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High unmet cooling hours for finished basements served by central cooling systems

asked 2020-06-07 17:08:19 -0600

y.tanaka's avatar

updated 2020-06-08 11:58:17 -0600

Using ResStock v2.2.4 with OpenStudio v.2.9.0 and EnergyPlus v9.2

I've found that homes modeled with a central cooling system, versus room-by-room, always results in a high amount of unmet cooling hours for the finished basement zone. I don't understand why that seems to be the case, especially considering the location I'm modeling in is in Northern Michigan.

The following is my results.csv that shows the number of unmet cooling hours for homes with a heated basement, with a centralized AC unit. image description

The following is the same results.csv, but instead, showcasing buildings modeled with a room-by-room AC unit. Notice how the number of unmet cooling hours are much more reasonable. image description

The following is a screenshot for one particular building's unmet hours by zone: notice the condition of the finished basement zone.

image description

Here is the HVAC Load profile for the same building: image description

The above is similar for all buildings with a finished basement. In a location like Northern Michigan, I really don't understand why this seems to be the case. Is there any way to fix this via PAT tool, or the options_lookup.tsv? Is this just a by-product of centralized cooling, where the finished basement will over heat? Why is that so? The same thing will happen if I modeled with an ASHP, or any centralized cooling equipment for that matter.

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answered 2020-06-08 10:09:19 -0600

updated 2020-06-08 11:56:45 -0600

With central cooling or heating, there's only one thermostat and it is in the living space zone and not the basement, so the basement will almost always be colder or hotter than the setpoint. Which direction and how much depend on how the basement supply airflow, as determined under design conditions, compares to the load in any given hour.

There are assumptions in the model about heated basements, like the amount of passive air exchange with living space, internal gains, infiltration, etc., that likely affect the balance between over and under conditioning, but it is difficult to know what the balance should be and how to improve upon the assumptions.

Edited to add: Note that we are in the process of transitioning ResStock to then OpenStudio-HPXML workflow, which uses only one conditioned zone (basements are not separate thermal zone). So this will no longer be an "issue" using that workflow.

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What methods would you suggest to tackle first in correcting for this issue? Is there a way to alter the basement supply airflow using ResStock? And likewise for the other variables you mentioned (passive air exchange, internal gains, infiltration, etc.). Would I have to alter the HPXMLtoOpenStudio\resources ruby files?

y.tanaka's avatar y.tanaka  ( 2020-06-08 10:35:50 -0600 )edit

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Asked: 2020-06-07 17:08:19 -0600

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Last updated: Jun 08 '20