heating system sizing for shared heating affected by envelope upgrade

[lixiliu]

In EUSS AMY2018 results, the primary heating system size is reduced by the Basic Envelope upgrade and the Enhanced Envelope upgrade when the heating system is:

build_existing_model.heating_fuel  build_existing_model.hvac_heating_efficiency 
Electricity                        Shared Heating                              

Here are their differences (just size_heating_system_primary_k_btu_h is different):
baseline

             upgrade_costs.size_heating_system_primary_k_btu_h  upgrade_costs.size_heating_system_secondary_k_btu_h  upgrade_costs.size_heat_pump_backup_primary_k_btu_h
building_id                                                                                                                                                             
12138                                                     4.79                                                0.0                                                  0.0  
13909                                                     9.72                                                0.0                                                  0.0  
20242                                                     7.26                                                0.0                                                  0.0  
31899                                                    15.66                                                0.0                                                  0.0  
55736                                                     8.57                                                0.0                                                  0.0  
...                                                        ...                                                ...                                                  ...  
514416                                                   25.44                                                0.0                                                  0.0  
514830                                                   23.43                                                0.0                                                  0.0  
532502                                                    9.62                                                0.0                                                  0.0  
548026                                                   12.23                                                0.0                                                  0.0  
549042                                                   15.40                                                0.0                                                  0.0  

[62 rows x 3 columns]

upgrade

             upgrade_costs.size_heating_system_primary_k_btu_h  upgrade_costs.size_heating_system_secondary_k_btu_h  upgrade_costs.size_heat_pump_backup_primary_k_btu_h
building_id                                                                                                                                                             
12138                                                     3.91                                                0.0                                                  0.0  
13909                                                     8.04                                                0.0                                                  0.0  
20242                                                     4.69                                                0.0                                                  0.0  
31899                                                    11.17                                                0.0                                                  0.0  
55736                                                     7.10                                                0.0                                                  0.0  
...                                                        ...                                                ...                                                  ...  
514416                                                   16.67                                                0.0                                                  0.0  
514830                                                   18.87                                                0.0                                                  0.0  
532502                                                    8.74                                                0.0                                                  0.0  
548026                                                    9.95                                                0.0                                                  0.0  
549042                                                    9.93                                                0.0                                                  0.0  

I do not see cooling system size difference between the baseline and envelope upgrades.

I think there has been previous effort to fix HVAC sizing to baseline level when an upgrade does not change the HVAC system, is this shared heating option an oversight?

[joseph-robertson]

Pretty sure it's because of this line, heating_capacity = nil when heating system type is "Shared", in BuildResidentialHPXML: https://github.com/NREL/OpenStudio-HPXML/blob/master/BuildResidentialHPXML/measure.rb#L4688.

Nothing jumps out at me as to why we need to autosize a shared heating system. @shorowit?

[shorowit]

Sorry for the lengthy response....

This gets a bit complicated. All shared systems will get modeled as individual systems (with efficiency adjustments) -- so the provided capacity, which is the capacity of the shared system, ends up replaced by an autosized capacity to represent the individual system.

For a shared boiler, the provided capacity of the shared system is completely ignored. For some other shared systems, the provided capacity of the shared system is only used to determine the efficiency adjustment. In all cases, again, the individual system that is modeled is autosized.

In the context of ResStock, where you don't model real buildings, we could theoretically allow the capacity of the shared system to be entered and simply divide by the number of dwelling units served for the capacity of the modeled individual system. But in a real building, dwelling units may differ such that some need more of the shared system's capacity than others, so it's not a great general solution.

I'm tempted to suggest that ResStock just model the shared boiler as an individual boiler because the efficiency adjustment is very straight-forward and not dependent on capacity. That would solve this issue. But it won't work for other shared systems where the adjustment differs based on capacity.

[shorowit]

If you went that path, you would just have to call https://github.com/NREL/OpenStudio-HPXML/blob/c28a3a350f143206b9060313e7079475ac91c995/HPXMLtoOpenStudio/resources/hvac.rb#L1789 from ResStockArguments measure, to get the default shared boiler electric auxiliary energy (EAE), and then pass that into the BuildResHPXML measure. Or if you are not modeling WLHPs, the numbers are fixed and could simply get added to options_lookup.tsv.