I live in Minneapolis where the temp is -15 degrees right now. All our faucets work great (including the upstairs vanity sink) except our upstairs bathtub faucet suddenly doesn't have pressure - only a trickle of hot water comes out and it didn't improve by leaving it on for 30 minutes.

Is this a frozen pipe or some other issue? Water coming out is hot so wouldn't it have melted a frozen pipe?

Thanks,
Steve

If you are getting a trickle of hot water for 30 minutes its not frozen. I think junk broke loose and is clogging your mixing valve. I am assuming this is a one handle valve, right?