Creating a Thermal Model

A 3D model is simply a way of encoding a set of information so that it can be interpreted by a computer program or converted for use in a mathematical algorithm. Unfortunately computers are not particularly good at actually ‘understanding’ things, particularly complex geometry, so they cannot really guess what you may have intended when you created your model. Instead, they can only work with exactly what you have input and apply a set of fundamental rules in the interpretation of each individual model element. As a result, these rules need to be straightforward and completely unequivocal - meaning that there can only ever be one way of interpreting each condition. Thus, a lot depends on your ability to properly encode your building information according to these rules.

When creating a thermal model, you must remember the following:

  • Each zone a thin-skinned prism
    Create each thermal zone as a complete 3 dimensional prism, with planar surfaces on all sides even if they are specifically set as openings. Use only a single planar object per boundary surface, irrespective of material thickness.

  • Rooms and zones
    You should really use a separate thermal zone for each room in your building. However ECOTECT is very flexible, allowing you to amalgamate multiple rooms into one zone or divide one room into multiple zones. It is therefore quite important that you understand what effect these different zoning options are likely to have on your results.

  • Adjacent and Overlapping Surfaces
    To indicate that two zones are next to each other and share common elements, you need to position them such that the shared elements are both adjacent and overlapping. You should check that they show up as dotted during inter-zonal adjacency calculations.

  • Some Element Types and Important
    ECOTECT doesn’t really care about the assignment of most elements in your model. However it does care a lot about FLOOR, WINDOW, VOID and PARTITION elements as these have very specific roles in a thermal analysis.

  • External Shading on the Outside
    External shading systems that should not contribute thermal mass or solar radiation to any zone must be placed on a non-thermal zone. Any one will do, one of your own creation or the Outside zone.

  • Use Underground and Adiabatic Surfaces
    Boundary surfaces on thermal zones that overlap planar objects on non-thermal zones are either treated as underground in the case of the Outside zone, or as adiabatic for all others.

Why Can’t I Just Use My CAD Model?

Unfortunately traditional CAD drawings simply do not contain the kind of material, operational and spatial information required by building simulation and analysis tools. Whilst some of the information in a 3D CAD model may be suitable for lighting and shadow analysis (where colour and transparency are the critical properties), it is almost entirely unsuitable for detailed thermal and acoustic studies. For a more detailed explanation of this, see the CAD Geometry vs Performance Analysis article.

In any thermal analysis, what you will most likely require are the internal temperatures for each room or space - from which comfort, gains and loads can be derived. However traditional CAD models have no real concept of ‘spaces’, they exist solely as a by-product of the layout of a series of disassociated geometry in the form of prisms and polygons. Without human interpretation, it is difficult if not impossible to automatically extract important information such as room volume, floor area or which windows affect which space. All of these are fundamental to any thermal analysis.

A Different Approach

Creating a thermal model in ECOTECT requires a very different approach. Thermal models are based first and foremost on the spatial arrangement of discrete zones, as spaces are termed in ECOTECT. All building elements must belong to a zone, so it is relatively simple to count up the number of floors, walls or windows within each zone. Using zones and their elements, ECOTECT is able to automatically calculate which zones are adjacent to each other as well as the exact surfaces through which inter-zonal heat exchange takes place. Also, zones can be assigned their own set of internal conditions, including occupant, lighting and equipment loads as well as thermostat set points or comfort limits.

Why Does It Work This Way?

As a zone-based approach is likely to be very different to what you have been used to, and will usually require you to make some abstractions or simplifications of your building, you will need to not only know the rules but actually understand them so that you can make the right decisions as you generate your model. The section attempts to explain the reasons behind the different rules and how they are applied.

Zones and Zoning

A thermal zone is defined in ECOTECT as a homogenous enclosed volume of air. In most cases this will be a single room, assuming that the air within it is able to mix freely so that the concept of an ‘average’ room temperature makes some sense.

Figure 1 - 'House', a sculpture by Rachel Whiteread in London's East End, demonstrates quite clearly how ECOTECT thinks about zones.

In certain cases, such as a bedroom and en-suite or kitchen and dining area, it may be appropriate to group two or more rooms into a single zone, simply because they are subject to the same thermal effects and will likely act as a single air volume. Alternately, in the case of a large space with windows on all sides, it may be more appropriate to subdivide the one room into multiple zones in order to detect differences between the north, south, east and west sides.

Figure 2 - Example apartment divided into multi-room zones.

For example, in the apartment shown in Figure 2, each en-suite can exchange air only with its associated bedroom. Thus, any significant difference in temperature between the two spaces will likely be short-lived, with the heat balance quickly restored by convection either through the open doorway or the air vents provided. This is true also of the kitchen and living areas, so it makes sense to use only three zones for the whole apartment.

If the kitchen area was served by a window on the other side of the building, then there would likely be some localized differences between it and the living area. In that case, it would probably make more sense to zone the kitchen and living area separately, with a shared VOID element between them.

In the same way, if you consider the open-plan office floor in Figure 3, even though it is one large space it has windows on all sides, so there will likely be significant localized temperature variation at different times of the day. If we modeled it all as one big zone, the higher temperatures due to morning sun on the east side would be balanced by lower temperatures on the shaded sides. This would happen again in the evening with low level sun heating up only the west side. If we looked at hourly overall average zone temperatures for the whole day, they would probably all look absolutely fine - so we would remain blissfully unaware of any localised overheating and assume that all parts of the zone were perfectly comfortable.

Figure 3 - Example office floor with a multi-zone room.

The usual way to approach such a situation is to divide the floor into five different zones, one along the perimeter of each of the four sides and one in the centre. The depth of each perimeter zone depends on the floor-to-ceiling height and the likely depth of solar penetration, being typically equivalent to 2.5h, where h is the ceiling height. Any deeper and you start getting the same type of averaging effects on zone temperatures. You can use a smaller depth but anything less than 1.0h tends to over-emphasise the effects of solar gains.

Whilst ECOTECT itself imposes no limit on the number of zones you can use in a building model, some of the 3rd part tools it exports files to may. Thus, if you intend exporting models to tools such as EnergyPlus, HTB2 or ESP-r, you should first check their documentation for any such limitations, noting that these tend to vary with different versions. Also, the more zones you have in a model, the longer calculations will take and more data you will end up having to deal with.

So Which Side of the Wall Should I Trace?

Almost all 2D plans and sections show wall thickness around the perimeter and between rooms. Ideally you would trace along the centre of each wall as the thermal model requires only single-plane boundaries around and between zones. The centre line gives the closest approximation between surface area exposed to outside conditions and an accurate internal volume, as well as making all the shared surfaces of adjacent zones meet up perfectly. However, wall centre lines are rarely drawn so there are usually no accurate intersection points to snap to, which kind of defeats the purpose of quickly loading in a building plan and tracing over it.

You basically have two options here, each with their own advantages and disadvantages:

Option 1 - Use Internal Wall Lines

This means tracing the internal boundary of each room and then setting the surface adjacency tolerance to accommodate each separation distance. When performing inter-zonal adjacency calculations, ECOTECT will then look for any parallel surfaces that are closer than the tolerance value and assume that they actually overlap.

Figure 4 - Tracing zones using internal room dimensions.

When zones created like this are extruded in height, a model similar to that shown in Figure 5 is created. It may look as if the wind whistles down the gaps between each zone however, if you have set the adjacency tolerance, ECOTECT does know that this is not the case.

Figure 5 - Thermal model with gaps between zones to accommodate wall thickness.

This solution is useful if you want to use the same model for accurate lighting and shadow calculations as it properly represents the internal surfaces of each zone. Then all you have to do is extrude all the windows outwards to represent their wall thickness and either make the extruded surfaces partitions or move them to a non-thermal zone. This way you have a model suitable for lighting, thermal and acoustic analysis.

Figure 6 - Model with extruded windows to simulate wall depth.

Option 2 - Use Top-Left or Bottom-Right

This requires choosing right at the start between left/right and top/bottom, and then consistently using the selected sides of each wall in each zone. All your zones will be slightly offset but still almost exactly equivalent to having used centre lines.

Figure 7 - Tracing zones using the right-bottom sides of each wall.

Inter-Zonal Adjacency

The process of determining which zones are immediately adjacent to each other is done by looking for co-planar surfaces on different zones that overlap each other. For example, if the ceiling plane of zone 1 overlaps the floor plane of zone 2, then ECOTECT will assume that zone 2 is above zone 1 and that the ceiling/floor intersection area represents a heat flow path between the two. Similarly, if the east wall of zone 1 overlaps the west wall of zone 2, ECOTECT will assume that the two zones must be next to each other and share a wall which also serves as a heat flow path.

The implication of this is that each zone in a model must have a complete set of bounding surfaces on all sides. As explained in the main help file, imagine that the space within each zone were full of water. No matter how you shake, tilt or spin the zone around, no water should be able to leak out.

If a zone is meant to have an opening to the outside or another zone, then you should model that opening as a VOID object so that ECOTECT will know it is meant to be there and deal with it appropriately. If the opening is between two zones, then the other zone should obviously have a matching VOID object, this way ECOTECT will interpret that you intend a free flow of air and heat between the two.

Simplifying the Modelling Process

Imagine a situation where you need to analyse an apartment in a multi-storey tower block or a unit in a large row of terrace houses. For absolute accuracy you would need to model the whole building or row in order to fully account for heat flow paths between floors or party walls. However, in many cases it is possible to assume that temperature differences between adjacent apartments/units will be insignificant and that the major influences on each zone’s thermal environment will be its internal conditions and those areas of the building fabric exposed to the outside. If this assumption is reasonable in your particular case, then you may only need to model the actual apartment/unit you are interested in.

To make this work, you must be able to let ECOTECT know that it is to ignore heat flow through certain elements in the model. This can be done by either assigning those elements as PARTITIONS or by creating a non-thermal zone immediately adjacent to them.

Partitions and Adiabatic Surfaces

PARTITION objects in ECOTECT are special in that they are assumed to be completely internal to a space and, as such, are never checked for inter-zonal adjacency, exposure to outside or for the incidence of solar radiation. They serve only to add internal mass to a zone. You can use PARTITIONS for screens, half-height walls, office partitions and even furniture.

Figure 8 - Internal partitions within a zone.

If you change one of the boundary elements of a zone to a PARTITION, you are essentially telling ECOTECT to ignore everything except for its thermal mass effect, making it an adiabatic surface (one through which there is no heat exchange). Even if it is directly exposed to the outside or adjacent to another zone, there will be no heat flow through it and no calculation of incident solar gains. However, it is also removed from the surface area calculations for the zone to which it belongs. If it was previously a CEILING, ROOF or WALL then this is not a problem apart from the possibility that a ‘Surface Area Too Low for Calculated Volume’ warning may be reported. However, if it was previously a FLOOR then the zone will have a reduced total floor area, affecting both the distribution of internal gains and the calculation of heating and cooling loads per m2. To avoid such problems, you can instead use an adjacent non-thermal zone.

Non-Thermal Zones and Adiabatic Surfaces

Non-thermal zones do not participate in volume, temperature or heat load calculations. They are used to hold items such as construction lines, fences, trees, shading devices and other non-participatory geometry. Elements on non-thermal zones will both shade and reflect light onto other objects in the model, but incident solar radiation and overshadowing calculations are not performed for them. No attempt is made to calculate their volume and the objects they contain will not have shading masks generated for them.

When a boundary surface on a thermal zone is found to be immediately adjacent to another surface belonging to a non-thermal zone, ECOTECT will interpret it as an adiabatic surface. It’s thermal mass will already have been attributed to the zone but, as non-thermal zones have undefined internal conditions, no heat flow across the surface is calculated. Also, because some or all of the surface is adjacent to another, its exposed area and shading mask will have already been properly calculated, thereby reducing solar gains by the appropriate amount.

As volume calculations are not carried out on non-thermal zones, they do not have to be in any way geometrically complete in the same way as thermal zones. Thus you are free to use just enough objects in the non-thermal zone to overlap the required building surfaces. Similarly, you do not have to use different non-thermal zones for different sides or floors in a building. You may choose to have just the one non-thermal zone adjacent to any number of thermal zones.

Figure 9 - Adjacent objects on a non-thermal zone used to indicate that side walls are adiabatic.

Having said that, it is usually better if you create your non-thermal zones so that they actually look like parts of the adjacent building instead of just using individual adjacent surfaces. This is because it is much more obvious to other people when they look at your model to see what you are doing. Without this extra geometry (even though it serves no other useful purpose), it is difficult for anyone else to distinguish the fact that you have a series of overlapping objects on different zones and that the affected surfaces are not actually exposed to outside conditions.

Figure 10 - 'Complete' adjacent non-thermal zones, shown as semi-transparent to visually indicate their role as 'phantom' zones.

You will note in the above example that the extra geometry is semi-transparent. This is done for no other reason than to communicate visually the idea that these are ‘phantom’ parts of the building. It is important to note first that the actual overlapping surface are NOT semi-transparent - if they were then some amount of solar radiation would pass through and fall on the adjacent surfaces. Secondly, if the ‘phantom’ parts of the building ever cast shadows on any of the real parts, then it is not appropriate to make them semi-transparent as this will reduce their calculated shading effect.

The Outside Zone and Earth Berms

Surfaces that are adjacent to objects on the Outside zone are special in that they are considered to be underground. This is also true of any exposed surfaces that project below the ground plane (Z=0).

This allows you to quickly create underground buildings by moving them downwards, or simulate sloping ground and/or earth berms by creating objects adjacent to the underground surfaces and placing them on the Outside zone.

Figure 11 - An example earth berm up the side of a building, constructed using objects on the Outside zone.

Ground temperatures are automatically calculated in ECOTECT based on both the annual average temperature and a 60-day running average. A series of 10 graduated depth bands are used to interpolate between hourly temperatures at ground level and the annual average temperature 2m below. During the inter-zonal adjacency calculation, the percentage area of each underground surface within each depth band is recorded for use in hourly heat-flow calculations. For horizontal elements adjacent to the ground, the method outlined in ISO 13792:2005(E) for floor slabs is used.

Zone Settings

Each thermal zone in ECOTECT is assigned a set of internal operating conditions. It is important that you become familiar with these settings as they can have an enormous effect on both temperatures and loads in each space.

These operating conditions include the number of people in the space, heat gains from equipment and lighting, ventilation and infiltration rates, comfort and thermostat temperatures as well as operating times. Many of these parameters can be assigned complex schedules that control their operation on an hour-by-hour basis over the whole year.

Figure 12 - The Zone Management dialog in ECOTECT.

One very important setting is the type of heating, ventilation and air-conditioning (HVAC) system used within the space. These are chosen at a highly conceptual level (air-conditioned, heated, cooled, naturally ventilated, etc), however it is important to note that ECOTECT itself is concerned only with comfort (for passive buildings) and space loads (for mechanically serviced buildings) - not with the detailed analysis of supply systems installed to meet those loads. Whilst it is possible to enter a coefficient of performance (COP) for any system to estimate overall energy requirements, real HVAC systems have variable efficiencies that depend on complex factors such as flow rates in ducts, fan efficiency curves and a whole range of equipment sizing issues.

ECOTECT’s aim is to allow the designer to generate the most efficient building with the lowest possible space loads. It is typically the mechanical engineer’s job to meet any residual loads that cannot be met using passive means with the most efficient and appropriate mechanical systems design.

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