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Fan Coil Unit FCU Vs Air Handling Unit AHU

The fan filter unit is a terminal purification equipment that combines a fan and a filter (high-efficiency filter (HEPA) or ultra-high-efficiency filter (ULPA)) to form its own power. To be precise, it is a modular terminal air supply device with its own power and filtering function. The fan sucks air from the top of the FFU and filters it through HEPA, and the filtered clean air is evenly sent out at the wind speed of 0.45m/s±20% on the entire air outlet surface. In this instance, the only real difference would be that the air handling unit operates by bringing in fresh air from the outside, whilst the FCU generally recirculates and conditions the air in the space it serves. The high side being the main supply duct from the air handler to the inlet of each VAV box. The main is considered upstream of the VAV box, while downstream of the box is considered the low-side supply.

At present, air handling units mainly have several forms such as ceiling type, vertical type, horizontal type and combined type. Among them, the ceiling-type air handling unit is also called a hanging cabinet; the combined air handling unit is also called a combined air cabinet. A DOAS terminal is a fan-powered air terminal mounted in the ceiling plenum, with a chilled water cooling coil mounted at the inlet from the ceiling plenum. Heating coils (hot water or electric) are included in DOAS terminals which serve spaces requiring heat. Due to their simplicity, flexibility and easy maintenance, fan coil units can be more economical to install than ducted 100% fresh air systems (VAV) or central heating systems with air handling units or chilled beams. FCUs come in various configurations, including horizontal (ceiling-mounted) and vertical (floor-mounted) and can be used in a wide range of applications from small residential units to large commercial and industrial buildings.

A concealed fan coil unit will typically be installed within an accessible ceiling void or services zone. It is quite common for the return air not to be ducted and to use the ceiling void as a return air plenum. On the other hand, because the central air handling unit provides all cooling to the spaces, it is required to move a much greater volume of air. Thus, the floor space required for air handling equipment and vertical duct risers (shafts) is much more significant when compared to the other systems. The heating hot water valve will modulate open providing a range of heating hot water flow (GPM) to meet the heating load.

I suspected before that higher fan power of VAV results in more temperature increase through fan and hence more space cooling requirement. But after manually change the delta T through fan, the effect is not very obvious that the space cooling vav vs fcu increase proportion drops from 32% to around 28%.seems it is not the main cause. Lately I am modelling an office & hotel complex through eQUEST with energy conservative strategies added one by one to test the energy saving effect.

  1. These controls may be individual or combined into one control and are designed to protect the compressor.
  2. After dust removal, dehumidification (or humidification), cooling (or heating) and other treatments, it is sent to the room through the fan, and the original air in the room is replaced when it enters the indoor space.
  3. All the zones on a floor of a high-rise maybe fed from the same air handler, but each zone can adjust its CFM according to their specific needs.
  4. This toolkit provides contractors with additional tools and insights for use on the job.
  5. In some systems the supply air temperature could be increased to a temperature that is just cool enough to cool the most-demanding zone with its VAV box set to maximum flow, thereby saving additional energy.

This pressure change is picked up by a static pressure sensor in the main supply air duct. The air handler will deliver a constant temperature of 55ºF (13 ºC) supply air to the VAV boxes. While the supply air temperature stays constant the volume (CFM) of air will vary based on the total demand of all the zones on the system. There are several control strategies to adjust the speed of the fan which we’ll discuss below. To model the air cooled heat pump as heating source for hotel, I modelled 2-pipe heat pump loop at water-side and set it as the heating source at air-side (cooling source as chilled water loop). The loop operation is set as snap with the snap temp as mean temp of indoor heating and cooling design temperature (otherwise much unmet hour occur).

How a Variable Air Volume VAV System Works

When the temperature sensor in the space calls for cooling it sends a command to the VAV box controller which then adjust the supply air flow rate (CFM). The adjustment is done by an actuator rotating the VAV box inlet damper either open or closed in increments. Variable air volume is more energy efficient than constant volume flow because of the reduction in fan motor energy due to reducing fan speed (RPM) at partial load. As the cooling or heating demand is reduced because of a mild temperature day, the VAV Air Handler system can reduce the amount of air flow (CFM) by reducing the fan speed.

Pros and Cons of VAV and Dedicated Outdoor Air Systems (DOAS)

• VAV systems are very cost effective from a chilled-water and heating-water-pumping perspective. Because the unit transfers heat to the space using forced convection, the coil heat transfer area required is reduced as compared to a chilled beam. The reduction in coil heat transfer surface area results in lower coil water side pressure drop and lower pumping energy. Forced convection heat transfer also yields higher water side delta T which also results in lower pumping energy.

Using a DDC control system with VAV boxes that have a flow station and temperature sensor at the supply air discharge the system can determine the amount of reheat. The use of this strategy is required by Title-24 (California) and ASHRAE 90.1 for system that have DDC to the zone level. The static pressure setting in the main supply duct is reduced to a point where one VAV box damper is nearly full open. This would require that the VAV box actuators can report their damper position, best performed with an analog output. The pressure sensor in the duct will send a signal to the Variable Frequency Drive (VFD) causing the supply and return fans to slow down or reduce its RPM. If the pressure in the duct decreases because the VAV boxes are opening due to the need for additional cooling, the pressure sensor will send a signal to increase the fan speed (RPM).

Fan coil systems are comprised of water-to-air coil air handlers connected via a two or four-pipe insulated water loop. Fan coils require complex chillers and boilers to provide water loop fluid in a particular temperature range (i.e., chilled water for cooling and hot water for heating). Two-pipe fan coils have a major disadvantage as control is substantially limited to whatever mode the system is currently set at (i.e., cooling or heating). A four-pipe version can be installed that requires both chilled and heated water to be available at the same time.

FCUs are commonly used in HVAC systems of residential, commercial, and industrial buildings that use ducted split air conditioning or with central plant cooling. FCUs are typically connected to ductwork and a thermostat to regulate the temperature of one or more spaces and to assist the main air handling unit for each space if used with chillers. The thermostat controls the fan speed and/or the flow of water or refrigerant to the heat exchanger using a control valve.

• VAV system generally have lower maintenance requirements as compared to the other systems. This is because there is no chilled water coil at the VAV terminal and, where non-fan powered VAV terminal are used, there is no filter at the unit. All the routine maintenance (filter replacement, condensate pan cleaning) is centrally located at the air handling unit, minimizing maintenance needed within the occupied spaces. As the pressure increases in the main supply duct because the VAV boxes are closing their dampers and are adjusting their dampers towards the minimum open setting, the air handler supply fan VFD slows down the fan. The opposite will happen due to the VAV boxes opening because of increased demand and the dampers are opening, in this case the VFD will cause the supply fan to speed up when the pressure in the main supply air duct drops. As the VAV boxes open or close due to demand called for by the temperature sensor in the space, the pressure in the main supply air duct will either increase or decrease.

Variable Air Volume, or VAV, is one of the most common types of HVAC systems used in large commercial buildings today. A typical system is usually comprised of a large air handler, central ductwork system, and a relatively large equipment room. Conditioned air is distributed throughout the building via a central ductwork system and is regulated via dampers in each space.

Fan coil unit description

For a single VAV air handler that serves multiple thermal zones, the flow rate to each zone must be varied as well. The VAV box regulates the flow (CFM) to a zone in relationship to the demand of the temperature sensor in the space. In high-rise buildings, fan coils may be vertically stacked, located one above the other from floor to floor and all interconnected by the same piping loop. Fan coil units linked to ducted split air conditioning units use refrigerant in the cooling coil instead of chilled coolant and linked to a large condenser unit instead of a chiller. They might also be linked to liquid-cooled condenser units which use an intermediate coolant to cool the condenser using cooling towers.

It has been widely used in high-clean places such as operating rooms, animal laboratories, crystal experiments and aviation. When the VAV boxes are connected to a building automation system that monitors the function and status of the boxes there are various options for control. As you can see in the diagram above the VAV Damper goes from a minimum of 30% open, whatever the minimum required to meet ASHRAE 62, all the way to the damper being 100% open. These motors are sometimes called DC motors, sometimes EC motors and occasionally EC/DC motors. On the other hand FCU is the equipment used to condition the space it can either be a DX or Chilled water system. Where r is the annual rate, i the periodic rate, and n the number of compounding periods per year.

The minimum CFM setpoint can be somewhere between 30% and 50% of the maximum cooling setpoint. Minimums are set by some code jurisdiction so that the minimum ventilation rate is always achieved. In California see Title-24 Sec 120.1 Requirements for Ventilation and Indoor Air Quality. This primary supply air will also bring a percentage of mandatory ventilation air (Outside Air).

DC/EC motor powered units

It follows that if one has to choose between receiving $100 today and $100 in one year, the rational decision is to cash the $100 today. This is because if you have cash of $100 today and deposit in your savings account, you will have $105 in one year. The operation of evaluating a present value into the future value is called capitalization (how much will $100 today be worth in 5 years?). The reverse operation which consists in evaluating the present value of a future amount of money is called a discounting (how much $100 that will be received in 5 years- at a lottery, for example -are worth today?).

Alternatively a condensate pump may be employed where space for such gravity pipework is limited. Water-source and geothermal heat pump systems are comprised of individual packaged units that transfer heat via a single- or two-pipe water loop. Each unit can be used in either heating or cooling mode year-round and loop temperature is maintained via a boiler/tower combination or earth-coupled loop. Each zone has complete control of its heating/cooling mode and each unit is independent from the others.

nicvosFan Coil Unit FCU Vs Air Handling Unit AHU