HVAC control valves
HVAC control valves have a substantial effect on system efficiency. In a perfect world, control valves would never “hunt” or “dither” (continually open and close trying to find the correct position), they would not leak when closed and they would not restrict flow when fully open. Alas, in the real world, valve controls are difficult to “tune” for stable control unless they are relatively small and the smaller the valve, the greater the pump pressure/energy it will take to move adequate water at full building load. Real world valves also leak water even when they are “closed” if the pump pressure is higher than the valve close-off pressure or if the valve seal fails. Three-way valves allow the same water flow no matter what the building load so the pumps always run at full flow and waste energy. Poorly selected and poorly maintained valves will cause excessive water flow (indicated by a low temperature differential) along with simultaneous heating and cooling.
Here are some valve characteristics to consider when selecting a control valve:
“Lift & Lay” valves use a shaft to push or pull a stopper against a sealing surface. This type of valve works well with pneumatic actuators and the valves are traditionally sized to drop as much pressure as the coil they serve, when fully open. These valves are fairly linear in their position to flow characteristics so they are appropriate for a pneumatic system to control. The valve bodies are relatively large compared to the amount of water they can pass. In a “normally closed” valve, a spring pushes the valve closed while an actuator forces the valve open. Too much pump pressure can push a “normally closed” valve open. Seals must be serviced periodically to prevent leakage. The push pull stem bushings also tend to leak. This is not a cost effective valve to use with a modern DDC control system using electric actuators.
Rotating valves are generally best for electric actuators and come in four typical configurations: ball, characterized ball, shoe and butterfly.
Ball valves are appropriate in sizes from ¼” to 2” full port. They provide tight reliable shutoff (if the water is free of particulates) and they easily pass a lot of water flow for the body size. These valves are not very linear in their position to flow characteristics so it takes DDC controls, an electric actuator and careful tuning to make them control well.
Characterized Ball valves are a little more expensive than traditional ball valves and come in the same size range. They provide tight reliable shutoff (if the water is free of particulates) and they easily pass a lot of water flow for the body size. These valves are very linear in their position to flow characteristics so it does not take much tuning to achieve stable control.
Shoe valves come in sizes up to 6” and can be used for 2-way and 3-way applications. They are linear and easy to control but they do not provide tight shutoff. This is the appropriate valve to use from 2 ½” to 6” if good control characteristics are desired and tight shutoff is not required. They are more expensive than butterfly valves in 2-way applications but are a more economical choice for 3-way applications.
Butterfly valves are affordable for 2 ½” and larger applications. They are not as linear as shoe valves or characterized ball valves but they do OK in the middle control range. They do not provide tight shutoff unless a “bubble-tight” valve is selected and a bubble-tight valve requires a lot of torque from the actuator to get it out of the seat. It takes DDC controls and an electric actuator to control these valves well at low flow.
If DDC controls and electric actuators are used, size rotating valves to provide a full open pressure drop of about 25% of the pressure drop through the coil at full design flow. Large butterfly valves are often selected at line size.
Steam Valves have always been difficult to control and provide stable temperature control. Some facilities have used two valves of 1/3rd and 2/3rd capacity with better results than one valve but still not great control. The result has been excessive simultaneous heating and cooling. It is now possible to purchase steam valves with 100:1 turndown ratios. The payback can be much less than 1 year.
Pressure Independent Flow Control valves are expensive but can be cost effective if a facility is striving for a high temperature differential operation.