Fault Detection: When Fan Coils Fight

Managing a building with hundreds of Fan Coil Units (FCU) is not for the faint of heart. Especially when temperatures rise or plummet, the effectiveness of your maintenance and management of these systems is really put to the test. Problems get exposed and people get uncomfortable!

However, data analysis and open protocols, such as BACnet, have changed the game. The operation of FCUs can be analysed efficiently at scale using automated tools. Rather than waiting for seasonal conditions and comfort complaints to draw attention to problems, data analysis will help you to flush out and resolve issues in a manageable process that reduces maintenance and energy costs, whilst improving comfort.

What is FCU fighting?

“Fighting” can be best described as when closely located units, i.e. with supply ducts within 5-6m of each other, are doing opposing actions. For example, one unit may be supplying air at its maximum supply temperature, whereas its closely located neighbour is supplying air at its minimum temperature. The net effect might be acceptable comfort levels, but at the cost of a lot of wasted energy.

Fan Coil fighting normally occurs in large open plan areas that are served by multiple units. The open plan area may be one large temperature controlled zone or it may contain multiple smaller zones each with its own setpoint, either way fighting can still occur.

The only way to effectively identify a potential fighting issue is when you have both a spatial context and the relevant temperature information for the zones and units.

What causes FCU Fighting?

One scenario occurs when two closely located zones, in an open area, have slightly different setpoints or the same setpoints but with narrow deadbands between heating and cooling. Spill over of heat or cooling from neighbouring zones causes the FCUs to respond by polarising their cooling and heating outputs. As can be seen in the chart below, some units are supplying at over 30°C and others at 15°C.

Fan Coil Unit Chart

Another scenario that we have identified concerns the control strategy of Slave units, where the slave adopts the valve position of the Master unit. As opposed to adopting the calculated supply air temperature setpoint from the Master, thus allowing it to modulate its valves accordingly and independently. Various reasons can cause supply air temperature to diverge even when valve positions are synced, such as water and air flow into each unit or a mechanical issue can cause one unit to behave differently.

In the chart below the valve profiles are perfectly aligned for all units in one large zone, whereas there is a vast difference between supply air temperatures.

Fan Coil Unit Chart 2

Can't my BMS solve this?

In short, no. In fact in the examples above the control strategy is causing the issue. Manual scrutiny of your HVAC data could help you to identify issues, but when you have hundreds of FCUs the task becomes infeasible. Continuous monitoring and data analytics is the most efficient and cost effective way to improve the operation of your building and reduce its carbon footprint.

Client Feedback

"Thanks to OPNBuildings the HVAC upgrade project for €1.1M was a success. Throughout the construction stage snags were easily identified, trouble shooted and addressed. On handover there was confidence the building was achieving air quality and thermal comfort and the necessary tools were in place to support the continuous improvement in building operation with fault detection and easy to use dashboards. OPNBuildings was the most important quality driver in this project and represented excellent value for money."

- Robert Ryan Facilities Project Manager at Ervia