposed system for that zone is "exhausting or transferring air from mechanically cooled thermal zones in the proposed design." It sounds like in your case this oversight is (incorrectly) causing those zones to be dropped onto a system 9 or 10. I've filed an issue so that it gets on the list to be fixed.
Third thing, I think those zones should be attached to the primary system on the floor unless they meet the internal loads or operating hours exception for being pulled off onto a separate system. Either way, keep the thermostats the same as in the proposed.