You have a trip in vain situation. It used to be a full refund and an ecert. Not sure now. You are doing nothing wrong and UA if a mechanical issue that causes you to miss your connection should pay for your hotel as an elite. I would be very surprised if you didn't take the flight and stayed in IAH that they wouldn't give you original routing credit at the very least and a refund or some portion of it.

I found this in a search as I have never had a trip in vain but know many who have. Not in the last 2 years however. There is a good precedence for refunds in the past before the merger.

The short answer is you are in title to a involuntary refund of the un-flown portion of your ticket due to a schedule irregularity per the CoC Rule 24(E)(2)(b)
Quote:
In the event a Passenger misses an onward connecting flight on which space is reserved because the Delivering Carrier did not operate its flight due to a Schedule Irregularity or Change in Schedule, the Delivering Carrier is responsible to arrange for carriage of Passenger or to make a refund
and Rule 27(A)