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1 March 2009

Masterclass: Fan Coil Units Part 39

IN LAST month’s article we looked at the need to specify various elements of a fan coil unit based system together with the precautions and considerations the specifier, contractor or manufacturer must bear in mind if a successful installation is to be achieved. We now continue in more detail with these important criteria in mind in our coverage of this increasingly popular and important approach to comfort conditioning.

Mike Creamer.

Airside control fan coil units

Airside control fan coil units have been available for many years offering an alternate control method to the more traditional waterside control fan coil unit.

Although utilising this alternate method of controlling the units' cooling or heating output to the space or zone to be conditioned, this type of unit is available in similar model types, ranges and capacities as its waterside counterpart. These could include vertical or horizontal orientated units and either basic chassis or cased models as required (the latter for vertical units only). This also applies to the range of application variations and accessory options too.

In basic terms, the difference in control equipment is that the airside unit typically utilises an actuator motor coupled to either a gear drive assembly or a linkage arrangement. These in turn move damper equipment that in turn controls the path of the air to initiate heating or cooling as opposed to the valves that are used on a waterside control machine.

The following stages describe and illustrate the cooling, bypass (ventilation only) and heating operations of airside control more clearly (see fig 1):

Heating mode:

Airside control fan coil units - cooling damper closed, heating damper in the open/control mode. This means the air being re-circulated is passing predominantly over either the heating coil or through the bypass channel depending on the amount of heating required.

Bypass mode:

Airside control fan coil units - cooling damper closed, heating damper closed. All the air being re-circulated is passing through the bypass channel and is not affected by the cooling or heating coils at all.

Cooling mode:

Airside control fan coil units - cooling damper open/control mode, heating damper closed. This means the air being re-circulated is passing over the cooling coil or through the bypass channel depending on the amount of cooling required.

This is of course a very simplistic illustration. In point of fact, with the controls equipment available in today's marketplace, the control of the air to the space is much more responsive to the demands made. They are equally subtle in providing the appropriate air treatment by virtue of the fact that the mode dampers modulate, to allow mixing to take place within the unit, and thereby more closely match specific demand and supply.

Airside/waterside control fan coil units
The following stages describe and illustrate the cooling, bypass (ventilation only) and heating operations of waterside control more clearly (see fig 2):

Fig 1 Heating Dampers


Heating mode:

Waterside - cooling valve in bypass mode, heating valve in the open/control mode thus allowing low-pressure hot water to pass through the heating coil and affect heating.

Bypass mode:

Waterside - both cooling and heating valves in their bypass positions. Therefore neither coil is in circulation.

Cooling mode:

Waterside - cooling valve in open/control mode thereby allowing chilled water to pass through the cooling coil and affect cooling. Heating valve in bypass mode.

Comparison of advantages and disadvantages

When compared to its waterside counterpart, airside control fan coil equipment is claimed to be more efficient. This is due to the airside control fan coil equipment being more effective in its thermal control operation because the unit maintains the space condition by mixing quantities of heated or cooled air with a proportion of re-circulated air. As stated earlier, the controls can respond very quickly and accurately to changes in space temperature.

Waterside control units, on the other hand, are less responsive due to the fact that they rely on the control-to-valve response time. The unit output not only being related to the temperature of the space but also the condition and rate of the chilled water subsequently being supplied (following valve initiation), thereby, possibly operating with a certain degree of lag. For instance, when a waterside unit changes over from heating to cooling, the unit has to dissipate all the heat remaining in the heating coil before it can commence the cooling operation (and vice versa for the changeover from cooling to heating).

Airside control fan coil equipment is also potentially easier to install and commission. With this type of system the water flow is allowed to circulate relatively unrestricted with the airside mechanism, factory pre set, operating and modulating to maintain the space temperature. This is contrary to the waterside system that requires proper and accurate balancing in order for the 4-port valves to control the system correctly.

Airside control systems shouldn't suffer from a water-related problem. Waterside control units utilise a valve to control the flow of water through a small clearance section within the valve. This section can be prone to blockage or sticking, when the water system concerned has not been thoroughly and frequently maintained, thereby rendering the system or unit out of design parameters.

The airside control system has no requirement for 4-port valves. This means that the overall system has a reduced resistance requirement, which in turn gives savings on the pump or pumps due to a lower duty being necessary.

It is simple to use, service and maintain and can be manually set up should the electrically operated controls fail.

Airside control systems have, of late, been the subject of various in-depth investigations and reviews with regard to net performance. Prospective users of this type of system must ensure that they have been soundly advised on the available capacity within the space to be conditioned.
However, Airside control systems can, in some instances, be unsuitable in certain applications due to physical space restrictions.

Unfortunately, because of the area required to occupy and operate the damper mechanism, the unit can be larger than usual, particularly with the vertical style unit, and this may prohibit its use for instance under a particular sill height.


Fig 2 Waterside Controls




Like all forms of heating, ventilation and air-conditioning equipment used in today's building services environment, the airside control fan coil unit has its place for applications within the range of solutions for heating and cooling problems.

As can be seen from the above descriptive passages, this type of equipment can offer benefits in both the initial capital cost of design, application, installation and the ongoing expenditure relative to efficiencies in some areas.

However, it can lose out in not being able to fit some installation situations and the efficiency can be called into question if the initial equipment design hasn't been 100% or the net output hasn't been clearly stated and taken into consideration.


With thanks to Mike Creamer of Business Edge who revisits his Masterclass series of articles, updating and adding to the information which proved so useful to readers when the series was first published over ten years ago. In this reincarnation, the series will cover both air conditioning and refrigeration and serve as an on-going source of technical reference for experienced personnel as well as providing a solid educational grounding for newcomers to our industry.



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