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.

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!