Low Cooling DSE value for a case with ResStock 3.2/OS-HPXML 1.7

[sugirdhalakshmi]

We are getting Low Cooling DSE: 0.02836323 while running one building (NREL\Team OCHRE - Validation\OS-HPXML_1.7\test_100\buildings\up00\bldg0000001) using os-hpxml-1.7branch. The building is located in MA, duct location: attic-vented and has room air conditioner for cooling and furnace for heating. The cooling fan flow is 108 cfm, so the cooling duct leakage is 22 cfm for supply and 11 cfm for return (very low values). ASHRAE152 spreasheet is attached for reference.

152faip2003_prc3_39.xls

[mnblonsky]

Oh I didn't realize it has a room AC. Should we not be calculating cooling DSE for those? It seems unnecessary. I imagine that may not fix the whole problem, but it would at least fix this case.

[jmaguire1]

It's not only a room AC, it's only supposed to condition 20% of the space. I'm not sure we're even set up to handle that? But that's part of why the airflow rate is so low here. In OS-HPXML they basically model the HVAC as fully conditioning the space and then reduce the energy consumption by the % conditioned. I'm not 100% sure but I'd guess this is playing into why the DSE calculation is so low. Here's the element of HPXML setting this to only condition 20% of the space:

<CoolingSystem>
              <SystemIdentifier id='CoolingSystem1'/>
              <UnitLocation>conditioned space</UnitLocation>
              <CoolingSystemType>room air conditioner</CoolingSystemType>
              <CoolingSystemFuel>electricity</CoolingSystemFuel>
              <CoolingCapacity>2797.0</CoolingCapacity>
              <CompressorType>single stage</CompressorType>
              <FractionCoolLoadServed>0.2</FractionCoolLoadServed>

That said, room ACs can't be ducted. There are ducts in this building, but they're only used for cooling. That'll be true for all room ACs, but you can also tell from the HPXML file because the duct cooling component load is 0:

                <Heating>
                  <Total>88509.0</Total>
                  <Ducts>31224.0</Ducts>
                  <Windows>2582.0</Windows>
                  <Skylights>0.0</Skylights>
                  <Doors>258.0</Doors>
                  <Walls>14641.0</Walls>
                  <Roofs>0.0</Roofs>
                  <Floors>0.0</Floors>
                  <Slabs>11968.0</Slabs>
                  <Ceilings>1999.0</Ceilings>
                  <InfilVent>25838.0</InfilVent>
                </Heating>
                <CoolingSensible>
                  <Total>10112.0</Total>
                  <Ducts>0.0</Ducts>
                  <Windows>1557.0</Windows>
                  <Skylights>0.0</Skylights>
                  <Doors>94.0</Doors>
                  <Walls>1822.0</Walls>
                  <Roofs>0.0</Roofs>
                  <Floors>0.0</Floors>
                  <Slabs>0.0</Slabs>
                  <Ceilings>1167.0</Ceilings>
                  <InfilVent>2382.0</InfilVent>
                  <InternalLoads>3090.0</InternalLoads>

So the fix we actually need here is to say any room AC is not ducted and skip the DSE calc (set it to 1.0). Hopefully a pretty quick fix.

[jmaguire1]

This also makes me feel better about seeing such a low value! It's an infeasible case/bug outside of our DSE calculation. So I think we can still use the duct validation effort #20 to come up with an appropriate minimum value we can cap DSE at.

[sugirdhalakshmi]

Adding other few test files with bad cooling DSE to this issue: 'NREL\Team OCHRE - Validation/OS-HPXML_1.7/bad_dse_may2024'

The duct location were attic - vented and basement - conditioned. The heating equipment is Furnace and cooling equipment is Room AC. The heating DSE values are okay. But the cooling DSE values are bad because the room ACs are assigned to have ducts. DSE should be set to 1 for Room AC. For basement - conditioned duct location, using "basement with insulated walls" option from ASHRAE 152 spreadsheet looks good for heating.