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[EnergyPlus_Support] Re: Exhaust Fan Increases Cooling Load



No. As James summed up, the fan takes air from the zone as it should, but the makeup air must come from somewhere. Ultimately it will come from outside either through your HVAC or through airflow network cracks (if using the AFN) or from (if you specified it) other ventilation objects. This is always a tricky object to include. For starters you should output the mass flow rates in the 4 AHU connections to ODA, Exhaust Air (called relief air in e+), Return Air and Supply Air. Make sure you understand on which nodes these exist, for example Supply Air is the mixing box outlet node if you don't have a secondary mixing box at your Terminal Unit (the air damper to control the air delivered to a particular zone) as is common in the USA. Return air is easy as the node after the mixer in the return path is the first unambiguous oppertunity. At the mixer you can also do this, of course. When e+ relieves less air than it takes in from outdoors to fuel the supply, an unbalanced air flow warning is issued for the AHU, although this is probably quite common (at least in Germany). 

In your specific case I assumed that, as James said, your AHU was supplying more ODA to make up the exhaust air, because your system is set up like that. As I stated, it doesn't have to be like this, if other zones get less air because of the exhaust fan that's also okay. But you need to specifically constrain your system to limit the ODA. 

3rd party e+ software could do well with a graphical GUI that displays the airflows.

James often says that he ALWAYS outputs the nodes to monitor airflow. I should as well. You should as well.  

--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "iluminasol33" <iluminasol33@...> wrote:
>
> Thanks for the tip Jean. So, just to make it clear, when I add the exhaust fan into a zone then the air that goes trough it comes from the outside? So how can these fans be modeled correctly? 
> 
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Jean marais" <jeannieboef@> wrote:
> >
> > 
> > Ha! And where does the makeup air come from? Outside. Now you have to cool more outdoor air. 
> > 
> > This is correct. If you want to incorporate the effect of exhausting hot air directly from source to achieve reduced heat gain to the space (eg using kitchen hood equipment) then this is best reflected where you specify the heat gain, for example a equipment heat gain with 50% "lost" would reflect (roughly, depending on manufacturer specifications) the effectiveness of a cooking exhaust hood equipment. 
> > 
> > For recessed lighting, I believe one can specify the fraction going directly into the exhaust air (about 30%, from memory). 
> > 
> > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "iluminasol33" <iluminasol33@> wrote:
> > >
> > > Dear forum,
> > > 
> > > I have a particular problem that when adding a zone exhaust fan in one zone with high internal loads, the cooling load increases. I was expecting it would decrease since the fan is taking warm air out so it helps the HVAC system somehow. For that zone I have a DX cooling coil, in a Unitary Furnace: Cool Only. 
> > > 
> > > I would appreciate any help. Thanks in advance.
> > > 
> > > Santiago
> > >
> >
>




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