Just to clear up what "refundable" means ...
If you pay more taxes than you owe, via withholding or estimates payments, you get a refund of your overpayment.
Some tax credits are called "refundable" because you can get back money that you did not pay in. For example, the Child Tax Credit or the Earned Income Tax Credit.
Other credits are not refundable. You cannot get back more than you put in.
The ACA subsidy in non-refundable. If your premium subsidy was too small all year and then you reconcile at tax time, you can be reimbursed for your insurance premium
that you overpaid. But you will not have that reduced to less than zero and get back
more than what you paid in premiums.
https://www.irs.gov/pub/irs-pdf/f8962.pdf
Column A (Annual Enrollment Premiums) is compared with Column D (Annual Maximum Premium Assistance) to come up with Column E (Annual Premium Tax Credit Allowed - smaller of Column A or D). So your Annual Premium Tax Credit is limited to the Annual Premium Amount.
I hope I've explained that well enough, if not chime in and clear it up.
Nice to see this thread is back. It was missing late yesterday.