The answer to 'why is my supply line no hot water' is that there could be several common causes, including a malfunctioning water heater, a blockage in the supply line, or a problem with the hot water valve or plumbing fixture. Symptoms may include only cold water coming out of the faucet or fixture, or the water taking a long time to heat up.
Some potential solutions include checking the water heater temperature and thermostat settings, inspecting the supply line for any clogs or damage, and ensuring the hot water valve is fully open. If the issue persists, it's best to call a professional plumber to diagnose and repair the problem, as they have the expertise and tools to properly identify and fix the root cause.