Exhaust fan modeling

bmatthews · Posts: 30

I can't get DB to model a bathroom exhaust fan at 75 cfm running 24/7 in an apartment with separate ptac and separate radiator heating. I want the exhaust fan cfm to equal infiltration of outside air.

Thanks
Brian

Laurent_Support · Posts: 292 Location: Vincennes

Hello Brian,

Currently in DB, you set a ACH. You should enter your value at your bathroom zone level.
Then fresh air is supplied and been exhausted at your ACH value.

Regards,
Laurent

bmatthews · Posts: 30

Yes I do that, but the exhaust fan power loads of apartment towers is considerable and it doesn't seem like setting the Air change/hr. seems to reflect this.

Laurent_Support · Posts: 292 Location: Vincennes

Hello Brian,

A reason that your ach is not what expected could come from the HVAC system you choose. For example (it could not be your case), in HVAC Compact mode you could choose VAV system.

About fan power loads do you check the Fan heading, there are parameters which can much change the energy consumption.

If you need more detailed info, please give some extra info or post your model with indications on what you feel inadequate.

Thanks,
Laurent

bmatthews · Posts: 30

From the menus I have surveyed, I can't find a setup that reflects 2 different fans serving a space. One fan being an exhaust fan in the bathroom. Another fan serving the Ptac, and no fan on the radiant heating. The ones you mention seem to relate to one fan per zone. It looks to me like in order to model a simple apartment I must include natural ventilation to account for the exhaust fan exhausting air from the apartment, and outside fresh air finding its way back into replace the exhausted air. Am I wrong?

Thank you again.

Andy Tindale · Posts: 2432 Location: Stroud

DesignBuilder does not need to account for the flow of air out of the building because this does not affect the heat balance (we must of course account for the fan energy consumption though). It is the flow of air into the building that is important. If your building uses 2 fans to achieve the flow rate (Supply and exhaust) then you could account for the exhaust fan energy consumption by increasing the supply fan fan pressure drop (bear in mind that the exhaust fan is not in the inlet airstream, so 'Fan in air %' should be 50% max).

Andy

bmatthews · Posts: 30

Yes, but not if the two fans are on completely different schedules. The AC unit fan (controlling cooling) runs according to cycling cooling requirements of the room. The Exhaust fan for the bathroom I need to have the choice of 24/7 operation (and thus have an impact on cooling/heating of the room) or 2 hours a day operation (which has a dramatically less impact on heating/cooling loads). Again, please correct me if I am mistaken.
thanks
bmatt

Andy Tindale · Posts: 2432 Location: Stroud

There is currently no way to have two fans serving a single space, although I suppose you could split a zone into two each with a fan and have a virtual partition between the two (if fan energy is an important part of your model).

bmatthews · Posts: 30

Typical apartment air operation in Nyc is:
100% fresh air to hall, crack under door is vent to make up air lost through kitchen/bath exhaust, separate heating and cooling fan system in room space independent of exhaust fan(s).

There by the design builder modeling should be:
VAV Draw Through Fan has cfm and schedule of the sum of exhaust vents in large multi zone area served. So it is, in essence, a large exhaust fan. At the zone level, the local Series PIU Fan has schedule and operation of the zones heating and cooling demands. The reheat coil at this zone level does local heating and cooling...

Now for the (possibly stupid) question, can I trick the reheat coil into doing cooling as well so it behaves like a separate heating and cooling unit for the zone or am i barking up the wrong tree?

or should I go with your suggestion, or do natural ventilation modeling through walls with vents and keep hall as the air plenum for the apartments?

Again, thanks so much for clearing this up.

Thanks again for the aid!

Andy Tindale · Posts: 2432 Location: Stroud

bmatthews · Posts: 30

Thanks so much.

Following your earlier suggestion, of putting a virtual partition in a drawn new zone with in the apartment zone areas just to handle the separately scheduled exhaust fan in the bathroom/kitchens, I run into a problem.

Ideally I would like to draw a 1'x1' small zone in the corner and apply the new exhaust fan and characteristics just to that zone to minimally affect the larger zone (gains/area) loading. But this runs into the limits of input into the zone. For instance, a 3586 sf floor area group of apartments require 1800 cfm of scheduled exhaust, making a 11612 ach exhaust fan in a 1x1x9.3 cubic foot room, max through design builder is 50 ach I think. I run into similar "limits of input" for cfm/sf and cfm/person. Question: Does E-Plus have similar limits of input? should I go directly to E-Plus to define this?
If yes, how do I relate zones in Design builder to Zones in E-Plus to keep track of such changes?

Instead of this method,
if I draw the exhaust zone with virtual partition to be the real size of the kitchen and bathroom, I loose the schedule control of the heating and cooling energy /sf for this area. Heating and cooling will be fully obliged to follow the only one exhaust fan schedule. Last time I tried this I got large error files because no heating and cooling could occur without a fan to make it happen. Deleting this new exhaust zone floor area removes a significant interaction between room gains and heating and cooling loads.

Again, I may be missing something obvious and I greatly appreciate the advice! I will try some of the other nice suggestions you gave as well.
Bmatt.

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