For optimum heat exchange and occupant comfort while remaining energy efficient, Fan Coils require good control of air volumes and water flow rates. However, manufacturers of FCUs often only publish flow requirements for maximum load conditions. What is unknown is the optimum water flow when air volumes are adjusted by the occupant or the DDC controller. Also, when using non-PI valves, differential pressure fluctuation in the hydronic system will impact the flow-rate through the fan coil and therefore the thermal energy supplied to the room.

Reciprocal influence with several consumers is eliminated due to the dynamic balancing. Dynamic balancing is carried out automatically at each operating point. Changes in differential pressure cause minimal changes in the flow rate.

A FCU is commissioned to the following values, design water flow is set to 0.11 l/s (0.029 GPM), with 23 °C (73 °F) air entering the unit. As the system is dynamic after a short period of time, the pressure in the system increases due to changes in other locations of the hyrdonic system. In this specific FCU, the increase in pressure has caused the flow to increase which results in overflow of the FCU, consequently lowering the delta T and lowering the efficiency of heat transfer.

As a result an occupant of the space may have found the air flow bothersome and manually set the fan speed at low, as space temperatures rise the valve is commanded further open by the controller until the air is simply unable to remove any more energy from the heat exchanger. This results in the return water temperature being cooler than expected. With the water flowing too quickly for the air flow we have a classic recipe for low delta T, this is effecting not only this unit, but is reducing the capacity of the central plant, and the overflow is significant point of consumption for the pumps with our flow now at 0.14 l/s (0.037 GPM).

    

We have the same FCU working as expected; design water flow is set to 0.11 l/s +/10% [0.029 GPM], with 23 °C [73 °F] air entering. Even if the pressure rises from 2 bar to 3 bar, the flow rate hardly increases. Flow rate inaccuracy is vastly improved offering some plant savings, but not as accurate as an electronic valve. User fan speed adjustments still effect the optimum heat exchange which will result in low delta T and poor off coil conditions.

There is the same FCU working as expected, design water flow is set to 0.11 l/s [0.029 GPM], with 23 °C [73 °F] air entering. Again the system delivers a pressure increase due to changes in other locations. The electronic flow sensor in the EV detects the flow increase before the room sensor can register an increase in space temperature and allows the valve to close slightly to avoid reaching the saturation zone.

If the FCU has been held on low fan speed again. As space temperatures rise the valve is commanded further open by the controller which would have resulted in the air being unable to remove any more energy from the heat exchanger. The EV‘s fixed Delta T Manager recognizes the diminishing delta T and overrides the control signal and does not allow the valve to open further, this eliminates the over flow condition.

The EV continually measures the flow and calculates if it needs to compensate or not, based on the control signal input as well on the Delta T Manager set point. The Delta T Manager has slowed the water down to ensure the heat exchange is optimal for the air volume set by the user. This saves pump energy and keeps the system permanently optimized.

In the age of easily controlled EC/DC fans, variable air volume FCUs represents one of the most energy efficient methods of air conditioning a space. However, the control of fan speed vs valve position is a matter of debate.The energy valve when configured for power control provides a completely linear response, meaning fan speeds can be directly tied to the valve output, simplifying the controls process dramatically.