Introduction

IDA ICE includes several templates for air handling units. You can change them to your needs and save new templates into the palette.

Standard air handling unit

The preset air handling unit (AHU) consists of the following components: a controller for the supply air temperature setpoint, a return air fan, a heat exchanger (heat recovery), a heating coil, a cooling coil, a supply air fan, and separate schedules for operating the fans and the heat recovery system. For more information on the components, see: Air Handling System

The system delivers temperature-controlled supply air at a specified pressure. Key parameters of the individual components can be viewed and adjusted via the respective dialog boxes.

The supply air temperature setpoint is linked to the heat exchanger, the heating coil, and the cooling coil. All three components have separate ideal control loops and operate independently to maintain the specified supply air temperature. First, the ambient air is pre-conditioned in the heat exchanger using energy from the return air. If heat recovery is insufficient, the heating coil raises the air temperature to the setpoint. When cooling is required, the cooling coil lowers the supply air temperature. Neither coil has a capacity limit.

Following air conditioning, the supply air is brought up to the required system pressure by the supply air fan. In doing so, the fan may increase the air temperature. This temperature increase can be modelled either as a fixed temperature rise (default setting) or by considering the motor and drive losses in the air flow.

The return fan discharges the stale room air to the outside via the heat exchanger and, together with the supply fan, ensures the required air flow rate for the system. The operating times of the fans and the heat recovery system are controlled via separate schedule.

Enthalpy wheel AHU

Compared with the standard air handling unit (AHU), this air handling unit is fitted with an enthalpy wheel for heat recovery. All other components and their control functions are the same.

The enthalpy wheel is used to transfer both sensible and latent heat between the return air and the ambient air. As a result, in addition to the temperature, some of the moisture contained in the return air is transferred to the supply air. In winter, this humidifies the supply air and reduces the need for heating and humidification. In summer, it can reduce the cooling load and, where applicable, the need for dehumidification.

Mixing box (recirculation)

Compared with the standard AHU, the heat exchanger is replaced by a recirculated air mixing box (MIXBOXCTR). All other components and their control functions are the same.

The recirculated air mixing box mixes ambient air and return air (recirculated air) before the air flows through the heating and cooling coil. The proportion of outdoor air is controlled based on the specified supply temperature and the minimum can be defined via a separate schedule. Regardless of the operating mode, a specified minimum ambient air flow rate is always maintained to ensure the supply of fresh air required for hygiene purposes.

The setpoint for the supply air temperature is linked to the mixing box and the heating and cooling coils. The mixing box utilises the thermal properties of the recirculated air to pre-condition the supply air. If the pre-conditioning is insufficient, the heating or cooling coils take over to maintain the specified supply air temperature.

ASHRAE Toolkit (limited capacity)

Compared with the standard AHU, the heating and cooling coils are modelled using the macros hc and cc. All other components and their control functions are the same.

These coils are no longer regarded as ideal components with unlimited capacity, but as physically described heat exchangers with limited heating and cooling capacity.

For the heating and cooling coils, in addition to the nominal capacity, parameters such as the maximum water mass flow rate, the design conditions on the air and water sides and the pressure drop can be defined. Based on these parameters, IDA ICE determines the actual achievable heating or cooling capacity under the respective operating conditions.

Supply air only (no return side)

Compared with the standard AHU, this version does not have a return air side. The system only has a supply air duct with a heating coil, a cooling coil and a supply air fan. It does not include a heat exchanger for heat recovery or a return air fan.

Return air only (no supply side)

Compared with the standard AHU, this version does not have a supply air side. The system consists solely of a return air fan with an associated schedule for fan operation. Air conditioning components such as heat recovery, heating coils, cooling coils or supply air temperature control are not included.

Return air temperature control

Compared with the standard AHU, the supply air temperature is controlled in relation to the return air temperature and is thus dynamically adjusted. The basic system layout of the remaining components is the same.

The heating and cooling coils are controlled via separate PI control loops. In each case, the measured exhaust air temperature serves as the reference variable, which is compared with an adjustable setpoint. Different setpoints for the return air temperature can be defined for heating and cooling modes.

The supply air temperature can be limited to a minimum and maximum value. The supply air temperature is restricted to the respective range between this minimum and maximum value. This ensures that the desired room temperature is achieved without the supply air temperature becoming unacceptably high or low.

Return air temperature control and CO₂ control

Compared with the AHU with return air temperature control, this version also includes a CO₂-controlled recirculated air mixing box. The basic system configuration, comprising heat recovery, heating coils, cooling coils and PI control, remains unchanged.

In addition, the proportion of fresh air in the mixing box (MIXBOX) is controlled based on the measured CO₂ concentration in the return air. The mixing box operates using proportional control and continuously adjusts the proportion of fresh air between 0% and 100%. If the CO₂ concentration is below the lower limit, recirculated air is predominantly used. If the CO₂ concentration rises, the control system gradually increases the proportion of ambient air until, upon reaching the upper limit, only fresh air is supplied. In addition, the minimum fresh air supply can be defined in a schedule.

By adjusting the proportion of fresh air to meet demand, the required air quality is ensured whilst reducing the energy required for air conditioning.

Night ventilation control

Compared to the standard AHU, this AHU has an additional control unit that sets the setpoints and controls the operation of the fans for a night ventilation. The basic system configuration of the remaining components is the same.

Night ventilation is activated when passive cooling is possible. The exact characteristics of the control system are described in detail in the macro (text box at the bottom of the macro).

During night ventilation operation, the setpoints for air conditioning are automatically adjusted. The heating setpoint is lowered, while the cooling setpoint is raised. As a result, the heating and cooling coils and the heat recovery system remain largely inactive during the night to take advantage of passive cooling.

Compared to the standard AHU, this expansion enables passive nighttime cooling of the building. By utilizing cool ambient air, the accumulated heat loads within the building can be dissipated and the mechanical cooling demand for the following day can be reduced, thereby lowering the energy consumption of the HVAC system.

Return air humidity control (steam)

Compared to the standard AHU, this AHU has a steam humidifier in the supply air. This is controlled based on the humidity of the return air. The basic system design of the remaining components is the same.

The supply air temperature continues to be controlled as in the standard AHU. In addition, the relative humidity of the return air is continuously monitored and compared to an adjustable setpoint. If the measured humidity falls below the setpoint, the steam humidifier is activated, and water vapor is introduced directly into the supply air stream.

The system takes additional constraints into account. These include, among other things, the maximum permissible relative humidity of the supply air, the steam temperature and a minimum ambient air temperature below which the steam humidifier may not be activated. In addition, a temperature differential (deadband) can be added to the supply air temperature setpoint for the cooling coil.

Compared to the standard AHU, this system configuration allows for the control of both temperature and indoor relative humidity. This helps prevent excessively low humidity - particularly during the heating season - and improves thermal comfort and indoor air quality. At the same time, the integrated limiting functions prevent excessively high humidity levels and an unnecessary additional cooling load.

Electric heating coil

Compared to the standard AHU, this model is equipped with an electric heating coil instead of a water-based heating coil. The rest of the components is the same.

Unlike the water-based heating coil, the electric heating coil does not require connection to a heating water circuit. Heat is generated directly by electrical energy and transferred to the air. The heating capacity is limited to a maximum output. If this output limit is reached, the supply air temperature setpoint may not be fully reached.

Extra heating coil (dehumidification)

Compared to the standard AHU, this AHU has an additional heating coil located behind the cooling coil to reheat the supply air after dehumidification. The other remaining components are the same.

When there is requirement of dehumidification, the supply air is first cooled below the dew point in the cooling coil. This causes moisture in the air to condense, thereby reducing its absolute humidity. The setpoint of the cooling coil is set via a separate setpoint (corresponds to the dew point temperature of the desired (absolute) humidity).

Since this process cools the air lower than the supply air setpoint, it is then reheated in the second heating coil to the preset supply air temperature.

Compared to a standard AHU, this system configuration enables active dehumidification of the supply air while not providing a too low supply air temperature.

Indir. & dir. evaporative cooling

Compared to the standard AHU, this AHU has two humidification units for direct and indirect evaporative cooling. The other components and controls remaining the same.

Direct evaporative cooling takes place in the supply air stream. Water is introduced into the air stream, causing some of the water to evaporate. The heat of vaporization required for this is extracted from the air, which lowers its temperature while simultaneously increasing its relative humidity. To prevent excessive humidification, evaporation is limited by a maximum relative humidity and a minimum supply air outlet temperature.

In addition, indirect evaporative cooling is integrated on the return air side. Here, the return air is cooled adiabatically. The lower return air temperature improves heat transfer in the heat exchanger and results in additional cooling of the ambient air without altering its moisture content. Indirect evaporative cooling is activated only when the outside temperature exceeds the given threshold.

Compared to the standard AHU, this system configuration enables energy-efficient cooling of the supply air, as a significant portion of the cooling load can be covered by adiabatic evaporation instead of mechanical refrigeration. This leads to a significant reduction in the ventilation system’s energy consumption, particularly in dry and warm climates.

Indirect evaporative cooling

Compared to the AHU with indirect and direct evaporative cooling, this AHU has only indirect evaporative cooling. The basic system configuration remains unchanged.

In comparison to the AHU with direct and indirect evaporative cooling, this system configuration has the advantage that no moisture is introduced into the supply air. As a result, the supply air humidity remains constant regardless of whether the evaporative cooling is in operation. This is particularly advantageous for applications with high requirements for indoor air humidity.

Separate setpoints for hc, cc, hx

Compared to the standard AHU, this AHU has separate setpoints for the heat exchanger (hx), the heating coil (hc) and the cooling coil (cc). The basic system design of the remaining components is the same.

Separate setpoints allow the individual components to be controlled individually and independently.

Supply temp function of return

Compared to the standard AHU, this AHU controls the supply air temperature directly based on the return air temperature. The basic system design stays the same.

A characteristic curve defines the relationship between return air and supply air temperatures: Based on the current return air temperature, a corresponding setpoint for the supply air is calculated. This allows the supply air temperature to be continuously adjusted to the building’s thermal requirements, rather than maintaining a fixed temperature value.

Evaporative humidifier and cooling dehumidification with return air control

Compared to the standard AHU, this AHU enables full humidity and temperature control. In addition to the standard configuration, both an evaporative humidifier for humidification and a cooling and additional heating coil for dehumidification and reheating of the supply air will be used.

Control is achieved via separate setpoints for supply air temperature and humidity, depending on the return air temperature and humidity. The measured temperature and humidity are continuously compared with an adjustable setpoint. If the air is too dry, the humidifier is activated via a PI controller to increase the humidity of the supply air. Minimum and maximum relative humidity values are considered to prevent over-humidification.

If, on the other hand, the humidity is too high, the cooling coil is used for dehumidification. To do this, the supply air is cooled below the dew point so that moisture condenses out of the air. The air is then reheated to the desired supply air temperature via the second heating coil. Dehumidification is also controlled by a PI controller and operates within defined temperature limits to prevent excessive cooling of the supply air.

The operation of the heat recovery system as well as the supply and return air fans is controlled - as with the standard AHU - via corresponding schedules. The humidity and dehumidification control work in conjunction with the temperature control to ensure that, after air treatment, both the desired supply air temperature and the specified room humidity are maintained.

It is important that the evaporative humidifier is properly sized and that the parameters in the model are set correctly. The exact control logic is described in the “Controls” macro.

With the control of the temperature and humidity, this AHU is particularly suitable for buildings with high requirements for temperature and humidity stability, such as museums, laboratories, or production areas.

Steam humidifier and cooling dehumidifying return air control

This AHU works similar as the system above, only that it has a steam humidifier instead of an evaporative humidifier.

Depending on the different humidifier, the control slightly differs. It is detailed described inside the “Controls” macro. Also, here it is important that the steam humidifier is properly sized and that the parameters in the model are set correctly.

Exhaust air with liquid heat recovery

This AHU is intended exclusively for return air handling with liquid-based heat recovery. The system consists of a return air fan and a liquid-based heat exchanger, which is used as a heat source for an external energy supply system. There is no supply air.

The heat contained in the return air is transferred to a heat transfer fluid and can then be utilized within another energy system. The heat exchanger is pre-configured to be connected to the plant and functions there as a free heat source. If necessary, the recovered heat can be supplied, for example, to a heat pump or another energy generation system within the plant.

This system configuration is particularly suitable for buildings in which the heat contained in the return air is to be made available to a central energy supply system.

Exhaust air with DX heat recovery

This AHU is designed exclusively for return air extraction and features an integrated direct-evaporator heat pump. The ventilation system consists of a return air fan. The external heat pump system is not part of this ventilation unit but is modelled separately within the plant model. There is no supply air.

The room air is returned via the return fan and flows through an evaporator heat exchanger. In this heat exchanger, heat is extracted from the return air and transferred to a refrigerant circuit. The recovered heat is then made available to an external direct-evaporator heat pump.

This system configuration is particularly suitable for buildings in which the heat in the return air should be used for energy via a heat pump. Unlike a conventional air-to-air heat recovery system, the heat is not transferred directly to the outside air but is fed into the heat pump’s refrigerant circuit. This allows the energy contained in the return air to be flexibly utilized for various heating applications and reduces the building’s overall energy demand.

Ground duct supply air

Compared to the standard AHU, the supply air is preheated or precooled by a ground duct that is connected before the heat exchanger. The air temperature is logged after the ground duct and by a sensor inside the duct, which is shown in the “AHU temperatures” diagram. Otherwise the AHU is identical to the Standard Air Handling unit.

Since this model includes the ground with a large thermal mass, a longer dynamic startup time than set by default is needed. This time could vary from case to case, but could go from several months to one year. Periodic startup must never be used with this model, so heating and cooling load simulations must not use synthetic weather (one day data), but instead a period from an ordinary (annual data) climate file, which uses dynamic startup.

Double clicking on the GroundDuct symbol in the Ground ICE macro opens the Ground duct form. Further information about the parameter settings in the form is given here.

Internal liquid heat recovery

Compared to the standard AHU, instead of a direct air-to-air heat exchanger, this AHU contains a liquid-based heat recovery. The basic system design of the remaining components is the same.

The heat recovery system consists of two air-to-liquid heat exchangers, one located in the supply air stream and the other in the return air stream. Both heat exchangers are connected via a circuit with a circulation pump. The thermal energy from the exhaust air is transferred to the heat transfer fluid in the exhaust air heat exchanger and then released to the incoming ambient air in the supply air heat exchanger. In cooling mode, the heat transfer process occurs in the opposite direction, so that the ambient air is pre-conditioned by the cooler return air.

Heat recovery is controlled via the fluid mass flow in the circuit and the circulation pump. Depending on the operating mode (heating or cooling), the mass flow rate of the heat transfer fluid is adjusted to achieve the desired heat recovery. Additionally, parameters such as the type of heat transfer fluid, its freezing point and the mass flow rate can be specified. To prevent freezing, the temperature of the return air is monitored so that heat recovery can be limited or shut off under critical operating conditions.

Compared to a standard AHU, liquid-based heat recovery offers the advantage that supply and exhaust air can be routed separately, since no direct air contact between the two air streams is required. As a result, this system design is particularly well-suited for applications where mixing of the air streams must be prevented or where the supply and return air ducts are structurally located far apart from one another.

Limited Capacity Air Handling unit

The air handling unit is designed in a way that it can be used with the Swegon Wise water system

This Air Handling Unit has limited Capacity on all main components such as heat exchanger, heating coil, cooling coil and the air flow of the fans. To make the Air handling easy to use, all equipment are automatically sized. The heat exchanger is automatically set to match the total supply airflow. The automatic sizing can be overwritten by opening the component in question and right click on the grey boxes selecting mapping and break link.

The air handling unit will never deliver more air than can be seen when opening the form for the supply fan.

The AHU contain several control options, cycling for heating, Night ventilation for cooling, return air temperature control and standard temperature control.

Night cycling for heating provides the option to heat the building by starting the ventilation system during the night, even if the fan operation schedule is off. The supply air setpoint is set to a higher value (modulated up to the value given by Max supply air temperature) and the fans are started when any room temperature drops below its setpoint.

When using cycling for heating several parameters need to be set. First Select what zones to be included in cycling control, by default all zones in the building is selected. The other parameters are; room air temperature for start of cycling and Maximum supply temperature when cycling.

When using night ventilation The daily (normal) fan operation is governed by the Schedule 'Normal fan operation'. This schedule should not attempt to run the fans during the night. Night (flush) ventilation is instead given in the 'Night vent operation' schedule. The default schedule 'Nite vent timetable' has rules to run the night ventilation Mo-Fri, May 1 through Sept 30. Fans may operate in night vent mode between 22:00 and 07:00.

The night ventilation will be on when the following conditions are all fulfilled:

- Outdoor temperature is above 12°C (change at 'Outdoor temp limit').

- Outdoor air is at least 2°C below return air (change at 'Benefit limit').

- Return air is above 22°C (change at 'Return air limit').

You set the parameters in the form for night ventilation parameters.

While night flush ventilation is active, the supply air setpoint to the heat exchanger and the heating coil is lowered by 10°C (change at 'Setpoint shift'). The setpoint for the cooling coil is set 20°C higher (change at 'Setpoint shift') to avoid any mechanical cooling in AHU while night flush ventilation. Note, that the chiller must turned off or change cooling setpoints of the spaces to be sure that there is no mechanical cooling in spaces.

When using Return temperature control, the standard temperature control is overridden. The parameters to set are; set point for return temperature, maximum supply temperature and minimum supply temperature.

Heat exchanger cooling recovery enables an air-to-air heat exchanger to precool incoming air whenever possible by using exhaust air.

Automatic sizing of heating and cooling coils.

For heating, the settings are as in the red rectangle of the figure below.

The heating power is then calculated as

Power=(“Dim Supply temperature” – “Dim outdoor temperature”)*1007*1.189 * ”Max supply air flow” /1000.

The mass flow in the valve is than calculated as Flow=Power/”Dim tem drop, liquid”.

For cooling the settings are as in the red rectangle of the figure below.

The cooling power is then calculated as

Power=(“Hin_rat” – “Hout_rat”) *1.189 * ”Max supply air flow” where Hin_rat and Hout_rat are enthalpies calculated from the inlet and outlet temperatures and humidity at the rating point. The inlet humidity is given and the outlet humidity is estimated dependent on the average coolant temperature.

The mass flow in the valve is than calculated as Flow=Power/”Liquid temperature rise at rating”.