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What input is needed?

Local units

Room units

Local heating or cooling is supplied to the zone by room units. All room units are listed in the zone form. Some units such as ideal heaters and coolers, do not have a given location in the room. These can be introduced directly into the list in the zone form. Most hydronic units may, on the other hand, be located on a specific zone surface and they are instead inserted by dragging them onto a surface. Most hydronic units can also exist without a specific position in the zone, i.e. they can be dragged directly into the zone form. Note, however, that all radiative units still require an explicit surface area, which is given in the Geometry tab when the input form of the unit is active.

The temperature setpoint for cooling devices is normally taken directly from the Control setpoints, max value for Temperature, but for VAV systems an offset, set in System parameters, is added to use air cooling first. The corresponding value for heaters is the Temperature min value. However, it is also possible for the Expert edition user to define any controller for an individual device.

An overview of all local heating and cooling units you find here: Room units for cooling and heating

Ideal heaters

Ideal room units should be used to condition the zone when no detailed information about an actual room unit, such as a fan coil or active chilled beam, is available or this amount of detail is unmotivated. They have no given physical location on any room surface and are not connected to the plant of the building. They do have a maximum capacity parameter, enabling the user to experiment with limited heating/cooling capacity. However, this parameter should normally be set to a large enough value to always cover any foreseen need. However, do not set the value to an unrealistic number, e.g., 100 times the reasonable heating load. This will result in poor control action.

Ideal coolers

The ideal cooler works much like the ideal heater, but with cooling instead of heating. Unlike the ideal heater, the cooling has no radiative part. Instead, cooling by condensation may occur, which is determined by the coil temperature that is set by the user.

Simple fan coils

The Simple fan coil is connected to the cold-water circuit and, optionally, also to the hot-water circuit, although it could also be electrically heated. The cooling or heating is calculated and delivered as convective heat to the zone air without simulating the actual air flow. The design conditions for the water circuit are preset but can be altered in Outline.

Fan electricity use is specified as a percentage of cooling (or heating) power. This percentage can be set to zero without affecting the operation of the device. Setting the fan electricity to zero in a Simple fan coil is actually the computationally most economical way of heating or cooling the room by water from the Plant.

Fan coils

Fan coils are connected to both the cold-water and hot-water circuits. Unlike the simple fan coils they may also be connected to supply and return air terminals that can be inserted by the user anywhere on a room surface. When using the Climate zone model with stratification the location of the terminals will impact the results. The zone air temperature at the height of the return air terminal will be used for the control of the device.

Water radiators

All hydronic devices with a radiative component use a mathematical model that allows them to have a given position on a zone surface. This enables the computation of correct local radiation near the device (when Climate zone models are used).

Heat emission from hydronic heating devices is calculated using

P = K*l*dT^N,

where l is device length and dT is the temperature difference between the water and the zone air. K and N are constants characterizing a device of a certain height (or width for ceiling devices).

The figure below has a radiator inserted on a wall surface. Its main form has been opened and a dialog for the Device type has been opened from the form. Often the values of K, N and Height come from a database, which in the form can be opened under “Use manufacturer’s data”. The data for the Device type can also be altered and saved by the user with a new name. The user must then only enter the surface area (given graphically or in the Geometry tab) and the Design conditions, from which the mass flow rate at full power is calculated.

The warm radiator surface that is exposed to the zone is defined by the box that is drawn when the unit is inserted on a wall surface. The size of this box has meaning for the heat transfer. While the total emitted heat is always given by the expression for P, the division between radiation and convection is calculated based on the surface temperature - which is connected to the water temperature - and the given exposed surface. The zone “sees” a warm surface and calculates the radiation and convection from this surface. The main part of the remaining heat (needed to complete P) is emitted convectively directly to the air (corresponding to convection behind a radiator). A small part of the heat goes to increase the temperature on the portion of wall behind the heating device. The heat transfer coefficient between the device surface and the wall behind is considered in the basic case to be completely dominated by radiation and is calculated by the model.

A radiator on an external wall, its standard form, and a dialog for the Device type.

To facilitate adding new heating devices without direct knowledge of K and N, a Simplified model data input is given (see the form in the figure above). The values for design mass flow and K are calculated from the power given by the user at the specified Design conditions. The user also provides a value for N in this case. Note that K is calculated from the information given in the alternative input. This is thereafter the value for K. If the size of the graphical box is later changed, the device is likely to have an unintended maximum power. Note also that the actual maximum heating capacity of the device will vary with actual room and supply water temperatures. The Design conditions are only used to calculate K and the mass flow rate at full capacity and have no impact on the simulated supply and return water temperatures.

The actual water supply temperature is given by the boiler; the return temperature will vary with the actual massflow through the unit, the current room temperature and the size (K value) of the heating unit. However, if actual temperature conditions are made to coincide with those given in the dialog, exactly the given power will be emitted.

Electric radiators

Electrical radiators work much like the Water radiator, but with electric instead of water-based heating. The model is however simpler, and the surface temperature is simply obtained from the heat balance between the supplied power and the calculated heat emissions from the front and back of the radiator.

Hot objects

The hot object is aimed at modeling free-standing heat sources, e.g. a stove, in the zone. Model-wise it is similar to an Electrical radiator, but instead of attaching it to a wall, it has a three-dimensional geometry that can be imported to any place in the room. In the Climate zone, the surfaces of the object then become part of the radiation exchange between the room surfaces, using view factors. Without imported geometry, or in an Energy zone, the radiation distribution will instead be area based. Convection to the room air is also calculated for all the surfaces.

Cooling devices

The Cooling device is used for radiative and convective units. Cooling units operate completely analogous to waterborne radiators, with the exception of a few differences in input, which are discussed here.

A cooling panel on the ceiling surface, its standard form, and a dialog for the Device type.

Here, the height of a radiator corresponds instead to a Module width, to which K and N refer. The total length is calculated as the given box area divided with Module width (as is the case also for the water radiator). There is a difference in that the heat transfer coefficient between the back of the device, and the surface behind (often the ceiling) is given directly in the main form. If an (arbitrary) negative figure is entered, the heat transfer coefficient is calculated in the same way as for the heating device, i.e. as if all heat transfer were done by radiation. This is a good approximation for a device that has no insulation at all.

The dialog for alternative input has somewhat different parameters for cooling units. Absorbed power and temperature differences between air and water may be given for two points on the power curve, from which both K and N can be calculated. For max power, the temperature rise of the water is also given.

Heating/Cooling panels

The heating/cooling panel is a combination of a Water radiator and a Cooling device in the same unit, but with a few differences compared to the separate devices. It does not have a database to store user defined products and is mainly used for branded products in the program.

Heating/Cooling floor

If a floor heating/cooling object is inserted on the floor of a zone, the floor construction for this area is divided into two parts, above and below the heated layer. Between the two, a heat exchanger model is inserted corresponding to the piping layer. Sometimes a floor heating circuit will heat the room below almost as much as the room it belongs to.

The floor coil model assumes that the active layer can be treated as an infinitely conductive plane in the floor slab, i.e. all 2D effects are disregarded. Heat transfer is calculated with a logarithmic temperature difference between the fluid and this plane of constant temperature. The user supplied total heat transfer coefficient between the fluid and the plane, includes

1. Convection between medium and tube wall
2. Heat conduction through the tube walls
3. “Fin efficiency” corresponding to the distance between immersed tubes or actual fins.

The modeling approach will in steady state correspond to the Resistance method of the standard EN 15377-1.

The floor coil circuit can have its own three-way valve and pump circuit keeping the mass flow constant (default). Emitted power is then controlled by varying the supply water temperature. PI, P or on-off control can be selected (PI is default). A further alternative Always on is also available, which will keep both the boiler mass flow and the coil circulation mass flow permanently at their design values. Control must then be maintained by the boiler temperature controller.

In the case where no separate coil pump circuit is used, the four control options will instead act by limiting the mass flow through the coil, or keeping it constant at design conditions in the Always on case.

See the sketch below for an example:

Sketch of the two coil mass flow options for controlling the power

In the upper option, the flow in the circuit varies from 0 to mmax. In the lower option, there is a constant circuit flow (mcirc) and a variation in the mixed-in flow between 0 and mmax, which thereby controls the temperature. The flow rates are calculated according to the equations in the figure. The example mass flow rates apply to 1 m² of underfloor heating circuits. The supply temperature to the underfloor heating installation is the temperature coming from the plant.

Electric floor heating

Electric floor heating works similar to Heating floor, but with electric instead of water-based heating.

Active beams

Active beams serve both as supply air terminals and as cooling devices with significant convection. Their performance depends on the amount of supply air that is passed through, but they normally retain a heat transfer contact with room air also in the case of zero supply air flow. The radiative coupling with the room is neglected in the present model.

Two input data options are available: Simplified and Manufacturer’s. The latter means that the performance parameters K and N are given as functions of air flow. This alternative is mostly used when data is automatically imported from an on-line manufacturer’s database. The Simplified option is based on two user supplied performance points, at design conditions and at zero flow (figure below). N is for this case set to 1.0.

Active beam form

In the constant flow (CAV) case, the given Design air flow is regarded to pass through the beam whenever fans are running. For VAV, the constant Design air flow will pass through the beam whenever there is sufficient air into the room and surplus air will feed directly to the room without first passing the beam. If the VAV flow is insufficient to serve the beam with the full requested Design flow, the flow through the beam will be reduced accordingly.

The total Design air flow through all beams must not exceed the total for the zone. If beam air flow is less than the requested total for the zone, the remaining part is regarded as being supplied through conventional terminals.

When the flow to the room is increased by forcing the central fan (or similarly reduced) the beams will still keep their requested Design flow, as far as possible.

For the Climate zone model with stratification the distribution of the convective energy to the different layers is calculated depending on the height location of the Active beam, which by default is close to the ceiling.

Hydronic reheat coils

Reheat coils heats a room by adding heat to the mechanical supply air. It is controlled by a room sensor. The data at design conditions gives the max water flow rate.

Electrical reheat coils

The Electrical reheat coils works in the same way as the hydronic reheat coil, but with electrical heating of the supply air.

Air terminals

This section covers the models and input which determine the airflows through the building. IDA ICE enables the user to take account of natural ventilation, i.e. flows driven by wind pressure and the stack (chimney) effect.

In the simplest case, each zone has three paths for airflows: through the supply and exhaust terminals and via leakage through the envelope. When two zones are placed adjacent to each other and there is an opening in the common wall between them or when windows are open or when additional leaks have been added, additional flow paths are created. However, let us start with the simplest case.

The mechanical ventilation terminals are always of VAV (Variable Air Volume) type. This means that as long as there is sufficient pressure head from the fans, a given flow is maintained as requested by the zone itself. In CAV (Constant Air Volume), the control signal is kept constant and a constant flow will be maintained, irrespective of pressure. In a VAV system, a controller regulates the flow with respect to temperature, carbon dioxide, humidity or pressure levels in the zone.

Since, in the simplest case, flows through two of the three paths are given, the size of the third flow, through the leak, is important only for the pressure in the zone, and not for net flows. Note that if the size of the leak is much too small, an unrealistic pressure may build up in the zone. Such a pressure may become so large as to affect the air psychometric calculation routines and may then be reported as a condensation problem, when, in fact, it is a pressure problem.

It is also possible to define additional given in/exfiltration flows, i.e. balanced flows into and out of a zone. Since these incoming and outgoing (mass) flows are always equal, they will not affect the pressure of a zone. The given in/exfiltration will only act to exchange heat, moisture and CO₂ with the ambient.

Air flow input forms

The figure below shows the zone form. The input fields in the Ventilation section govern air flows. Also involved are Controller setpoints.

The zone form

Clicking on the "Controller setpoints" link field opens the dialog (figure below).

Control setpoints

In the zone form, the user can select System type (CAV, VAV with temperature control, VAV with CO₂ control or VAV with humidity control, VAV with both temperature and CO₂ control, and VAV with pressure control, schedule controlled VAV etc.). For CAV, the required airflows are given directly in the zone form . Leak area gives the size of the combined envelope leak . (In the model, the combined envelope leak area is distributed on all external walls, 1 m above floor level. The height is only relevant for natural ventilation situations.) Both Leak area and Given additional in/exfiltration are by default computed automatically based on information for the whole building that is given in the Infiltration form which is reached from the Building form.

In the combo box Controller setpoints (figure "The zone form"), the user can select an object with a collection of relevant zone-level control setpoints. The Control setpoints dialog (figure "Control setpoints") provides input data for zone climate quality requirements. Here, only those parameters, which are of importance for airflows, are discussed. For a VAV system, the given minimum value of Mechanical exhaust airflow provides the lowest allowable airflow, with the maximum value providing the highest one. (figure "The zone form", an exhaust only VAV system has been specified.) In the case of CAV, as was already pointed out, the desired flow is given directly in the zone form. If the chosen CAV flow falls out of the quality range provided in Control setpoints, a warning is issued when the simulation is started.

The other values in figure "Control setpoints" may impact on the corresponding VAV control scheme. If the user selects CO₂ control, (see figure "The zone form"), the airflow is varied in proportion to the CO₂ content of the air in the zone. For the VAV with CO₂ control scheme, a CO₂ value, which equals or exceeds the given max-value (in figure "Control setpoints"), results in the maximum flow through the exhaust terminal, with a minimum CO₂ value producing the given minimum airflow. The humidity control function is entirely analogous with respect to relative humidity (assuming supply air will dry the zone).

The option ‘VAV with temperature control’ functions somewhat differently. Here, the maximum comfort temperature value is used. Forcing of the VAV flow begins somewhat below (normally 1 °C) the indicated maximum value (P control). Full exhaust flow (see figure "Control setpoints") is reached at somewhat above (normally 1 °C) the maximum temperature value. The throttling range is normally 2 °C, but can be selected at the building level under System parameters.

This scheme will assume that the supply air is able to cool the zone, i.e. if there is a need for heat and the supply air is warmer than the zone, this will not be recognized by the controller.

The option ‘VAV, temp+CO2’, on the other hand, will be “smart” enough to both heat and cool with the supply air. It relies on PI controllers instead of P ditto, and will therefore not have any offset error. In addition, it will also force air flow if needed to maintain CO₂ level at the maximum limit. (The minimum limit is not used.)

Pressure controlled VAV is normally used for return air flow control where some other VAV method is used to supply air into adjacent zones. It will attempt to maintain zone pressure, within the given range, with respect to ambient pressure using a proportional controller (measured as pressure drop in the local ambient leak). In the example in figure "Control setpoints", the zone is maintained between 10 and 20 Pa below ambient pressure.

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