The answer to 'water heater installation won't flush' is that a clogged drain line or sediment buildup in the tank is the most common cause. This can prevent the water heater from draining properly during a flush or maintenance cycle. Symptoms include slow or no drainage, standing water around the tank, and difficulty flushing the system.
To troubleshoot, first check the drain valve and drain line for any blockages. Use a plumber's snake or auger to clear the line. If the issue persists, you may need to flush the tank to remove sediment buildup. This is a job best left to a professional plumber, as it requires draining the full tank and can be dangerous if not done correctly.