My work jargon includes a lot of acronyms, which sometimes causes me some internal angst when I have to write documents that include these acronyms (which is often).

How do you folks assign indefinite articles to acronyms? In English, "a" is typically used before a subject that begins with a consonant:

Example: a cat.

And "an" is typically used before a subject that begins with a vowel:

Example: an owl.

We just like having consonants separate our vowels, I suppose. But in writing, is it more important to pay attention to the letters in an acronym or the sound that is associated with pronouncing those acronyms?

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For example, HEPA is an acronym for High-Efficiency Particulate Air filter. It's pronounced the way it looks: heh-pah. However, HVAC is an acronym for Heating, Ventilating and Air Conditioning systems, and it's pronounced like this: aich-vack.

So even though both of these acronyms begin with the letter "H," they're pronounced with a consonant and vowel sound, respectively. So should I write "a HEPA" and "an HVAC"? Should both of them be "a HEPA" and "a HVAC"?

Help!