We’ve covered a(n) historical already: an before an H word is, essentially, a historical holdover we can do without; but do it if it makes you feel better.
We have, however, safely abandoned the an that used to go before words like eulogy, one, unique and unit.
A reader has asked a related question about abbreviations. Is it an LLB or a LLB? A LCBO outlet or an LCBO outlet?
I think the classical rule was to use whichever form of the indefinite article would be appropriate if the abbreviation were spelled out in full: a LLP for a limited liability partnership..
In speech, however, people naturally began to say things like an MP – and you can do this in writing too, especially where the abbreviated form is more commonly used than the full version.