Drain clams, reserving up to 2 cups of clam juice. If needed, add enough water to clam juice to make 2 cups.
Add diced bacon to the Instant Pot. Using the display panel select the SAUTE function.
Cook, stirring occasionally, until the fat has rendered, but the bacon is not crispy.
Add onion, butter, thyme and celery seed or celery. Deglaze by using a wooden spoon to scrape the brown bits from the bottom of the pot, then saute until onion is soft, 3-4 minutes.
Add garlic, salt and pepper and cook for 1-2 minutes more.
Add clam juice and potatoes and stir to combine.
Turn the pot off by selecting CANCEL, then secure the lid, making sure the vent is closed.
Using the display panel select the MANUAL function*. Use the +/- keys and program the Instant Pot for 4 minutes.
When the time is up, let the pressure naturally release for 4 minutes, then quick-release the remaining pressure.
Using a slotted spoon, remove about half the potatoes to a bowl and set aside. Remove thyme sprigs and discard.
Use an immersion blender to process soup mixture until smooth. Return reserved potatoes to the pot along with the clams and half and half.
Cook and stir on the WARM function until chowder is heated through. (Switch to the SAUTE function if needed, but do not boil.)
Garnish with snipped chives
Notes
*The MANUAL and PRESSURE COOK buttons are interchangeable.