The answer to 'backflow preventer no hot water' is that a malfunctioning backflow preventer can prevent hot water from reaching your fixtures. This is because the backflow preventer is designed to prevent contaminated water from flowing back into the clean water supply, but if it's not working properly, it can restrict the flow of hot water.
Some common causes of this issue include a buildup of sediment or mineral deposits in the backflow preventer, a broken or stuck check valve, or a problem with the relief valve. Symptoms may include no hot water at all, or very low water pressure and flow. In some cases, you may also notice water leaks around the backflow preventer.