The answer to 'why is my backflow preventer no hot water' is that a malfunctioning backflow preventer can block hot water from reaching your fixtures. This is because the backflow preventer is designed to prevent contaminated water from flowing back into your clean water supply. If the backflow preventer is not working properly, it can restrict the flow of hot water through the system.
Some common causes of this issue include a stuck or broken backflow preventer valve, a buildup of sediment or debris, or a problem with the water heater itself. Symptoms may include no hot water at all or reduced hot water flow and pressure. To solve the problem, you may need to clean, repair, or replace the backflow preventer, or address any issues with the water heater.