HOU is both HOU and IAH.
That's very strange, and, as far as I've seen, it's not any any of the lists of these codes (
like this list).
Want "proof"?
- Try a ticket from IAH to XXX. The fare is going to be HOU -> XXX.
- Search for HOU to XXX. IAH -> XXX flights will show up.
Contrast this with DAL. DAL does not include DFW.
Edit: This can actually be annoying if one wants to look for flights out of HOU (Hobby) and not HOU (Houston Metro).
From Expedia:
Houston, TX (HOU) – This includes Houston (EFD-Ellington Fld.); Houston (HOU-Hobby); Houston (IAH-Bush Intercontinental); and Houston (SGR-Sugar Land Munc.)