It's not as simple as that. In general English usage, when we have an acronym, we tend to look at whether it sounds like it starts with a vowel. So for instance

It's not as simple as that. In general English usage, when we have an acronym, we tend to look at whether it sounds like it starts with a vowel. So for instance the acronym RTFM is pronounced "arr tee eff em", which starts with a vowel. Which brings us back to this one - both "a" and "an" are valid, because it depends whether you're pronouncing your "h" as an Australian "Haich" or a British/American "Aich".