What are DRV's and when do we use them?

A double regulating valve with a primary measuring element for determining and adjusting hydronic fluid flow in an HVAC system. an effective means of ensuring occupant comfort in buildings incorporating hydronic heating or cooling systems. Correct HVAC system operation requires that a specified minimum flow rate of heated or chilled fluid be delivered to every part of the building. Accurate circuit balancing assures this "design flow" is available to each terminal unit throughout the system, for all anticipated operating conditions.

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.