The lowdown regarding vav control for fancoil units
The primary benefits of this proportional control, closed-loop system are increased comfort and energy savings.
Constant air volume fancoil unit control - Three-speed motors and two-position water valves:
Prior to the development of modulating variable air volume (vav) fcu motor controls, the prime option in changing fan speed had been through two- or three-speed fan motors, via a manual “low-med-high” switch.Recently, the switching process of the fan speeds has been automated. One model offered switches the fan speed on a two- or three-speed motor, from its high speed to its low speed, when the room temperature comes to within 4°F of set temperature.
This offset helps to diminish the overshoot and undershoot (fluctuations) inherent with the two-position (on-off) water valve, installed on most fcu’s. However, unless the required balance point can be reached within the low fan speed, you still have to deal with the differential between the on and off positions of the water solenoid valve and the cycling of the fan from low speed to high speed.
“Timed” and “heat anticipation” functions on two-position controls (chilled water or dx) greatly reduce the overshoot.
Modulating water valves further reduce overshoot and undershoot conditions by varying the amount of chilled water circulating through the fancoil. This further reduces the differential. However, thermal resistance produces a lag time in supplying the required change in conditioning capacity.
Prior to vav-fcu control, the modulating valve proved itself to be the most effective method to achieve an accurate balance point. The disadvantages of this system are that they are overly mechanical (prone to wear out), expensive, and more complicated to install.
Vav-fcu basics
Motor control: Due to a relatively new type of motor speed reduction method, efficient vav control for retrofits or new installations is now available.This new type of microprocessor control slows the motor not by varying frequency, but by lowering the voltage and current to the motor. This allows the motor to consume less energy and run cooler over a continuously variable range of speed.
Note — Some manufacturers do use frequency drives, which lower voltage and raise amperage as they lower the motor speed.
Temperature sensing: Precise temperature sensing, combined with response times of about 1 sec, allow a room temperature change of approximately 1/10th of a degree to be countered by a change in fan speed within 3 or 4 sec. Some electronic temperature transducers also digitally display the temperature of the remote temperature sensor to within 1/10th of a degree.
Remote sensor placement versatility: The temperature sensor now becomes a versatile component of the system, whether it’s a building automation system or stand-alone vav. Primarily it can be a remote component. In many cases the sensor can be placed at the air intake of the fcu or air-handling unit. In other situations, it may be practical to have the sensor placed at a distance from the fcu. Using remote sensing, the control unit or “setting panel” can be placed near a heat source such as a reading lamp on the nightstand in a hotel room, with no adverse effect from the heat generated by the lamp.
Proportional control, or reaching that balance point: With accurate temperature sensing and precise fan motor control in its toolbox, the microprocessor-based proportional control is better able to achieve an accurate balance point.
When the capacity of the fcu and the load are more accurately balanced, the temperature fluctuation in the conditioned zone is further reduced. This also minimizes the amount of energy required to achieve and maintain the required temperature.
Energy savings: Explain the energy-saving aspect in an easy-to-understand way: “This type of system uses the exact amount of energy required to achieve and maintain the required room temperature.”
You also save energy at the fan motor (on non-frequency drives). This is minimal compared to the amount saved at the water chiller-heater.
For those of you who have clients outside of the United States, in countries where it is legal to re-bill tenants for energy used, the following may lead to further energy savings. With the new type of motor control that lowers voltage and current as it slows the fan speed, the input current to the controller is proportional to the fan speed.
Some building managers have taken advantage of the fact that the fan speed of the fancoil unit is also relatively proportional to the amount of energy consumed at the water chiller, and have placed amp meters on the vav-fcu controllers.
Dramatic decreases in the amount of energy consumed at the water chillers have been noted once tenants were individually billed for the amount of energy consumed.
Increased comfort: We all know the irritation of having to tap on a thermostat to compensate for the differential and resulting temperature fluctuation; or the distraction of listening to clicking relays and fans winding up and down to one of two speeds. Proportional controls reduce temperature fluctuations to the point where they are virtually undetectable by human skin.
Noise: Some vav controls have soft-start functions, in which the initial voltage and current are gradually applied to the motor. In addition to saving energy, this method reduces noise and prolongs motor life.
Dehumidification properties: A more constant dehumidification capability is a major selling point in high-humidity areas. A vav system using fan control to vary the conditioning capacity depends less on the water solenoid valve.
Most systems are able to slow the fan to a speed below the lowest of a three-speed fan in order to achieve the balance point. This type of operation allows the water valve to stay open much longer than with conventional systems. Chilled-water circulation and fan operation equal constant dehumidification.
Setback recovery: In high-humidity areas, setback recovery to a comfort level is quicker. Since vav-fcu fans are able to operate at a speed below that of a three-speed fan, the water solenoid valve can remain open longer, in some cases never closing. While this valve is open and the fan is blowing, the room is being dehumidified.
If a key card system, occupancy sensor, or building automation system indicates that the room is unoccupied, the setback temperature can be maintained while dehumidification is ongoing. When the room becomes occupied, the temperature level is reached quickly while the humidity rate is already at comfortable level.
“Over-efficient” water chiller in a high-humidity area: Hopefully you’ve never experienced this first hand, but what happens when a water chiller is running too cold in a high-humidity area, connected to a cav-fcu and two-position valve control?
The temperature reaches the top of the differential, then the valve opens. The water is so cold that the humid room reaches a bone-chilling humid cold before it has a chance to be dehumidified. When it reaches the bottom of the differential, the valve closes and the room returns to hot and humid in short order.
Proportional vav controls take into account the chilled-water temperature as one of the controlled variables, when striving for that balance point.
Vav and zone control via local or building automation control: Vav-fcu’s, for the most part, have a single temperature sensor for each fancoil unit. When dealing with more than one fcu conditioning a common area or different zones, all vav-fcu controls can be set (via local control or building automation) to maintain the same temperature. This makes movement between areas controlled by different fcu’s more comfortable due to all zones being the same temperature.
To save energy, areas that aren’t being used can have their fcu’s shut off or set to a fall-back temperature. Some situations may call for remote control and monitoring, such as in a hospital or classroom.
This can be done with certain stand-alone systems, or more complex building automation systems.
Links
