Duct cost multiplier - unconditioned area

[shorowit]

Resolves #[issue number here].

Pull Request Description

Converts "Duct Surface Area (ft^2)" cost multiplier to "Duct Unconditioned Surface Area (ft^2)". Provides consistency w/ ResStock-HPXML. Follow-up to #532.

Only buildings where the primary duct location is living space or finished basement are affected; they now return zero (instead of non-zero) values for the cost multiplier.

Checklist

Not all may apply:

• Unit tests have been added or updated
• All rake tasks have been run, and pass
• Documentation has been modified appropriately
• Any new options are added to project_testing
• project_testing runs without any failures
• No unexpected regression test changes
• All tests are passing
• The changelog has been updated appropriately
• This branch is up-to-date with develop

For more information on how to perform these checklist items, see the documentation's Advanced Tutorial.

[shorowit]

Verified that results perfectly match ResStock-HPXML for 30 project_national samples.

[joseph-robertson]

Seems to zero out the duct cost multiplier for some national project samples.

[lixiliu]

@shorowit - if you end up sealing ducts in conditioned space, would you expect there to be substantial energy saving?

I found an EUSS building (372868) in MN that has an envelop upgrade containing only duct sealing: 30% leakage, R-4. -> 10% leakage, R-8. The result saving is 1924 kWh, which is surprisingly high to me. For context, the heating system is ASHP, SEER 15, 8.5 HSPF of 12.61 kbtu/h cap, 6 deg F offset for heating at 68 deg F setpoint, -3h day & night.

But because of the ducts are in conditioned space, "Duct Unconditioned Surface Area (ft^2)"=0, leading to $0 upgrade cost. I understand the motivation for not air-sealing the ducts in conditioned space (and we could limit the apply_logic for ducts going forward), but am quite surprised by the energy saving.