That's not how I read the 12th Amendment. It provides for a special voting mechanism in the event that there is not electoral majority, but says nothing about the Speaker being named President.
The Electors shall meet in their respective states and vote by ballot for President and Vice-President, one of whom, at least, shall not be an inhabitant of the same state with themselves; they shall name in their ballots the person voted for as President, and in distinct ballots the person voted for as Vice-President, and they shall make distinct lists of all persons voted for as President, and of all persons voted for as Vice-President, and of the number of votes for each, which lists they shall sign and certify, and transmit sealed to the seat of the government of the United States, directed to the President of the Senate. The President of the Senate shall, in the presence of the Senate and House of Representatives, open all the certificates and the votes shall then be counted.
A contingent election is the procedure used in presidential elections in the case where no candidate wins an absolute majority of votes in the Electoral College, the constitutional mechanism for electing the president and the vice president of the United States. A contingent election for the president is decided by a vote of the United States House of Representatives, and the contingent election for the vice president is decided by a vote of the United States Senate. The contingent election procedure, along with the other parts of the presidential election process, was first established in Article Two, Section 1, Clause 3 of the United States Constitution, and then modified by the 12th Amendment in 1804.
Pursuant to the 12th Amendment, the House of Representatives is required to go into session immediately after the counting of the electoral votes to vote for president if no candidate for the office receives a majority of the electoral votes. In this event, the House is limited to choosing from among the three candidates who received the most electoral votes. Each state delegation votes en bloc, with each state having a single vote. A candidate is required to receive an absolute majority of state delegation votes (currently 26 votes) in order for that candidate to become the president-elect. The District of Columbia, which is not a state, does not receive a vote. The House continues balloting until it elects a president.