Posted August 14th, 2012, 11:46 PM
Going back to the OP's question;
IAH-DEN-YVR, you will clear Canadian Immigration and Customs at YVR, although there will be a passport check at DEN when you board (it may also be at IAH when you check in), in order to confirm you have the correct paperwork to enter Canada (which in fact is mostly to confirm you'll have the correct paperwork to return the USA -- Canada won't let you in if you don't have a USA passport which is required to return to the USA). Luggage will be checked through all the way from IAH.
YVR-DEN-IAH; you will [pre] clear USA Immigration and Customs prior to entering the transborder wing [E gates area] of YVR. You will arrive in DEN as if you were a domestic arrival; and your luggage will be checked through.
However, this comment contains some things which need clarification.
Where ever you land first is where you clear. We flew from San Francisco to Vancouver and cleared into Vancouver. On the return trip we cleared US customs back in SF.
The Boston Family landed in Toronto on the way to Vancouver and cleared there, not Vancouver. On the way home they flew via Seattle and cleared there and then flew onto Boston.
Flying BOS-YYZ-YVR of course they would clear on entering the country and before flying a domestic flight; on which there would be no controls when you get off the flight.
Flying YVR-SEA-BOS, they would have pre-cleared USA Immigration/Customs in Vancouver prior to entering the transborder wing's E gates, and then departed out of the E94-96 section of the airportand arrived as a normal USA domestic passenger in SEA (at the N gates for the AC/QK flights and the B2 gate complex for the AS/QX flights).
It's extremely (<1%) unusual for a Vancouver-SFO flight to clear customs at SFO; are you really sure that's what happened-- although the USA does still have the right for a cursory inspection, and still requires pre-cleared flights to fly into an AOE [Airport of Entry] in the USA.
Was it a special charter flight?
Almost all US bound flights clear USA Immigration and Customs prior to entering the transborder [E gates] part of YVR. When you land you are essentially treated as a domestic arrival
Exceptions to this are:
5th freedom flights; PR's YVR-LAS, CX's YVR-JFK
Late departures; UA's redeye flight YVR-ORD
To see if you'll pre-clear in YVR, look up your flight on http://YVR.CA/
if your flight leaves from an E gate, you will have pre-cleared USA Immigration/Customs in Vancouver; and should arrive in the US as if you were a domestic arrival.
Also, I wonder about your note about customs delays at Vancouver. Normally the worst delays are at the [departing] USA pre-clearance. What time of day and day of week did you arrive that it was more than an hour? It's never ever been more than 20 minutes for me (on the Canadian side), and never more than about 90 minutes on the USA pre-clearance side.
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