expressions - Should one use "an" or an "a" before an acronym beginning with the letter H?

Consider the sentence:

He offered to be an HSC.

vs:

He offered to be a HSC.

In the example above, HSC stands for Health Service Consultant. If one were saying the sentence aloud, one would say an HSC because an obviously proceeds the spoken letter H, but the H actually represents the word Health, at which point the sentence should perhaps read a HSC because one would obviously say a Health Service Consultant. If one considers what the letter H actually stands for, in a strange way, both versions could perhaps appear to be incorrect depending on whether the acronym is viewed by the reader solely as a contraction or as its real meaning.

I would surmise that an HSC would be the more correct grammar, but I'd like opinion.