HVAC in attic

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Trying to understand why the pipe is taped closed. Any idea why this is done ?

That is an auxiliary drain connection.

As the air-conditioning coil is attached to a gas furnace, the drain line is in a negative atmospheric pressure and draws unconditioned air into the equipment. It should be capped off.

The primary and secondary drains are installed incorrectly should be trapped with secondary downward directed into the pan and they tape them trying to prevent air loss which filled traps should accomplish

Texas also requires insulation of the attic exposed condesate drainlines due to excessive heat and moisture in our attic