macroflow link in the UK Compliance DSM calculation

[Blackeagle]

Hi

I need to run PartL Compliance calculation via DSM method for naturally ventilated building , I am using the same model that I created for the purpose of overheating assessment , all openings (including windcathers) are controlled by temperature and CO2 , My observation is when I link macroflow for DSM calculation ; i realised that the carbon emission from the building increases (against the option without macroflow linked) I understand that the heating energy will increase as a result of control strategy of the openings so I cannot see any benefit of linking macroflow as the software already assign natural ventilation rate for zone that are either not air-conditioned or mechanically ventilated , so i assume I am doubling natural ventilation losses ( one from the vent rate attached due to activity type , and one from macroflow) my question is within the legal frame of UK compliance and EPC calculation , although i use macroflow for overheating calculation ; do I still need to link or use macroflow in DSM calculation ? would it be ok to issue epc without this linked?

My Comment as follows
I understand that having macroflow linked will help cooling energy and it will reduce internal temperature due to benefit of actual vent control strategy (temperature control ) however it will also increase heating load because of heat loss from macroflow opening , If there is no legal requirement to use this option in the dsm calculation , I would think to not use macroflow unless it provided positive benefit ( like energy reduction from cooling becomes greater than energy increase in heating , so overall carbon emission reduces)

[Complex Potential]

Hi Blackeagle

Generally I try to leave macroflo off for compliance models unless I am dealing with a complex, multi level space like an atrium where I need to let air move from one zone to another to avoid massive heat build up at high levels.

The official SBEM guidance states that you approximate the effect of natural ventilation in compliance modelling by including a free cooling allowance of 5AC/H which is normally fine. What you want to avoid is introducing additional unregulated fresh air via window openings into compliance models because of the extra loads it generates which the target building will not have.

So in summary:

- if your building is simple, switch macroflo off
- if your building is complex, switch macroflo on BUT ensure that all external window and door openings are fully closed continuously

[Blackeagle]

Thank you Complex Potential for your response below,

I would like to ask you some questions or anyone who may answer

Does this mean I will need to assign 5ACH for every naturally ventilated spaces (through the Apache view system section where you define free cooling flow rate ) or it is only applicable when you have mixed mode strategy ( you use windows and mechanical comfort cooling combination ) using this in my opinion ; especially makes sense when you control natural ventilation opening via temperature in real building ( with this control ; you generally provide outside air in the room beyond min fresh air requirement) I don’t think this would be applicable where you control openings based on co2 only in real building so we should not use free cooling as the actual building does not use vent opening as sole of reducing internal temperature by introducing more air


Having looked through the DSM user guide; it has been suggested 0.5 l/s.m2 should be used for free cooling flow rate for the air conditioned rooms ; which generally means less than 5 ACH ( no great benefit) , do you advise 5ACH for every air conditioned space with natural ventilation strategy , or should i stick on above value in the form of l/s.m2?

I have also realised there is Auxiliary ventilation rate that is automatically attached to zone (although the room is natural ventilated) , does this mean this ventilation rate there ( names as aux vent ) actually represents fresh air requirement for the rooms? (controlled by vent profile) , can I override this control strategy to mimic actual vent opening strategy ( become same as macroflow except you have no control to define fresh air requirement ) , also how can i view natural ventilation losses in vista? , when nat vent is clicked nothing appears

Last question, you referred “when building is complex , switch macroflow on but close all windows and doors” , if I do that how can I use benefit of macroflow opening as it will be shut, I am assuming what you mean is teh building with atrium where is ventilated via H/L openings , so I guess you mean only use macroflow opening in the atrium and leave the rest kept shut? If that is true how can you maximize air flow when windows in the zone are kept shut , this may influence airspeed as it is generally air through the windows in the zone is discharged to the Atrium then elevated to high level to discharge from this zone

[PCully]

Hi,

The link for MacroFlo is intended as an extra means to show benefit of an advanced nat vent control system or technology. The general feeling I get is that it won't fit with the NCM framework in many instances and so most people tend not to use it in the Compliance model and instead keep it for Apache design work for overheating analysis etc. This surprised me but it seems that the most common effect will be that the heating load is driven up in these buildings and so this raises BER. It's there as an option if you have Controls specified in the MacroFlo that will benefit the BER or have an advanced system that you want to account for in the model but there is no requirement to activate the link in the Compliance model and instead you can specify a system as naturally ventilated so that cooling conditioning is not applied and the energy consumptions reflect it properly (this is specified for Apache on the Ventilation tab of the NCM System Wizard).

I think what Complex is referring to for atria in to utilise the ability of MacroFlo to model the bulk airflows internally in the building. If you specify the openings as sealed using the MacroFlo opening types then MacroFlo will still simulate internal bulk airflows so that indirectly conditioned spaces etc are simulated in more detail.

Phil

[JohnLloyd]

My take on MacroFlo and compliance models is that I only tend to use it to model internal convective flows but not for window openings.

So I will always turn MacroFlo on if I've got atria, double height spaces, different areas of the building open to each other etc. but at the same time I ensure all external windows and doors are set to be closed with no crack flow coefficient. Natural ventilation is then modelled using 5ACH of 'free cooling'.