The use of "an" as the indefinite article before a noun starting with "H" is perfectly correct usage (if a trifle archaic).
Apparently the use dates back to a pronounciation change in English from the "French" silent H as in hour, honour, heir etc. to the harder aspirated "Germanic" H as in house, hotel and hair etc.
In short,'an' is technically acceptible (but not generally used) with all such nouns while 'a' is only acceptible for those with the 'hard' H. As yes, I do still use an hundred, an historian although strangely, a hotel.

Walrus.