The answer to 'supply line no hot water' is that the most common cause is a problem with the water heater or the supply line connecting it to the faucet. This could be due to a broken or malfunctioning water heater, a clogged or damaged supply line, or an issue with the shutoff valve. Symptoms may include only cold water coming from the faucet and no hot water at all.
To troubleshoot, you can first check the water heater to ensure it is powered on and the thermostat is set to the proper temperature. If the water heater is working, inspect the supply line for any visible damage, blockages, or leaks. If the line appears fine, the issue may be with the shutoff valve or a problem deeper in the plumbing system that requires a professional inspection.