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Overestimating infiltration? #1101
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@ekpresent found a large reduction in heating energy when switching to a water heater that did not have a flue which started this discussion thread. |
My understanding is that the handling of a flue in our infiltration model is meant to be applied to a measured infiltration rate (or effective leakage area) that reflects the presence of that flue. A flue will behave differently as a function of stack and wind (under normal hourly operating conditions) than when pressurized during a blower door test, and I believe that that's what the infiltration model accounts for. If that is true, then really all it means is that we may be underestimating infiltration in homes with flues and overestimating infiltration in homes without flues because we are using the same infiltration distribution across both sets of homes, whereas we could assume higher infiltration rates in homes with flues vs homes without flues. But it has been a while since I've looked at this. If someone wants to dig into it, here are two good resources on our infiltration model: |
From @joseph-robertson's recent test, adding flue will result in about 37% increase in infiltration component load. |
On a related note, ResStock is using the OS-HPXML default site/terrain (suburban) and shielding (normal) for all homes, both of which impact the infiltration model. |
And we are adding an input for height off the ground that you could use for apartment units on upper stories. |
@afontani explained it to me this way yesterday, which is the first explanation that I've heard that would explain what I saw last summer without there necessarily being a baseline bug.
If all of this is correct, than the flue is not being double-counted in baseline. The small "however" is that we should probably assume the homes with flues have a higher distribution of baseline infiltration than the homes without them, and we don't. Right now we effectively assume that homes with flues have less infiltration everywhere else. The big "however" is that we should probably be modeling changes in infiltration (I guess in the YAML though that's clunky) when removing a flue, because right now the like walls or whatever are becoming leakier to compensate which seems like probably not how it works in reality. |
From @shorowit and @ekpresent :
Action Item: @afontani Close this issue and open a different issue (referencing this one) around upgrade logic and/or improve infiltration characterization if a home has a flue. |
Currently, ResStock sets infiltration with the Infiltration.tsv, but has other sources of infiltration (i.e. water heater and heating system flues and chimneys). There is a suspicion that the homes we are modeling are much leakier than the ACH values taken from LBNL's Residential Diagnostics Database (the data source behind the Infiltration.tsv). The data from LBNL is based on blower door experiments and would include the flues and chimney leakage area.
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