how to do local vent system with mixing

[golden eagle]

I am enquiring how to model correctly the local auxiliary ventilation with heat recovery option (or mixing ) in EPC calculation

I am working on a school project where local single fan will be used to provide fresh air in the classroom , the design incorporates 2 fans ,one providing fresh air from the atrium( through the stack ventilation at H/L) , and other extracting air into the same atrium , (this is the breathing building A500 serious fan ) so the mixing will occur in the stack or the main corridors used as a stack ,

1-my initial thought was the local vent unit serving into single zone and checking heat recovery box appears to be closet system configuration to represent above system design. I add HR option to avoid cold incoming air for the actual building as the air through stack vent will mix with extracted air from the classroom ( heat gain from occupant will increase incoming cold air temperature ) and I include 2 SFP information in the model one for the supply fan , and other for the extract fan.

Having applied this configuration in the model , I have realised from the output result ; the equivalent system in the notional building did not use HR option, it is therefore heat energy in the national is much greater than the actual building , as a result the actual building has shown significantly low carbon emission against the national building , which appears to me a bit unrealistic , the only time that national building applies HR option, when actual building uses centralized mechanical vent or mechanical vent system , so I am not entirely sure checking HR box option for the actual building is the correct way of modelling this system , in reality there is no HR but how can I avoid outside air coming in the classrooms without HR option , otherwise actual building will be penalized

2-or alternatively , the other way of modelling this would be simply selecting a single fan for the classroom without checking HR but at the system level checking air supply condition from the corridor (not from outside ) and applying extract fan as supply fan for the corridor atrium ( with total flow rate of extracted air in l/s ) and similarly selecting supply condition from the classroom . To include fresh air from outside , we then model H/L opening ( through macroflow ) to open by CO2 , I assume heat gain from the classroom should be cancelled out loss through stack vent opening so the heat energy should not be significantly increased

In the both options explained above; the equivalent system in the notional building will not use HR , and the result will always show in favour of actual building with large carbon emission difference between actual and the notional building . actual building carbon emission can be up to 40% lower than notional building .Although there is not so much difference comparing above system types against mechanical ventilation system with HR ( in terms of heat energy requirement ) , mechanical vet system with HR will show less carbon emission difference (TER /BER % difference would be only about 10% ) but selecting this as single fan and including mixing option or HR will result significantly large carbon emission difference . so it is very important to establish now how to do this correctly so that I can advise architect to improve building fabric U values to achieve better result if necessary (if the difference is only about 10%) , or if the difference is found to be 40% due to actual building using HR or mixing ; then there is not so much need for improvement

the only way to include HR option in the notional building is to model actual building ventilation system with centralized or mechanical ventilation system with HR , The corresponding system in the notional building would then use mechanical vent with HR and the difference in carbon emission would not be so significant

would you please advise me which way of above explained option is the most correct way of doing this? Or I should be doing differently all together ?

your help on this matter would be greatly appreciated

[RossThompson87]

That is a difficult one to model.

Are you just doing the EPC? In which case the TER is only relevant for benchmarking and I don't believe you can be audited on this. So perhaps don't stress about the notional building too much.

How does fresh air enter the atrium? If this is through a window or louver I'm not sure I would describe the system as having 'heat recovery', as the incoming external air to the building is not pre heated.

In which case I would just go down the route of a simple mechanically ventilated room with no heat recovery.

This won't give a great rating though...

Ross

[golden eagle]

Ross, Thank you for your reply ,

No , I need to do this for both UK compliance and EPC calculations via Apache sim method ( DSM)

Air enters through the louvre opening poisoned at the highest point of the stack ,which is also used as a main circulation corridor , yes I agree checking HR option would not actually represent what happens in actual design as there is no HR element in the fan , however this option avoids actual building consuming high heat energy to heat the air as the mixing option does .

If you want to mimic actual design philosophy ; you would then use a single fan supplying air from the corridor ( so you set supply air condition from the corridor at system level , in which way the incoming air temperature would be the maintained temperature of the NCM corridor ), for other fan that extracts air from the classroom to the corridor ; you would use this additional extract fan in the classroom or you would use this as supply fan for the corridor zone ( or stack ). you also enter weighted average SFP of the all extract fan in the classrooms and you enter total extracted air volume ( this is to ensure aux energy consumption due to extract fan operation included in the calculation ,

I don’t think heat gain from occupant will be added on this extracted air since you can override ventilation rate in NCM calculation , however you can manually calculate worst case scenario for the incoming cold air temperature after the mixing, this is to ensure you don’t introduce straight cold air from outside into the corridor . (create profile and assign temperature from profile for the incoming air supply condition )

you also need to include macroflow opening ( or louvre at H/L) by controlling it based on CO2 ( but it may not take into account co2 in the extracted air , so in which case open based on occupancy ) . Incoming cold air though the stack will meet extracted air from the corridors ( as heat gain received from the occupants. this approach requires macroflow checked in the simulation so it is a bit of long process but I think It is most correct way of doing it , I will need to do couple of test simulation in the meantime to see if the output makes any sense.

I don’t think that treating this system as mechanical vent without heat recovery would be correct for this design . otherwise you would not include benefit of mixing event occurring in the corridor , so the actual building shows high heat energy consumption , if option explained above is found to be not correct by the users in this platform ; I think the second closet option would be treating this system with HR as this way is very similar of what you do with mixing , ( both option heat incoming cold air )

my other concern is the way national building aux system is treated based upon selected options above, if you do select mixing option for actual l building. , notional building will not consider mixing ,so you get high heat energy in the notional building ( large difference between TER /BER) , however if you do select mechanical vet with HR for the actual building , notional building then uses HR so you get less heat energy ( small difference between TER/BER) , If you think about it , in both scenarios , you heat the incoming cold air through HR or mixing in order to save the heat energy , but why is the notional building being penalized if you do mixing , the other issue is it influences design team to do further improvement if needed , but mixing option ,you don’t need to so much as you get huge benefit by penalizing national building

in my opinion , BRE will need to consider this otherwise people involved in design process will find themselves doing nothing to achieve compliance or better e EPC rating