AUX VENT in the DSM calculation

[Blackeagle]

I would like to ask question about the Auxiliary ventilation rate in the DSM calculation.

Why does the software assign aux vent to naturally ventilated spaces ? are we allowed to turn vent profile off in the compliance calculation to disable vent rate in the actual building ?

Although I set zones to be naturally ventilated (through the NCM wizard system tab), the software still assigns aux vent rate to the rooms , aux vent rate are determined by software based on activity type assigned in the actual building , the same aux vent rate is also assigned in the national building for the same activity room, ( direct replica of the actual building ) , which means the national building will have the same heating loss as a result of aux vent , so at first glance there seem be no gain no loss situation , however BER/TER value will be different against the options when aux vent is included
(it may not have any % difference over TER ) , this may have an issue when final report send to BCO or EPC rating can be different.

I am aware that you can simply turn aux vent off from the apache view , but the guide say this is applicable for Apache HVAC module only and it should be only edited in the special circumstances ( I don’t know what they are )

I would be happy if anyone can add any comment or clarify this issue

[Complex Potential]

I am guessing that it is simply following the exact wording of the NCM guide but I agree that it makes no sense. For me, if we define the ventilation type as "natural" then the software should remove the heat loss/gain asociated with the aux vent from the air exchanges tab. At the moment, it appears as though changing the ventilation type only impacts the fan energy.

I would be interested in hearing if anyone has a more official explanation.

[Blackeagle]

Thank you Complex Potential for taking time and responding my question , you seem to be the only one in this whole entire forum that responds may be I need your direct email

In response to your comment ; you can set ventilation type from the UK NCM wizard as mech vent, air con, or natural vent , but no matter what you select , the software always assigns aux ventilation rate (fresh air requirement) to the zones .

Unfortunately having aux vent included ( in the natural ventilated zones) ; it will also add aux vent losses in the zones ( as appears aux gain in the vista variable) , so it does not only increae fan energy but also increases heating load too , I have tested this by myself and realised , you will get vent losses even though system is set to natural ventilation , if you turn off vent profile ; the ventilation losses become 0 , and this will give very different BER/TER value against the options with vent rate attached ,

My question is to establish if I am allowed to set vent profile to off continuous in the DSM calculation as the guide says it should only be edited in special circumstances

Let me know if you still have anything to add?

Thank you for your assistance again

[Terence]

Complex Potential is almost correct that the change of ventilation type between natural and mechanical affects only fan power, it also affects the type of demand control options available and whether heat recovery can be applied or not. Whether the space is naturally ventilated or mechanically ventilated the fresh air rate must be as per paragraph 45 of the NCM modelling guide i.e. based off the NCM activity database. For practical purposes it makes little difference whether natural ventilation is represented in the VE as an aux vent air exchange with no fan power or a natural ventilation air exchange, the thermal effect on the zone is the same. It would not make sense to remove this air exchange for naturally ventilated spaces, as this would mean naturally ventilated models would not be accounting for the heating load imposed by providing potentially cool fresh air to the space. An argument could be made that naturally ventilated spaces should have a natural ventilation air exchange applied instead of the auxiliary vent, but the only difference this would make would be to the labeling of the results in Vista, all compliance results/documentation would remain unchanged.

[Complex Potential]

So basically it's about the NCM assuming that fresh air would need to be provided somehow and so stipulates the loads associated should remain regardless of the ventilation strategy. Makes sense although the fact that it remains an "Aux Vent" air exchange is probably where the confusion is coming from.

[Blackeagle]

Terrance

Thank you for your response,

Based on your explanation ; I assume you are saying that I should not turn off /edit aux vent profile to off continuous as the model will need to account heat loss associated with natural ventilation system , however ventilation rate here is treated as constant during occupancy period determined by the NCM activity ( eg:office activity will have fresh air rate of 10 l/s/p , and occupancy will be based on people density determined by the same activity, so if this is a relatively big room , it will use large ventilation rate , which means more heat energy required , ) also as the ventilation rate remain constant energy requirement for heating due to ventilation will remain constant too , I don’t think this is correct for natural ventilated rooms as the fresh air goes into rooms should be varied by environmental condition outside, to be honest , in winter time natural ventilation is generally used as trickle ventilation and vent rate is 5 l/s/p as stated in the cibse guide which seem to be the half of flow rate that the software uses ,

My other question that relates to this ; there are conditions that you can link macroflow setting for the opening (created for the real building calculation ) , which then allows you to model ventilation by actual control strategy and profiles and rate based on outside conditions , but to avoid double counting, do you think If i need to turn off aux vent to off continuous mode for naturally ventilated rooms when macro flow is linked?

[Terence]

No, I wouldn't advise turning off the NCM Auxiliary Ventilation air exchange as it is from there the software gets the NCM fresh air requirement for the space, which is not necessarily related to a particular opening area.

You can of course add in extra ventilation yourself to account for natural vent through the windows using either MacroFlo or an Air Exchange. By means of comparison the Notional building will have 5 ach of free cooling applied in naturally ventilated spaces. In terms of estimating flow rate for a given window type, without using MacroFlo, you might find CIBSE AM10 helpful.