The answer to 'backflow preventer frozen' is that a frozen backflow preventer is a common issue during cold weather. This can happen when the water inside the backflow preventer freezes, causing the device to malfunction. Symptoms of a frozen backflow preventer include no water flow, leaks, or the device being stuck in the open or closed position.
To fix a frozen backflow preventer, you'll first need to thaw it out. This can be done by applying heat, such as with a space heater or heat lamp, directed at the backflow preventer. You can also wrap it in insulation or heat tape to help it gradually warm up. Once thawed, check the device for any damage and make sure it's working properly before turning the water back on. If the backflow preventer is damaged or continues to freeze, it's best to call a professional plumber to inspect and replace it.