Are you certain? First time I heard of a "U" series i5 that runs fanless. In fact, I'm convinced they do not. The latest Surface Pro is lauded for having a fanless i5 Kaby Lake - and that is a "Y" series 7W CPU. MacBook also has a Y series i5.
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If it's a U i5, it runs fans, always. If it's a Y chip, it doesn't have fans and you're using a 12" MacBook. It really is that simple.
Your computer is 41-45 C and the fans are working (silently) to keep it that way. I understand that you think they don't work because they are almost completely silent. But even on my 15", I can't hear them at 2000rpm, even if I put my ear to the keyboard. Apple makes silent fans - but if you have a MacBook Pro they are always working.
My 15" is also constantly between 40-45 C while working with non-demanding stuff. And I can't hear the fans, no matter how hard I try. They are working though. It's how these CPUs are built. Unless you have a 7W CPU, the fans are running. If you have a MacBook Pro, any MacBook Pro - you don't have a 7W CPU. Even the nTB model with a 15W CPU uses a fan all the time it's not sleeping or off, not to mention a TB 28W model.