None. Scientific theories don't travel on faith. Faith (as philosophers use the term) is the acceptence of a proposition in the absence of verifiable evidence or logical proof.
Scientific theories are accepted because -- and only as long as -- they are: (a) consistent with all relevant observations; and (b) make verifiable predictions. The history of science indicates that even widely accepted theories are overturned if evidence is found that contradicts the theory. The best-known example is the once universally believed idea of a steady-state universe, which recently gave way to the big bang theory. Today, there are no "steady-staters" clinging to their disproved theory on faith.
So if there are good scientific reasons why a theory should be rejected, it will be. No faith is involved.
Similarly, if one asks why we should have confidence (not faith) in scientific methods, one need only look at the example given by Brian Greene during his recent C-SPAN talk during a book-tour appearance. Greene recounts the story of one of his colleagues who is a expert in some aspect of quantum physics, who performed a 1000+ page-long calulation, based on current scientific theory, of some magnetic property of the electron. Essentially, the theory allowed his colleague to make a prediction accurate to TEN digits of the property.....
.... and then shortly thereafter, researchers doing experimental work actually measured this property of the electron, and lo and behold the predicted AND observed values were in agreement -- RIGHT DOWN TO THE TENTH DIGIT!
So, you are absolutely correct when you say that it's not about "faith" when scientists do science; it's about confidence that derives from extraordinarily accurate predictions made based upon scientific theory and borne out by experimental or observational measurement. It is the confidence one gets from repeated successes, which are only possible because we throw away or revise those theories whose predictions don't comport with reality.