Air-and-Water Systems (Fan Coil Units)

 Fan coil systems use terminal cabinets in each room serviced by 2, 3, or 4 pipes approximately 11/2" each in diameter. A fan blows air over the coils which are serviced by hot or chilled water or air. Each fan coil cabinet can be individually controlled. Four-pipe fan coils can provide both heating and cooling all year long. Most piping is steel. Non-cabinet units may be concealed in closets or custom cabinetry, such as benches, can be built. Air-water systems use the beneficial features from all air and all water systems. The energy is carried in the water and air is used, usually at high velocity, for ventilation.

SYSTEM DEFINITION

"An organized set of related components working together to produce a desired result"

The diagram below shows various configurations of fan and coil systems working together to cool and/or heat a space.

Fan Coils Control Schemes

2-pipe Auto changeover

A SPDT Aquastat™ is strapped to the supply pipe, and based on whether the water is hot or cold, selects the thermostat action. www.honeywell.ca/.../systems/ FanCoilsControlScheme.html source

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4-Pipe

Fan coil unit has two coils, one heating and one cooling. The user selects either the heating or cooling action. www.honeywell.ca/.../systems/ FanCoilsControlScheme.html source

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Fan Control

2-Pipe Constant Fan

Fan may be cycled with the valve as long as the system switch is not off. Two or three speed fan speed selection is common.

  www.honeywell.ca/.../systems/ FanCoilsControlScheme.html source

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1. Central equipment may be sized smaller by taking advantage of building heating and cooling diversity.

2. The system requires only piping installation which takes up less space than all-air duct systems.

3. It is usually easier to install wire and water pipes than ducts making this a good choice for retrofit applications.

4. Unoccupied areas of the building may be isolated and shut down, saving money.

5. Zones can be individually controlled.

6. The system can accommodate up to 100% outside air capability.

tristate.apogee.net/ cool/csfa.htm source

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Digital Thermostat

2-pipe fan coils have a single heating/cooling coil, and supply water is changed from hot to chilled on a seasonal basis. The user manually select heating or cooling action at the thermostat.

www.honeywell.ca/.../systems/ FanCoilsControlScheme.html source

Fan Speed Selection

Fan coil units usually allow the user to select two or three fan speeds for maximum comfort.

Since most fan coils are sized to the cooling load, and oversized for heating, the fan speed should always be left on low for heating, except when warming up a room from a low temperature.

Fans may be cycled with (or instead of) the valve, or run continuously when the System switch is not off. Some fan coil thermostats allow selection of this action by the user.

www.honeywell.ca/.../systems/ FanCoilsControlScheme.html source

 Standard industry uses are office buildings, hospitals, hotels, and high-rise structures due to the space savings of the system.

Picture source:www.trane.com/commercial/ library/vol31_1/fig5.asp

The Air Portion of System: source

The air-side of the system uses central air components such as air distribution duct and room terminal. The air supplied is constant volume (primary air). Primary air provides the necessary outside (fresh) air for ventilation. The primary air is pulled in through the building envelope by a large primary fan at velocities between 500-800 ft/min. The primary air for this system is usually supplied at high velocity (2500-3000 ft/min in risers and 1500-2000 ft/min in headers), which requires rigid spiral ducting and welded seams to eliminate leakage. When in cooling mode the primary air is dehumidified, to provide comfort and prevent condensation, by a central conditioning unit. In the winter, heating mode, the air is humidified, by the central conditioning unit, to limit dryness. Only a small percentage of return air is sometimes mixed with the primary air so in freezing conditions a preheater is necessary. Also filters should be higher efficiency due to the constant intake of outside air for the primary air supply.

The Water Portion of the System:source

The water-side consists of a pump and piping to supply water to heat transfer devices. The heat exchange surface (coil) may be integral with the air terminal (induction units) or separate (fan coil units). In some applications radiant heat panels can be used and in applications where there are large non-perimeter spaces water loop heat pumps can be used to transfer heat from the core of a building to the perimeter. These are less common. The water used can be chilled by direct refrigeration, by using chilled water from a primary cooling system, or by heat transfer through a water-to-water exchanger. Chillers usually supply chilled water anywhere from 35-48 degrees F. The temperature in each room is controlled by either the flow of water through the coil or by the amount of air passing over the coil. There can be one coil per conditioned area which is converted to heating or cooling depending on the season or two coils per conditioned space can be used to provide either heating or cooling at all times. In the winter, the heating capacity of the coil in a conditioned space must be great enough to heat the space and offset the cool primary air, which is provided.

1. Water has greater specific heat and density compared to air so the cross-sectional area of water distribution pipes is less than that of air distribution ductwork.

2. The amount of air supplied can be much lower and less space is needed.

3. Then return air system can be eliminated since the air supplied only has to meet outside air requirements plus any air exhausted.

4. The power required to pump water through the building is usually less than the fan power needed for supply air and return air systems.

5. Operating cost will be less.

6. The versatility of the system is high and matches that of versatile all air systems. Positive ventilation, central dehumidification, winter humidification, and good temperature control over a number of control zones are capable with the system.

            1.) Very good control is available over many zones.

            2.) Heating and Cooling can be available for all zones allowing variation of loads.

            3.) Space needed for distribution system is minimal.

            4.) A smaller central air-handling unit can be used.

            5.) Filtration, dehumidification, humidification is performed at a central location.

            6.) Outside air supply is positive.

7.) The total building load instead of the sum of room peaks dictates capacity requirements.

8.) Energy saving are achieved by using water instead of air to deliver heating and cooling to the space.

9.) Heating and cooling can be performed without ventilation when space is unoccupied.

10.) Cross contamination is reduced because recirculation occurs within rooms.

11.) System components have a long lifespan and distribution (ducts and piping) may last as long as the building.

1.) Design of between season operations is crucial as a result of the low primary air delivered.

2.) Change over of the system may need to be performed.

3.) The application of this system is mostly for perimeter spaces.

4.) System controls are more complex.

5.) Terminal unit filters must be changed often due to secondary airflow.

6.) The primary air is usually constant having no shutoff.

7.) Low dew point air must be provided because all dehumidification is done at central location.

8.) Low chilled water temperatures or chemical dehumidification may by necessary.

9.) The system should not be used in spaces with high exhaust required unless additional ventilation is supplied.

            A fan coil conditioner unit is a type of room terminal unit that can be used with water-only or air-water systems. In the air-water system the fan-coil conditioner unit provides the heating or cooling to a space while the primary air system supplies all ventilation. The fan-coil conditioner consists of a fan, filter and coil. The fan draws in room air and passes it through the filter and then by the coil to either heat or cool it and then returns it to the room to control the temperature and particulates. The coils have chilled water or heated water passing through them, which is supplied at a central location. The water can be heated or cooled by any method. Chilled water is usually supplied anywhere from 35-48 degrees F. The unit can also have a second coil for auxiliary heating so that heating and cooling can be offered simultaneously. Flow through coils, fan speed, and air bypass dampers can be used to control capacity. Units are available in standard capacities of 200, 300, 400, 600, 800, and 1200 CFM.

    Fan coil units can be vertical units which sit right on the floor or they can be horizontal units hung from the ceiling. The horizontal units can be above a dropped ceiling. Below are diagrams, some samples of performance for different types of fan-coil-units. Sizes and weights are given in some of the performance/specs charts             

http://arch.hku.hk/~kpcheung/new2001/ac/#0301

http://arch.hku.hk/~kpcheung/new2001/ac/#0301  

Horizontal fan coil unit for ceiling hung installation

http://www.barcol-air.co.uk/fan_coil-index.htm 

Wall mounted fan coil unit

http://www.klimaire.com/wallmnt.html