A double regulating valve, also referred to as a balancing valve, is used in plumbing to control water temperature. This type of valve is widely used in showers.
Double regulating valves are pressure sensitive. They react to pressure changes from the hot and cold water flowing into them. This causes the valve to either open or close a port, thus maintaining pressure balance. Balancing this pressure between the two water supplies keeps the temperature consistent.
When there is a sudden change in water, such as the change caused by flushing a toilet, a balance valve keeps the shower temperatures constant. This means that there will not be a sudden cessation in the flow of cold water when a toilet flushes.
Most new construction is equipped with balancing valves. Replacing an outdated valve in an existing house may require the help of a professional plumber.