There are two bathtubs with one faucet each.
95% of the time, I get less hot water than cold water when I turn the handle from cold to hot.
Which part should I replace?
Really need a little more info: What types of faucets do your have now?
If this is a tub/shower combination, then most shower faucets have a scald-guard feature. This is to prevent burns by turning the hot faucet too high. If you remove the handle, you'll see a gear-like ring, this is what limits how hot the faucet can run.
If you don't have very hot water all through the house, then you may need to adjust hour water heater to a higher temperature.
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