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After installing the geothermal heatpump, I noticed that certain rooms were staying a lot colder than others.  My HVAC contractor came out and performed an "air side" test which involves taking flow rate measurements at each duct register and determining if the flow rate is correct for the room size.

Turns out that those cold rooms were old achieving about 33% (73 out of 217 cfm) of the air flow they actually needed.  This was because the flex duct in the attic was undersized (6" instead of 10").  I asked them to give me a quote to install the correct size duct and they came back at about $5800.  Some googling told me the parts should be about $600 so I decided to tackle this myself.

The duct and collars came from Home Depot, the duct board from Walmart, and the dampers were from McMaster-Carr.  Once all the parts arrived, I dug into it and spent an entire day in the attic.  Fun stuff!

I didn't want to pay for another air side test afterward, but the air flow is significantly better and the rooms are all comfortable year round.