[Equest-users] Exhaust fans in appartment

Nathan Miller nathanm at rushingco.com
Thu Feb 19 11:12:32 PST 2015

We model that type of system all the time, and prefer to model the outside air as zonal-exhaust (infiltration).

The primary reason is that it ensures the space conditioning system sees the same vent load regardless of if you switch systems types (or have different systems in a baseline vs. proposed case for example). On jobs where I input it as OA on systems, I was never able to get the ventilation load energy use to line up when I compared, for example, electric resistance heat to PTACs, probably due to the slightly different algorithms employed for each system type.

The other nice thing is that it allows you to model the space conditioning fans as cycling to meet the load (schedule = 0 all the time, night-cycle-control allowed, fan operation = intermittent). If you introduce the outside air through the system inputs, often it will force the mechanical system fans to operate all the time to provide ventilation, but in your case, you already have the whole-house-fan to do that.

I believe any infiltration air assigned at the zone gets tacked on to infiltration air assigned at the space. If you want greater control of the net infiltration air, you’d probably have to do some math and decide when those values should stack, and when they would be “double counting” infiltration.

Nathan Miller, PE, LEED AP BD+C – Mechanical Engineer/Senior Energy Analyst
RUSHING | D 206-788-4577 | O 206-285-7100

From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Julien Marrec
Sent: Thursday, February 19, 2015 11:44 AM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Exhaust fans in appartment

I'm modeling an apartment building that has toilet and bathroom exhaust, and mechanical supply. Make-up air comes from trickle vents.
(Side note: the corridor has mechanical supply, much higher than 62.2 at 0.6 CFM/ft²,  and no exhaust whatsoever, so I expect some makeup air would come from there too, but I'm prohibited to capture this effect...)
I have been thinking about the best way to do this: whether I should assign this to a zonal exhaust fan (EXHAUST-FLOW) or whether I should specify the CFM exhausted as an outdoor air flow (OUTSIDE-AIR-FLOW).
I think the OUTSIDE-AIR-FLOW would be the least problematic if I only had to deal with the baseline, but in my proposed building I only have baseboards for heating, so this wouldn't work.
First, Am I correct in the above statements?
Second, if I do specify an exhaust fan in the following way:
   EXHAUST-FLOW     = 50
   EXHAUST-FAN-SCH  = "Fraction Always 1 Yr"

(I'm also defining exhaust systems like this for mechanical rooms in my basement)
Will eQuest actually take into account that it (he?) should add 50 CFM of outside air (through infiltration) as a load? Will eQuest also take that into account for the sizing of my zonal equipment?

Finally, will it interact in any way with the infiltration defined under Internal Loads for the Space?

Thanks for any clarifications you can offer.
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

LinkedIn (en) : www.linkedin.com/in/julienmarrec<http://www.linkedin.com/in/julienmarrec>
LinkedIn (fr) : www.linkedin.com/in/julienmarrec/fr<http://www.linkedin.com/in/julienmarrec/fr>
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