IES-VE User Forum

I am wondering if any of you have an idea about how to do hybrid ventilation in UK compliance Apache Sim calculation

The building I am working on utilizes mechanical and natural ventilation systems to deal With co2 and overheating compliance , I am aware that in SBEM tool, there is an option to select mixed mode ventilation which is beneficial and only being activated if mechanical cooling system is selected ( the idea here is to delay mechanical cooling requirement and save energy when environmental conditions are suitable to provide outside air through window or mechanical ventilation or both combined ) however when you hit the Apache SIM method(DSM ) , this option disappears ,

first of all; I am asking how DSM is dealing with mixed mode operations, if you don’t have mechanical cooling in the building , can we simply ignore this option although the actually building uses hybrid ventilation to assist overheating or CO2 level ? the problem with including hybrid option ( both for SBEM and if possible with DSM) it will increase heat demand in winter time and will have no energy benefit in summer time if you don’t use mechanical cooling ( you cant choose this option SBEM anyway if you don’t have a cooling system , but If you do go down the route of including this in DSM , it will have double impact both in winter and summertime due to increase in heat and aux energy ) , in Summary can we simply ignore hybrid vent in DSM calculation if you don’t have mechanical cooling system in the building , ? also does this cause any issue with building regulation compliance as mechanical vent is present but it is not being included in the calculation on the basis of not being operated constantly ( only operated when windows are not capable of providing enough air )

secondly ; if you meant to have cooling system and you would like to sue window opening or mechanical vent or combined to assist cooling in summer time ; am I allowed to modify/alter mechanical ventilation and window profile by use of macroflow profile and aux vent profile , the idea here to close window in winter time if macroflow is linked to apache sim calculation , otherwise heating consumption will be doubled as the NCM already allows nat ventilation loss in winter time , if the windows are not closed in winter , it will have additional loss due to this , so ideal solution would be is to profile it to open in summer time based temp or CO2 criterion and closed it during the course of all winter time ( although in reality you may be opening in winter time for min fresh air requirement ,) is this legitimate way of doing it? Do you think if may cause any issue with BCO or EPC accreditation? Similarly , if you want both mech vent window involved , can I edit mechanical ventilation profile to suit my control strategy otherwise it will be operated based NCM rule

your responses and comment this enquiry would be greatly appreciated

many thank in advance

Hi

First off, when it comes to SBEM/DSM compliance calculations or anything else based on the NCM, I think "mixed mode" is referring to cooling systems only (and if you don't have a cooling system, you wont see any noticeable energy improvements with mixed mode, only environmental ones). If you specify that your system is mixed mode under the apache systems cooling tab then I think the cooling setpoints for the applicable rooms in the actual, notional and reference buildings are increased to 27C. That's basically it as far as that option is concerned and it essentially means that the cooling component becomes a smaller contributor to your overall carbon emissions but, since the notional building gets the same treatment, this does not always result in a net improvement to your BER/TER ratio (if you have a particularly good cooling SEER it could go the other way in fact).

As far as summertime overheating is concerned, the SBEM/DSM methodology is not concerned with such things and so it will make no difference. You are right to be concerned about summertime overheating but an SBEM/DSM model is not the tool to be using to analyse it. Instead, you should be looking to create a bespoke overheating model to study how your building will perform in the summer, in addition to the SBEM/DSM model.

If you do have cooling and want to increase the benefits of the hybrid natural ventilation in your compliance model then it is technically allowable to include opening windows on the actual building via the use of Macroflo but only if you are using DSM. However, you would need to be very careful about the profiles and controls you apply to avoid massively increasing your winter heating demand when compared to the notional building and you would need evidence that your building controls will actually behave that way (so if you have a profile that opens on CO2 and keeps the windows shut if the outside air is below a certain temperature for example, you would need to have actuators on the windows because you could not rely on occupants to be so precise).

Theoretically, the above should be allowable for EPC and Part L2A compliance purposes but, since you are stepping outside of the "norm", an auditor may scrutinise the model more closely and so you had better be sure that you can justify all of the inputs.

CP

Thank you complex potential for your response


I understand what you said and agree with you the most of it. But just to clarify again first paragraph of my question , I was checking if I need to include mechanical vent system in the compliance calculation if it is only used to assist overheating? ( Sole purpose of this unit is to provide more air to offset temperatures in summer time but winter time it works as heat recovery unit ) . I understand there is not significant benefit of mixed mode if you don’t have mechanical cooling , however as the same system is being used in winter time for fresh air requirement , there will be fan power involved and the associated SFP , I think the only way of doing this , either to include this as mechanical vent as you normally do and ignore the hybrid part ( this way , you will loose energy saving in summer time ) or alter mechanical vent profile to mimic actual operation of the fan , both ways , you include SFP ,
however if the fan is only meant to be used in summer time ( like an extract fan located at the highest point of the building or in the stack to use as boost ventilation to help overheating ) , this then can be ignored in the compliance calculation( no SFP inclusion in the model)

I agree, you need a separate model to deal with overheating issue ,

Yes , IES user guide also states , you have to be careful when it comes to changing aux vent profile in DSM calculation, as you mentioned , if the vent profile is not being controlled in winter , this will have direct impact in heating energy so the best of doing it in my opinion , simply to use the same NCM profile for winter time ( this way , you would not have any drawback as the notional building uses the same profile and vent rate ) , but alter the summer vent profiles so that it can only operate as it is intended in real life , obviously , before this action taken , it is best to check with accredited scheme for approval

I would probably ignore the summertime mode based on what you say. Look at how the wintertime minimum fresh air is being provided and define your system on that. If it is via trickle vents in the façade, then it's a natural ventilation system, but if the minimum fresh air is being provided via ducts and fans, it's mechanical.