Modelling Hybrid Ventilation Units for part L2 and EPC purposes

Modelling hybrid ventilation units for part L2 and EPC purposes

Hybrid ventilation units have become very popular in recent years, often being used in priority schools as a lower energy alternative to mechanical ventilation or air conditioning. The units are available from a variety of manufacturers and often have a range of different ‘ventilation modes’ depending on the current internal and external conditions, they’re sometimes installed to help pass the BB101 assessment.

They are usually used in conjunction with openable windows and allow for air to naturally flow through the space when this will result in a comfortable environment for the occupants. When natural ventilation would result in an uncomfortable environment, either by high internal temperatures being reached or cold draughts near the windows the hybrid ventilation units change to a mechanically assisted mode. 

A mechanically assisted mode can help alleviate high temperatures or CO₂ levels by providing a boost to the natural ventilation rate via running an integrated fan, meaning more fresh air is pulled into the space. Alternatively if the external temperature is very low the units often have an air mixing mode in which fresh air is mixed with air from the occupied space before being supplied, this means that the space can be provided with fresh air without the risk of occupants experiencing cold draughts.

Many of the units also utilise a ‘night mode’ in which the unit provides mechanical ventilation to reduce the temperature in the occupied space overnight. This can make use of the thermal mass within in the space while the outside air is cooler but it’s not possible to open the windows.


Image from Monodraught’s hybrid ventilation brochure showing some of the different modes of operation for their units

As the fans work in conjunction with natural ventilation and are not needed at all times these units often boast low Specific Fan Powers (SFPs) and can save on energy consumption in the building when compared to a full mechanical ventilation system. The difficulty with modelling these units for Part L2 and EPC assessments however comes from the regulations assessment methodology. 

The L2 and EPC assessments are a comparisons against similar ‘notional’ and ‘reference’ buildings, these buildings will have the same building envelope size and location but will use set HVAC efficiencies. To make this possible the regulations outline that a HVAC system type for each space in the building must be selected from a list of pre-set options, for these units there are two possible options, naturally ventilated and mechanically ventilated. There is currently no option to model the hybrid units as they perform in reality for this assessment.

CO₂ emissions comparison for an actual, notional and reference building produced by Tas.

Modelling the spaces served by these units as naturally ventilated results in all fresh air requirements in your ‘actual’ building and the ‘notional’ building spaces to be provided naturally, thus no fans are included in the assessment and the benefit of the low SFPs achieved are not accounted for.

Unfortunately choosing to model the spaces as mechanically ventilated is also not ideal as this will result in having all air provided to the spaces in both the ‘actual’ and ‘notional’ building be supplied mechanically. As the units are not designed to work in this manner it is unclear if the low SFPs would still be applicable, additionally the units will be compared against mechanical ventilation heat recovery (MVHR) units which will be sized to provide all of the required fresh air mechanically and so are likely to have higher SFPs. This is an unfair comparison as the hybrid units do not necessarily have to be sized to provide the whole fresh air requirement to the spaces they serve. The heat recovery adds additional complexity as many of the hybrid units do not have mechanical heat recovery and so in some cases being compared against a space with MVHR units can have a negative impact on the results due to higher heating loads vs the ‘notional’ building.


Two CO₂ emissions comparisons for the same priority school building with hybrid units, the one on the left has the spaces served by the units modelled as naturally ventilated and the one on the right has them modelled as mechanically ventilated.

So how are you meant to model hybrid ventilation units for the purpose of L2 and EPC assessments if neither option is ideal? We asked CIBSE for clarification on the matter as they’re the governing body for all EPCs lodged in-house at EDSL and their response was as follows:

“We had this question a few times in the past, for project that are mostly schools where these appear to be popular.

We don’t have a definitive answer to this, we appreciate the arguments on both sides and the two different approaches.

The NCM modelling guide in paragraph 46 states that if a mechanical ventilation system only works on peak conditions and it otherwise it generally is natural ventilation, then the space should be considered naturally ventilated and any mechanical ventilation aspects ignored. This is the approach that the people who opt for the natural ventilation option would follow.

On the other hand, there is no clear line of what would be considered only working at peak conditions etc., the line is not clearly drawn, and because the system does indeed include a fan, a lot of assessor would consider this a mechanical ventilation system with a very low SFP.

We have advised in the past that we tend to lean towards the guidance in the NCM modelling guide, but generally advised assessors to liaise with Building Control to agree an acceptable approach.

I hope this helps a bit, I am afraid there is no clear cut right/wrong answer, these system appear to be too complicated for the NCM as it is structured today.”

So to summarise there isn’t a clear cut answer as not all applications of hybrid units are the same. If the majority of the ventilation to the space throughout the year is via natural ventilation and the mechanically assisted modes kick in during peak conditions (high CO₂ or temperatures) then we would suggest these should be modelled as naturally ventilated spaces for the purpose of Part L2 and EPC assessments. However if the majority of the ventilation throughout the year is provided by a fan assisted mode there is no clear option and it is advised to liaise the Building Control Officer (BCO) for the building.

The problem with modelling these hybrid ventilation units doesn’t come from limitations within the software package but with the current methodology set out by the regulations, perhaps in the future the methodology will be updated and there will be a clearer cut method for modelling these units in L2 and EPC assessments.


If you have an L2 or EPC assessment either involving or not involving hybrid ventilation units you would like assistance with, feel free to contact our consultancy department.

We also have a guide on how to model Breathing Buildings NVHR units if you would like more in depth information on this.

EDSL UK Weather Data Guide

UK Weather data sets guide

There are so many different weather files available these days and with a large number of assessments, each specifying weather data files to use it can be hard to keep track of which weather files are required for which assessments. We’ve put together a guide outlining most of the applicable weather files for assessments that are commonly undertaken using our software to help keep track of when to use each weather set. Here are some of the commonly used weather data sets:

CIBSE TRY (Test Reference year) – This is designed to be a typical year of weather data, it is composed of 12 separate months of data which are not necessarily from the same year, each month chosen to be the most average month from the collected data. The 2006 TRY files are based on weather data from 1984-2004, the 2016 TRY files are based on weather data from 1984-2013.


CIBSE DSY (Design Summer Year) – This represents a warmer than typical year. For the 2006 DSY files the year with the third hottest summer from 1984-2004 was selected. For the 2016 DSY weather files the methodology was updated and three files for each location were produced from the weather data recorded from 1984-2013:

DSY1 – A moderately warm summer.

DSY2 – A summer with a short intense spell.

DSY3 – A summer with a longer less intense warm spell.

CIBSE future weather files – These are predicted future weather files based on the UKCP09 projections and are available for a number of years with different emissions scenarios and percentiles. The percentiles represent that likelihood that the mean air temperature will be less than predicted. The CIBSE TRYs and DSYs are available for the following scenarios:
2020s – High emissions scenario – 10th, 50th, 90th percentile
2050s – Medium – 10th, 50th, 90th percentile
2050s – High – 10th, 50th, 90th percentile
2080s – Low – 10th, 50th, 90th percentile
2080s – Medium – 10th, 50th, 90th percentile
2080s – High – 10th, 50th, 90th percentile
PROMETHEUS Exeter University future weather files – These are also future weather files created using UKCP09 and were created as a set of future weather data for free distribution and use by industry and academics. The files are available from University of Exeter’s website and are available for the following emissions scenarios for a number of locations:
2030s medium (A1B)
2030s high (A1FI)
2050s medium (A1B)
2050s high (A1FI)
2080s medium (A1B)
2080s high (A1FI)

Assessments and the current guidance on weather data files given in them


L2 & EPC (2013): The (November 2017 update to the) NCM modelling guide states that the 2006 CIBSE TRY must be used. Please note that CIBSE are seeking endorsement for use of the new weather sets in compliance calculations so this may change to the 2016 TRY files in the future. CIBSE have produced a weather locations look-up EXCEL sheet which states which weather file should be used depending on the postcode of the building, this can be downloaded here.


TM52 (2013): it is suggested that an appropriate DSY weather file is used in the simulation. We would suggest checking which location weather file would be most appropriate for the building and initially running with a DSY1 weather file.

TM59 (2017): Developments should refer to the latest CIBSE DSY weather files and it is required to pass the assessment with the DSY1 file most appropriate to the site location, for the 2020s, high emissions 50th percentile scenario. Other files including the more extreme DSY2 and DSY3 files, as well as future files (i.e. 2050s or 2080s) should be used to further test designs of particular concern, but a pass is not mandatory for the purposes of the simpler test presented in this document.

BB101 (August 2018): The CIBSE DSY1 50th percentile range 2020s weather file most appropriate to the location of the school building should be used for the thermal comfort assessment.


BREEAM HEA04 thermal comfort – Adaptability for a projected climate change credit: PROMETHEUS project at Exeter University projected climate change weather files. For free running buildings the 2050s medium (A1B) emissions scenario DSY should be used. For mechanically ventilated or mixed mode buildings the 2030s medium (A1B) emissions scenario DSY should be used.

GN32 – Prediction of operational energy consumption: The current 2016 CIBSE TRY weather file may represent “typical” weather. For extreme weather DSY weather files should be used. The current DSY1 weather file is recommended to represent weather outside of London, whilst DSY data sets reflecting urban, semi-urban and rural locations are recommended for locations in greater London – guidance on this is given in TM49.


Heating & Cooling Loads: If not using cyclic or steady state design days for loads assessments, then a dynamic assessment could be used. If so then the modeller is free to make the most appropriate choice, taking into account the location of the building and if they want to allow for future changes in the weather. Often TRY files are used but DSY files could be used for cooling loads to account for warmer summers and worst case values.

Energy Models: There isn’t specific guidance on which weather file should be used, again the modeller should make the most appropriate choice for the building. A 2016 TRY file appropriate to the building location may give the best all round results but the modeller can use DSYs and future weather files to check the impact of warmer summers and future climate changes on the building performance.


ONC – A Calibrated Energy Model

What is a Calibrated Energy Model?

A Calibrated Energy Model (CEM) is a digital twin of a building created in a thermal modelling tool that is then fed inputs from actual metered data and BMS readings once the building is completed and occupied. This allows the thermal model to be calibrated with the actual installed values for efficiencies, energy use and HVAC component measurements, the model can then be used to predict the impacts of building control changes accurately. It can also highlight ‘problem’ areas in the building and BMS.

Although on more complex projects CEMs can take a lot of work and time to set up they can be extremely useful in testing how suggested renovations or HVAC control changes will perform before these are actually carried out. If we know that the CEM is predicting accurate results by comparing these to meter readings from the actual building, tweaking values in the CEM should give us a good indication of what effects these tweaks would have if carried out on the actual building. This could for example be used to test the effects of updating the VRF system on the CO2 emissions and costs of electricity in the building and thus be used to predict how long the payback period for the upgraded system would be. Another example could be to test how a number of different glazing specifications would perform for a building in need of an update to its façade; the most cost effective option for the long term could then be identified and installed

One New Change Project

EDSL worked in conjunction with a number of companies including Demand Logic, Landsec and NG Bailey to produce a CEM for the One New Change (ONC) building in London. To start with a 3D model of the building was created and the areas to be included in the analysis were then zoned. There were 2 major areas to be included in the analysis, the offices and the retails areas, with retail being the first two floors, offices above and some restaurant and hospitality on the roof.

EDSL worked in conjunction with a number of companies including Demand Logic, Landsec and NG Bailey to produce a CEM for the One New Change (ONC) building in London. To start with a 3D model of the building was created and the areas to be included in the analysis were then zoned. There were 2 major areas to be included in the analysis, the offices and the retails areas, with retail being the first two floors, offices above and some restaurant and hospitality on the roof.

As the building was so large and the HVAC systems utilised a number of different technologies the work on calibrating the systems was time consuming. The building uses 13 Ground Source Heat Pumps (GSHP) for heating and cooling, as shown in the geothermal ground loop pipework layout screenshot below. The water loops for these were set into the underground piles that support the building structurally.


The geothermal ground loop pipework layout schematic for ONC showing the location of all the ground source water loops:

Here is an annotated screenshot of the Tas systems file showing the waterside systems that were set up for the building:

The Retail units were calibrated using the metered data, this allowed for the internal gains to be tweaked over a number of iterations until the model was in good agreement with what was being observed in reality.

A phenomenon seen at ONC from the logged BMS data and meter readings was a substantial amount of simultaneous heating and cooling by the office FCUs in the building. As we were building a CEM this sub-optimal HVAC system behaviour had to be accounted for to compare old and new strategies, both before and after the Energy Conservation Measure (ECM) is implemented. Being able to model simultaneous FCU heating and cooling was an important part of the project.

Simultaneous heating and cooling can happen for a number of reasons. After extensive study of the FCU BMS logged data at ONC, there seemed to be two primary causes for this. Firstly, the control system in the FCUs was not sophisticated enough to control temperatures to the current settings, which were very exacting (effectively trying to control to within 1C, with only a 0.5C deadband). This was termed ‘hunting’ and had been confirmed and explained by the N G Bailey team, who put forward a proposal to cure this issue by widening the deadband.

Secondly, FCU setpoints were varying significantly within zones of the building. This means neighbouring spaces were at different temperatures and this leads to air mixing between these areas due to stack pressure effects. For example a typical FCU on level 4 had the following varying setpoint signal for the year (from 1st January to 11th June 2016).

The next chart shows the space/return temperature, so the unit is clearly attempting to follow this erratic setpoint control.

The approach to try and simulate the effect of ‘hunting’ was allowing the heating and cooling to have overlapping control bands. So, for example, heating and cooling control would be as follows.

This arrangement gives both heating and cooling between 22C and 22.25C and accounted for the hunting problem quite well. Once the model was calibrated the models simulated results were in agreement with the billing energy information to a fairly accurate degree.

The graph above shows the whole site monthly energy consumptions, the blue bars are the billing information for the building and the red bars are the simulated predictions from the CEM. (Two of the biggest differences can be explained – January billing data included part of December’s usage and June billing information did not include gas usage). The model was then also able to quantify the energy savings following an update on the humidity controls on the HVAC systems for ONC. The model was calibrated around July 2015 and so the July to December results are a better fit than the 2016 results. The building is always changing (so the results drift) which shows a calibrated model needs ‘maintenance’ to keep it up to date.

Implementing an Energy Conservation Measure (ECM) using the CEM

Below is a graphic showing the first ECM implemented at ONC using the CEM, the aim was to include for a change on humidity controls in the building and use the CEM to predict the energy savings from this change. The top left green box shows the original calibrated model performing close to the monitored BMS data. The red lozenge shows how the system would have carried on if the ECM hadn’t been implemented, notice that this includes for a lot of full cooling demand. The bottom right green box shows a Tas model with the ECM in place comparing well to the BMS data after the change, so it’s been implemented correctly and performing as expected. The vertical green line, where the two boxes touch at around April 19th, is when the ECM was introduced. So the lowest graph before the vertical line shows just how much could have been saved had the change been introduced earlier!

The ONC project was highly ambitious due to the sheer size of the building and the complexity of the HVAC systems, with a smaller less complex building the task would be much easier to carry out. A second project at Dashwood House was completed in less than a week. Although creating CEMs for buildings isn’t currently a very commonly undertaken task there may be an increasing need for these in the future, not only are there great benefits in the long run in terms of ensuring building efficiency and saving money on running costs, but as energy targets become more stringent and there is greater responsibility placed on building designers/managers to ensure/prove buildings are performing as efficiently as they’re designed to be. As developments in BMS monitoring and thermal modelling simulation software continue to be made, creating CEMs may become a simpler task to carry out.

If you found this blog interesting you may want to take a look at this somewhat similar article reviewing in use building performance from the November 2020 CIBSE Journal in which the thermal modelling was carried out by Hilson Moran using our software. Tas Systems allowed for the waterside systems to be modelled in great detail.


If you think that you would benefit from having a CEM created for a building contact our Consultancy department for help.

3D Visualisation

Using the 3D Visualisation to help sanity check your TBD file

Setting up the TBD correctly is a vital part of carrying out any thermal modelling in Tas, whether it’s for an L2 & EPC assessment, an overheating assessment or an energy model. It’s very important that the TBD file is set up appropriately for the analysis being carried out, as any errors or mistakes in the TBD could impact the results of the analysis and thus invalidate them. Ultimately it is the users responsibility to ensure that the correct constructions, internal conditions, apertures etc. are applied throughout the model, but Tas does have some great tools to assist with this.

The TBD file does give errors and warnings for a number of things when you run the pre-simulation checks under Tools in the toolbar, as shown in the image below.


The pre-simulation checks are a brilliant tool for highlighting common omissions and mistakes, it is a great place to start when performing a sanity check of the model but they are not able to decipher when the wrong information has been applied in the model. For example you would get an error message if no construction is applied to the windows but if you have applied the ground floor construction to your windows accidentally, the pre-simulation checks will not detect this.

The 3D Visualisation is a great tool to sanity check many components of your model and we strongly recommend using it to aid in checking your TBD is set up correctly. If you click on the 3D Visualisation icon in the toolbar (as shown in the image below) the 3D Visualisation window will appear with your building geometry displayed.

Only zoned areas of the building will be displayed in the 3D Visualisation. If there are areas of the building missing that you think should be included in the analysis you will to check these are zoned correctly in your 3D modeller file, then re-export and merge with your TBD file. Right clicking in the 3D Visualisation window will allow you to change the settings of the 3D Visualisation, you’re able to adjust the display colour of the visualisation to be based on a number of options including zone colour. 

Using the 3D Visualisation to check building elements

While the 3D Visualisation window is open you can expand the building elements folder in the tree-view and click one of the building elements, this will highlight where this building element is applied in the model. This is a quick way to check the correct building elements have been applied to the correct surfaces. If you have 2 external wall types, for example, with differing constructions and U values, you can highlight each of these separately and ensure that the model has these assigned in the correct areas.

By highlighting the “curtain wall N/E/W” building element I can quickly see that this element has been applied in the right locations and hasn’t been applied to the south facing curtain walling.

If you have any building elements for which you are unsure what construction to apply, you can use this feature to find where the building element is applied and then choose the appropriate construction.

Using the 3D Visualisation to check zones & internal conditions

Expanding the zones folder in the tree-view and selecting a zone will highlight this zone in the visualisation. You can quickly cycle through the zones using the arrow keys on your keyboard to check the zones in the model have been applied to the correct areas of the building. If you have any HVAC groups, Zone groups or Zone sets created in the model selecting these in the tree view will highlight all zones assigned to this group simultaneously. This feature can be useful to check the correct zones have been assigned to each group, for example if you have a HVAC group called “3F VRF” and see that there is GF zone applied to this in the visualisation you know this will need looking into.

Here I can see that there is a 1F zone assigned to the HVAC group “2F Office VRF group”, I can now look into ensuring the 1F zone is assigned to the correct HVAC group.

Checking the locations of the zones in the model using the visualisation is also a great way to find any non-contiguous zoning. You will likely get a warning about non-contiguous zoning in the pre-simulation checks if this is present in your model but highlighting the zone and its location in the 3D Visualisation is the quickest way of locating the issue.

Highlighting an internal condition in the tree view will highlight all areas where this is applied in the geometry, for larger models this may not be very useful as it will be hard to check the correct areas have this internal condition applied visually but it can be useful for smaller models as shown below.

By highlighting the toilet internal condition I can see there is a large space on the 3F that has incorrectly been assigned the toilet internal condition, this space should have the office internal condition applied.

Using the 3D Visualisation to check construction and aperture types

Selecting a construction in the tree view will highlight all surfaces this has been applied to. Cycling through the constructions using the arrow keys while the 3D Visualisation window is a fantastic way to sanity check the constructions assigned in the model. This feature is key to finding instances in which an incorrect construction has been assigned, for example if you have a different glazing specification for south facing glazing you can easily identify any cases of this being applied to the wrong façade(s). 

If you have a ceiling void modelled in the 3d modeller but have applied the default internal floor building element to this space and applied the default internal ceiling element to the space below this will likely come through as an internal floor/internal ceiling in the building simulator. It’s quite likely that the construction between a void and the space below will be different to the construction between one occupied floor and another and any cases of the incorrect construction being applied can easily be identified. 

Here I can see that the same internal floor construction has been applied between occupied floors and between the 4F and the void space above. I know that the construction between the 4F and the void is not a concrete floor and is plasterboard so I can amend this.

Selecting an aperture in the tree view will unsurprisingly show all instances where this aperture is applied in the 3D Visualisation, this is useful for checking you haven’t accidentally assigned the aperture to any incorrect building elements.

 “Top-tip”: if you change the colour of all openable windows to one colour in the 3D modeller file you can quickly check the correct windows have apertures assigned using this feature.

Because I know I made all of the openable windows green in the 3D modeller file I know that the blue window has incorrectly been assigned the aperture function in this model and it needs to be removed from this building element.

The 3D Visualisation is a great tool for quickly checking your TBD file for errors which could otherwise be easily over-looked. If you’re not currently a Tas user but are interested in trying out our software you can get a free trial from our website, because we are unable to offer our usual face to face software training our e-training  is currently all free!

CBDM Case Study

Using CBDM to improve daylight quality in my classroom design.

Climate Based Daylight Modelling (CBDM) has been around for a while now and it’s often a requirement that it’s carried out on buildings, particularly schools, to test the quality of the daylight that can be expected within the occupied spaces to help reduce electrical lighting energy and improve occupant well-being. Rather than simply taking a design, carrying out the CBDM assessment on it and tweaking the internal surface reflectance values and/or glazing transmission to achieve a “passing” result let’s look at using CBDM as an iterative design tool to find a daylight solution that provides desired results for our classroom. Starting with a very basic design let’s look at how we can alter this, investigate the effects on the daylight availability and compare the results obtained from 6 different classroom designs.

To start with a simple strip of six 1m x 1m windows was modelled along the façade for our fairly deep south facing classroom with a floor area of 70m2 as shown in the images below.

The CBDM report produces results in the form of Useful Daylight Illuminance (UDI) metrics which are as follows:

  • UDIs Supplementary Annual occurrence of illuminances less than the acceptable lux level.
  • UDIa Acceptable Annual occurrence of illuminances between the acceptable lux level and the excessive lux level.
  • UDIt Target Annual occurrence of illuminances between the target lux level and the excessive lux level.
  • UDIe Excessive Annual occurrence of illuminances greater than the excessive lux level.

UDIs corresponds to time where electric lighting will be required in the space. UDIa corresponds to time where the daylight is acceptable and electric lighting isnt required. UDIt corresponds to time where the desired target illuminance is met by daylight. UDIe corresponds to time where the daylight is excessive and may be too bright for occupants.

For this project the acceptable lux level was set to 100 lux, the target lux level was set to 300 lux and the excessive lux level was set to 3000 lux. These values may well change between projects and are set by the user in the CBDM tool before the calculation is carried out. The weather file used in this project was the 2005 CIBSE London TRY.

The CBDM report also produces results for the Spatial Daylight Autonomy (sDA); this is the percentage of area that is above a specified lux level for a specified percentage of the time or more (t/50% corresponds to the target illuminance for 50% of the occupied time) and the UDIa Min; this is the minimum result for the grid point with the lowest average UDIa result and so is useful in investigating the uniformity of the average UDIa result.

Classroom Design 1 Results:

Floor plans showing the average UDIs, UDIa and UDIe from left to right. As the UDIs reports areas that have lux levels below the acceptable level the light parts of the image correspond to areas that are not getting sufficient light and vise versa.

The average UDIt result of 25% from this assessment shows that the majority of the space is not achieving the target illuminance level of 300 lux for a significant portion of time, looking at the floor plans above it’s evident that not enough space is reaching the back of the classroom and you can also see a small patch by the windows that is occasionally getting too much daylight in the average UDIe floor plan on the right. The average UDIa floor plan in the middle shows a band around the middle of the classroom that is only achieving an acceptable lux level between 33-67% of the time.


One option to allow more daylight into the back and middle of the classroom is to increase the height of the windows, allowing a larger amount of daylight to enter the space and increasing the angle at which it can penetrate into the space. To investigate this the window heights were increased to 2m for design 2.

Classroom Design 2 Results:

Floor plans showing the average UDIs, UDIa and UDIe from left to right. As the UDIs reports areas that have lux levels below the acceptable level the light parts of the image correspond to areas that are not getting sufficient light and vise versa.

The change in window height resulted in an increased average UDIt and UDIa results of 36% and 65% respectively, this means that the space now achieves its target illuminance level and its acceptable illuminance level more often than in design 1. The average UDIe result has also increased to 11% however, this means that the excessive lighting level of 3000 Lux is now being reached more than before and there is an increased risk of glare and occupant discomfort.

If we look at the Minimum UDIa we can see that this is quite low when compared to the average UDIa, this signals that there are still areas in the space that are not achieving sufficient daylight and that the distribution of the light within the space could be improved upon. Inspecting the floor plans we can see we are still not achieving the desired daylight at the back of the classroom and the front of the classroom is often receiving excessive daylight levels.

To attempt to reduce the excessive daylight in the space an external shade system consisting of 6 fins at an angle of 165° was added in front of the south facing windows in the Tas 3D model, a screenshot of the 3D model showing the shades can be found below. An angle of 165° was chosen as this would block more daylight from entering the front of the classroom (where we are experiencing excessive lux levels) when the sun is positioned high in the sky, whilst still allowing light to penetrate into the back of the space when the sun is at a lower angle.

3D image from model showing shades included.

Classroom Design 3 Results:

Floor plans showing the average UDIs, UDIa and UDIe from left to right. As the UDIs reports areas that have lux levels below the acceptable level the light parts of the image correspond to areas that are not getting sufficient light and vise versa.

Introducing the shade reduced the average UDIe to 6%, it did however also reduce the average UDIa and UDIt results in doing so. Looking at the reduced UDIe figure and the UDIe floor plan informs us that the shade is successful in reducing daylight in the desired area of the classroom. If the UDIe had not come down this would inform us that the shade is not reducing the light entering the space at the times when excessive light levels are reached and that we need to look at refining its design, possibly by adding fins or adjusting the angle of the fins.

If the shade was not effective at reducing the average UDIe result we could use the CBDM results viewer to see at what time of the year the different parts of the space are too bright or dark, then use the display sun option in conjunction with shadows displayed to position the sun at the right location for that hour in the year and make an intelligent estimate for what sort of shade angle would rectify the problem we are trying to solve.

Our next step is to address the lack of daylight at the back of the classroom, one option to do this would be to add rooflights directly to the space, another possibility is to add windows into the top of the corridor and internal windows from the corridor to the classroom so that light can be shared between the corridor and the back of the classroom. To compare these two strategies the side-lit windows were reverted to the 1m tall windows and the external shades were removed so that the effect on the daylight levels reached at the back of the classroom by could be investigated. Both design options were run on identical classrooms at the same time to investigate which would be more appropriate.


Classroom design 4 includes two rooflights each 1.5m x 1m as shown in the model images below:

Classroom 5 includes high level windows into the corridor and internal windows between the corridor and classroom as shown in the 3D model image below:

*The roof constructions have been displayed as transparent to show the internal windows.

Classroom Design 4 & Classroom Design 5 Results:

Floor plans showing the average UDIs, UDIa and UDIe from left to right. As the UDIs reports areas that have lux levels below the acceptable level the light parts of the image correspond to areas that are not getting sufficient light and vise versa.

From the results it is clear that the rooflights are much more effective at introducing light into the back of the classroom with an average UDIa of 85% we know that the light in the space will be at acceptable levels without the use of additional lighting for a large portion of the year.

The floor plans clearly show the difference in the light distribution between the two methods if using shared light from the corridor was the desired approach the design would have to be refined, perhaps high level corridor windows aren’t sufficient or the internal windows need a larger area for the back of the classroom to receive sufficient light.


I was interested to see how the rooflights would perform in combination with the 2m tall windows and external shade system so this was set up as Classroom Design 6.

Classroom Design 6 Results:

Floor plans showing the average UDIs, UDIa and UDIe from left to right. As the UDIs reports areas that have lux levels below the acceptable level the light parts of the image correspond to areas that are not getting sufficient light and vise versa.

This provides the highest average UDIa and average UDIt of all of the design options modelled so far but the UDIe has also crept back up to 9%. It has to be considered if the small increase in average UDIa and average UDIt are worth the cost of installing the larger windows and shade system in addition to the increased risk of excess lighting. If it was decided that this is the desired design solution but the average UDIe of 9% was a concern then we could look at adjusting the external shade to further reduce excess light at the front of the classroom or add internal blinds onto the rooflights, these 2 options would address the areas of the classroom that are effected by excess daylight that can be seen in the floor plan on the right.



To summarize let’s look at all of the results together.

The initial design including 6 1m x 1m windows did not achieve acceptable daylight levels for a large proportion of the year, changing the height of the windows to 2min design 2 increased the acceptable daylight levels in the classroom but it also meant that excessive daylight levels were experienced more often near the windows. The shade that was added in design 3 reduced the excessive daylight along the south facing side of the classroom but also reduced the average UDIa and UDIt results we were trying to improve. Designs 4 and 5 were two options aimed at increasing the amount of daylight in the back of the classroom, design 4 utilized rooflights and design 5 aimed to share light with the corridor via high level windows. For designs 4 and 5 the windows were reverted to 1m x 1m windows and the shade system was removed, the rooflights proved a better option for our classroom and greatly increased the amount of time the classroom reached acceptable light levels with daylight alone. For Design 6 the rooflights were combined with the 2m tall windows and shade system, this did further increase the useful daylight in the space albeit slightly but it also increased the risk of excessive daylight and glare.


Using Tas CBDM tools distributed calculations makes is easy to quickly asses the daylight results for a number of different design options and find one that suits the space in question, being able to investigate the average UDI floor plans for the space allows a designer to clearly see which areas of the space are receiving the desired daylight and which areas could be improved on.

If you’re interested in carrying out CBDM assessments but don’t have a Tas licence you can contact us for a free trial of the software and learn how to carry them out or contact our consultancy department who can assist in carrying the assessment out on your building design.

Build2Perform 2020 Presentation

Our 2020 Build2Perform presentation on the future of building simulation

In-case you missed it at CIBSE’s Build2Perform conference last week here is Andrew Hilmy’s short presentation covering a variety of topics related to the future of building simulation. The video is only 6 minutes long but gives a quick insight into a wide range fast developing areas in the industry such as machine learning, more realistic modelling of sporadic occupant behaviour, smart glass, computational fluid dynamics (CFD) and calibrated energy models.


What is a TM54 assessment and should you carry one out on your building? 

I’m sure you will have heard about the performance gap between design stage estimations of energy consumption and real life energy consumption upon building completion, there can be a number of reasons for this such as the on-site management of the HVAC systems, occupants behaving unexpectedly, changes in the later stage of building design that weren’t accounted for or a poorly implemented “soft landings” strategy.  Another key reason for the performance gap could be that there wasn’t actually an assessment carried out to estimate energy usage of the building design!

In the UK an L2 assessment must be carried out on any new commercial building to ensure it’s design is efficient enough – this assessment is a comparison against a similar “notional building” that has the same geometry and location but uses standard efficiency values and constructions. Building occupancy levels, internal gains and running periods are assumed depending on the building category and cannot be adjusted by the assessor to reflect the actual values for the building. The L2 assessment isn’t intended to predict the energy usage of a building and even omits significant areas of usage such as small power, lifts and external lighting. Unfortunately the values produced by this assessment are sometimes incorrectly used as indicators of a buildings expected energy usage leading to a significant performance gap between design and reality as shown in the graph below.

This is where technical memorandum 54 comes in, it was published by CIBSE in 2013 with the aim of providing building designers and engineers with an approach to estimate the operational energy demand of a building using a DSM (Dynamic simulation model) tool; this can even be carried out at the design stage. It outlines methods on how to estimate the energy consumption of a building for each energy category (heating, cooling, auxiliary etc.) and contains guidance on including for margins of uncertainty on the estimations.

To carry out a TM54 assessment the building must be modeled in much greater detail then it would be for an L2 and EPC assessment a few examples of how are:

·         The actual building occupancy for the building should be modeled and this should account for people coming in early or staying later in the evenings – what if the building has more or less occupants than the design included for? This should be accounted for within the uncertainty margin calculations.

·         Electrical equipment included in the building should be assessed in detail, things like how many PCs per room, what power consumption each PC and monitor will have and will there be any additional equipment such as desk lighting should be considered.

·         External lighting, Lifts and small power are to be included in the results.

·         The HVAC should be modeled as closely as possible to how it will run in reality.

·         The assessment should include iterations of the model to show high and low end estimates for each energy usage category.

The graph below shows an example comparison of energy consumption estimations for part L, TM54 and the actual building (which can be obtained using metering over an occupied year) , it’s clear to see that the L2 assessment is not to be used for an actual buildings energy consumption approximation.

If carried out correctly TM54 can be used to aid in creating a building design that will meet energy usage targets and can be used to test the effects a change in the design would have on the resulting energy usage breakdown. This should enable the design to be adjusted to maximize efficiency within the budget for the building. It may also be possible to achieve additional BREEAM credits for a building if a TM54 is carried out and the post occupancy assessment shows that the building is performing as the design predicted.

If the building is already nearing completion and there is little scope to change the design TM54 can still give insight into the breakdown of energy consumption for the building, give potential occupiers a more accurate indication of energy use and identify areas of the building that are not performing to design expectations, perhaps due to HVAC controls that need altering.

Working in the industry of building design we have a responsibility to work towards reducing the performance gap where possible, striving towards more efficient buildings, reduced greenhouse gas emissions and a more sustainable future for ourselves and younger generations.


If you would like to read more TM54 can be purchased from CIBSE here:

If you would like a TM54 carrying out on a building this is something that our consultancy team may well be able to help you with or if you’re interested in learning how to carry one out yourself you can get a free trial of Tas and start learning how to use the software!

Importing DOE-2 INP

Importing DOE-2 INP (eQuest) files into Tas

You can now import DOE-2 INP files from packages such as eQuest directly into Tas. You can then easily perform daylight analysis such as calculating daylight factors and Climate Based Daylight Modelling (CBDM), and thermal/HVAC simulations without having to re-create the geometry and constructions in Tas.

Original geometry in eQuest
Geometry imported into the Tas 3D modeller

There are three routes you can take to import DOE-2 INP data into Tas:

  • Import into the 3D modeller (Tas3D)
  • Import into the Building Simulator (TBD)
  • Import into both Tas3D and TBD and merge the data

Importing into the 3D Modeller

If you want to import geometry and building element type data into the 3D modeller, you can do so via File  >> Import

You will be able to:

  • Perform daylight calculations (Daylight Factors, CBDM)
  • Change zoning, surface types & building element assignments
  • Export to the Building Simulator (TBD)

Importing into the Building Simulator

The Visualisation in the building simulator allows you to quickly inspect zones and their applied constructions/internal gains

If you want to import geometry directly into the building simulator, you can do so via the import utility:

  • Geometry, constructions & Internal Gains/Activity data will be imported (everything required for an annual simulation)
  • Weather data in Tas TWD or EnergyPlus EPW format can be imported at the same time 

You can therefore simulate the file immediately after importing with no further changes required!

How fast is it?

We created a 200 room model in eQuest and imported it into Tas3D and the Building Simulator. We included weather data, and the import process took 10 seconds.

Can I import the 3D geometry & building data at the same time?

Yes! If you nagivate to the Utilities folder in the Tas Manager, you can import the DOE-2 INP files into the 3D modeller and the Building Simulator at the same time. 

My INP file has inconsistent geometry. Can I still import it?

Yes! The import utility has options to account for thermal bridging and to automatically correct inconsistent floor and inconsistent roof geometry. 

Automatically correcting footprints for badly defined floors saves you time and gives you extra confidence in the validity of the model. 

Want to try it?

If you’re an energy plus or eQuest user and want to try the DOE-2 INP import, you can get a free trial from here which includes full, unrestricted access to the software. 

If you have any questions about the software and its capabilities, get in touch and we’ll be happy to help. 

Responding to coronavirus (COVID-19) together

During these are unprecedented times we wanted to let you know what we are doing to respond to the outbreak.

To protect the health and wellbeing of our employees and their families, all EDSL staff are working remotely. However our support and consultancy services remain fully operational and contactable via email and phone.

As countries all over the world put restrictions in place and encourage people to use social distancing, many of our customers will also be working remotely. We have implemented the following policies to help you during these difficult times.


Remote Working and License Access

During these times of social distancing and remote working users may need additional flexibility. Should you need an alternative method for sharing a license or if you no longer have the same access to your license, please contact and we will be able to provide you with free temporary licenses.


New Roaming Network Licenses

We also have new roaming network licenses in Tas 9.5 which are ideal for remote working and sharing licenses between different locations.


Training and Education

After a long break our in person training will resume in February, however we continue to make our e-training courses freely available for the time being.  Please note that you will need to sign up to enrol on a course.



Our support staff are here to help as normal. Please get in touch via email for the quickest response at Also visit our support page and FAQ sections for alternative assistance.


Student Access

As always, access to Tas is free for Acadmic and Research purposes. Get in touch with support for an academic license.


New Users/Trials

For new trial requests and additional licenses please visit Buy Online for options or contact