IES-VE User Forum

Before we dive into the specifics of auxiliary energy calculations, it's essential to clarify the scope of this guidance. While the Apache Systems tools described here can help you account for auxiliary energy, they are primarily based on the 2006 UK Compliance methodology.

For a comprehensive CIBSE TM54 assessment, the spirit of which is to capture a much more detailed and accurate picture of a building's operational energy, you should be using ApacheHVAC. This allows for a granular, plant-level modeling of building services, which is necessary to truly align with the detailed requirements of TM54. The techniques outlined in this post are useful for understanding the different components of auxiliary energy within ApacheSim but should be considered a starting point, not a substitute for the more detailed modeling required for a complete TM54 assessment.

If you're used to modelling and simulating buildings to show compliance with the Building Regulations and generating EPCs, you may not be used to the level of detail required for CIBSE TM54 assessments. One area where this is particularly evident is in the calculation of auxiliary energy consumption.

TM54 requires users to carefully consider auxiliary energy consumption and to use a detailed and accurate building model to capture it. Because you are using ApacheSim away from VE Compliance this uses the 2006 UK Compliance methodology which requires more thought on your inputs.

These options are found on the Aux Energy tab in Apache Systems and are explained further in the following FAQ: https://www.iesve.com/support/faq/pdf/A ... Method.pdf



The building level variable “Ap Sys aux + DHW/solar pumps energy” is the total sum of system energies associated with all fans pumps and controls. It is made up by the sum of 5 separate components that make up the auxiliary energy picture. I’ll go through an example for 2 zones in a test model to illustrate how that variable is calculated so that you can determine whether or not the inputs you have set are reasonable.

1. AEV (Auxiliary Energy Value)

The zone has an area of 100m2.



The AEV value is applied continuously as controlled by the highlighted profile.



The system has an AEV value of 1.002 W/m2 which is applied to all spaces served by this system.

The resulting plant energy consumption for this space is 1.002 W/m2 x 100m2 = 100.2W

2.) Ventilation SFP

As you can see in the system screenshot above, as well as having an AEV an SFP of 3.6 W/l/s has been set. In the screenshot below you can see the space has a supply rate of 80 l/s. This results in a supply fan power of 3.6 W/l/s x 80 l/s = 288W



3. Extract Ventilation SFP

This function is typically used to represent the auxiliary energy from an exhaust fan and is in addition to the previous function. It would commonly be used for modelling energy associated with an extract fan in a toilet where the extract rate exceeds the supply requirements.

This option was not used in this test case.



4. DHW Pumps Energy

This value would be expressed as a total power instead of being apportioned based on floor area. This again however has not been applied in this model.



5.) Solar hot water pumping Energy

As above this has not been applied in our model



In our test model we have two rooms, each with their own system. Room 1 uses the system above so the Total Auxiliary Energy is equal to AEV + System level SFP which gives 100.2 + 288 = 388.2W

Room 2 uses the following system with "Use SFPs" as the method so the Auxiliary Energy is just based on the system SFP which is as above, 288W



In our model only functions 1,2 (Aps Sys Aux) have any value associated with them and 3, 4 & 5 (Extract, DHW/Solar pumps
Energy) have no energy associated with them. As demonstrated above the zones will have a total of 388.2 + 288 = 676.2W of plant energy associated with it

So viewing the results in VistaPro the total “Ap Sys aux + DHW/solar pumps energy” you'll see it equals this value.

In the above example I focused on the building wide energy variables. You can also view the Auxiliary Energy associated with a particular system in VistaPro.

In this test each room has its own system so "Room 1" System will equal SFPs+AEV for all rooms it is assigned to and "Room 2" System is equal to SFPs for all rooms that is assigned to as well.



Further to this, users can also create and assign additional meters for a further breakdown. In this instance I created additional electricity meters, Meter 2 and Meter 3 and assigned them to each system under the Auxiliary Energy tab in Apache Systems (this is the tabular view).



And then you can view the energy consumption associated with those meters in VistaPro

More relevant information on these settings. The System Auxiliary Energy is always met by the Main System assigned to a space.



If you have a flow rate defined either in System Outside Air Supply or for an Auxiliary Ventilation air exchange then the fan power for supplying that air is associated to Aux Energy tab of the Apache System assigned as the Main System



, not the Aux. Vent System.



The Apache System you assign as the Auxiliary Vent. System is only used to condition the supplied air to the space when it is not supplied into the room at ambient conditions. See the description in the Apache View user guide below.



In my example looking at the results files in VistaPro below note each room has its own Main System and also Auxiliary Vent. System, the System Auxiliary Energy is only associated to the Main System though, not the Auxiliary Vent. System, which is zero.

AuxVentSystem_changedSFP.aps - I changed the SFP's in Aux Energy tab for the 2 Auxiliary Vent. Systems, the Aux Energy tab for Main System remained the same, result is no change to System Aux Energy.

AuxVentSystem_editflowrates.aps - I reduced the system outside air supply flow rate on room 1 and also the Auxiliary Ventilation flow rate assigned to room 2. The corresponding System Auxiliary Energy for both Main Systems reduced.

Hi Terence,
Thanks, this is a really useful explanation. I've got some rooms with 4-pipe FCUs for heating and cooling, and centralised MVHR. In the compliance version I know to include the FCU SFPs at room level. However this box disapears when I move to Apache. Am I missing something obvious? Do they just need to be included within the AEV?

I'm using the ApSys methodology.

Any help appreciated!