Account for baseline house duct size in the energy model

[jonwinkler]

The current approach for modeling fan power effectively results in upgrades having a higher power use for a given flow despite a targeted fan efficiency of 0.375 W/cfm in the upgraded home compared to 0.5 W/cfm in the baseline home. Comparing pressure rise vs flow curves shows that the fan pressure rise has increased for a given flow for the upgrade. Essentially the duct system has become more restrictive.

The first step is to expose fan W/cfm in OS-HPXML.

[joseph-robertson]

I'm starting to wonder whether a new "fan W/cfm" argument is the way to go here. Since the needed inputs to make the fan W/cfm adjustment include upgrade design max airflow, the ApplyUpgrade measure would need to either (A) somehow call apply_hvac_sizing from OS-HPXML first, calculate the W/cfm override, and then apply the BuildResidentialHPXML measure, or (B) apply the BuildResidentialHPXML measure twice (once to get upgrade_design_max_airflow and again with the calculated W/cfm value).

What if, instead of exposing a "fan W/cfm" argument, we exposed a "fan intermediate power" argument. This would be baseline_design_max_airflow * upgraded_fan_watts_per_cfm. Then the apply_hvac_sizing method gets updated to use this value (and upgrade_design_max_airflow) in the calculation of the fan W/cfm override.

Or maybe there's an easy way to get upgrade_design_max_airflow without calling the entire apply_hvac_sizing method?

@shorowit @jonwinkler

[joseph-robertson]

@jonwinkler and I proposed that we'd introduce two new arguments to BuildResidentialHPXML:

• hvac_distribution_fan_watts_per_cfm (https://github.com/NREL/OpenStudio-HPXML/issues/1530)
• hvac_distribution_design_max_airflow_for_adjustment

When hvac_distribution_design_max_airflow_for_adjustment is assigned, it (along with the newly calculated design max airflow) would be used to make an adjustment to hvac_distribution_fan_watts_per_cfm.

Edit: perhaps additional_properties could be used instead of introducing hvac_distribution_design_max_airflow_for_adjustment?

Another edit: perhaps we go with hvac_distribution_duct_max_airflow_cfm which receives the baseline value, and then another "multiplier" argument? This could also be useful with another heat pump sizing methodology.

[shorowit]

Why can't you apply the adjustment upstream (i.e., in ResStockArguments)? I don't follow why the adjustment needs to live in BuildResHPXML.

[joseph-robertson]

The adjustment is a function of the airflow that's calculated in the upgraded building.

[shorowit]

Maybe it would make more sense to be able to specify the heating/cooling airflows instead?

We already have HPXML extension elements for airflow rates (search hpxml.rb for heating_airflow_cfm and cooling_airflow_cfm) that we use for reporting purposes. We could start using them if they are entered in the HPXML file.

[joseph-robertson]

Would we need to be careful not to suggest that values entered for heating_airflow_cfm or cooling_airflow_cfm become the populated values in the HPXML file? Seems safer to use additional_properties where only the ApplyUpgrade measure assigns heating_airflow_cfm and cooling_airflow_cfm.

[shorowit]

Why wouldn't you want them in the HPXML file? I may be missing some nuance here, but I was specifically suggesting this approach so that the values do get entered in the HPXML and then used to create the model.

[joseph-robertson]

The adjustment to the fan W/cfm is a function of both the calculated existing and upgraded airflows. I'm suggesting we pass into BuildResHPXML the existing airflow (somehow), use OS-HPXML's sizing method to calculate the upgraded airflow (as usual), and then use those two values to make an adjustment to the fan W/cfm.

Here is some stubbed code for the adjustment.

[joseph-robertson]

@jonwinkler @shorowit @afontani

Sounds like it may be useful to expose, in addition to hvac_distribution_fan_watts_per_cfm, another argument for passing in the calculated maximum airflow rate for existing ducts -- something like hvac_distribution_duct_max_airflow_cfm. Analogous to system capacities, we can use ApplyUpgrade to assign the values corresponding to the existing building. This could make sizing heat pump upgrades (i.e., a new advanced HVAC sizing control input) more realistic where the maximum allowed airflow rate could be based on the baseline.

Another, maybe unrelated question, is when to apply the fan W/cfm adjustment.

[jonwinkler]

@joseph-robertson, Checking in to see if the hvac_distribution_fan_watts_per_cfm input been fully implemented.

@whiphi92 Mentioned EUSS would be interested in this fix as well.