The answer to 'why is my supply line no hot water' is that there could be several common causes, such as a malfunctioning water heater, a problem with the supply line itself, or an issue with the home's plumbing system. Symptoms may include lukewarm or cold water coming from the faucet, and the solution will depend on the underlying issue.
If the water heater is not properly heating the water, the supply line will not have hot water. This could be due to a broken thermostat, sediment buildup, or a problem with the heating element. Checking the water heater and making any necessary repairs or replacements is often the first step.