All about actuators

By Bob “Hot Rod” Rohr


We are surrounded by actuators in our everyday life. In your home, the trip lever on your toilet is one of the simplest examples, a hand-operated lever. In your vehicle, you may have power seat adjustment carried out by an actuator. Actuators are everywhere in factories with machine tools and robotics. This article discusses actuators that are used in hydronic and HVAC applications.

A definition may be as simple as: The actuator is a device which causes a valve or damper to operate. We hear the word actuator most commonly used to describe the electro-mechanical device found on hydronic manifolds. This thermal type of actuator receives 24Vac power, which in turn moves a valve mechanism. Inside this type of actuator is a “heat motor” mechanism. This motor is comprised of a wax substance that expands and contracts as electrical current is applied or removed.

Of course, I had to cut one of these copper capsules open. Inside I found a mixture of wax and copper particles for quick and accurate heat transfer. The actuators job is to enable flow through a piping circuit. It takes its orders from a room thermostat, for example. A thermal zone actuator is the term used to describe this type of actuator.

Another common use for an actuator is to operate a spring return type of zone valve, a ZV. In this application, when power is applied, a small synchros motor rotates to open a flapper and allow flow. Spring tension closes the valve when power is removed. This is considered normally closed, or an N.C., as it takes power to actuate or open.

Keep in mind these spring return valve actuators are also available in a normally open style, or an N.O. This would indicate that the valve requires power to shut off or close. Some engineers and designers prefer N.O. valves as they will be open in the event of a power loss in the building. This could help prevent a freeze up or allow some thermosiphoning of heat energy in a no-power condition.

Also, you’ll find them on wood-fired boilers to allow for an over-heat zone to open in the event of a power outage. The actuator is just one part of an assembly. Both the actuator and the valve or component it connects to can be engineered from a very basic to a more precise component with near perfect control and authority. If you are actually assigned or accustomed to acquiring all available arrangements of automatic actuators, adopt an attitude of authority andapply your aptitude.

Worth noting: In HVAC forced air systems an actuator can be attached to a damper in the ducting. This would allow for the system to be “zoned” and more closely regulated to individual space or room loads requirements. You may have come across HVAC systems that use pneumatic actuators. Several HVAC manufacturers supply this type of control. Clean, dry, oil-free air pressure supplied by small diameter copper or plastic tube is required to activate pneumatic valves and dampers.

Actuators can be attached to many ball-type valves. This turns a manually-operated valve into a valve that can be automatically operated. Look for mounting tabs on the valve body to connect the actuator. Generally speaking, a ball valve actuator will require power to open and close. There are manifolds that use linear type actuators. A similar type actuator is used on vending machines to drop your selection. This is a cool and unique approach to zone valve operation.


Simple but precise

Thermostatic radiator valves, TRVs, are another common example of an actuator activated valve. This is an actuator that does not require electricity or compressed air to function. A wax capsule similar to the thermal zone actuator allows the ambient air temperature surrounding the valve to operate. This is a fairly simple but also precise way to control flow.

TRVs are basically non-electric, modulating control devices, thermal actuators, if you will. They are also a proportional modulating valve. As the ambient temperature around the valve changes, so does the opening passage of the valve. It is a great valve for retrofitting, as it does not require wiring to the various valve locations. I feel it is an under-utilized product in the North American market. In most cases the actuator is a separate component on a valve, easily removed for service or replacement.


There are many types and brands of actuators available, be sure to consult with the manufacturer when you shop for actuators. Specifications such as voltage, operating time, stroke length, torque rating, and type of enclosure are some of the considerations.


Intelligent and sophisticated

Over the years, we have learned to build some intelligence into controls that drive actuators. These controls make it possible to modulate valve opening based on a signal from the control that give you more precise control of the piping or damper.

Taking the actuator yet another step forward, actuators can go well beyond the common on/off function. Products exist that can enable the flow, modulate the flow, have balancing characteristics, a bypass, purge and a strainer protection in one component. These are sometimes sold as coil kits. They can be used for quick and easy installation of heat and cool fan coils. These kits are sometimes supplied by the fan coil manufacturer. They are sized and configured for an engineered design application and are common on multiunit projects.