Exhaust fans in appartment

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Hi,

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/ft?, 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:
FAN-CONTROL = CONSTANT-VOLUME
EXHAUST-FLOW = 50
EXHAUST-FAN-SCH = "Fraction Always 1 Yr"
EXHAUST-SOURCE = INFILTRATION

(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.

Best,
Julien
--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

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

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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
www.rushingco.com

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I agree with Nathan's recommendation; that is also how we have also settled
on modeling whole house exhaust ventilation. With regards to infiltration,
we do model a nominal amount that is calculated using PNNLs infiltration
modeling guidelines and is input at the space as the component adjusted
with wind speed. The EXHAUST-SOURCE = infiltration will automatically
combine the space and zone exhaust in quadrature, which we believe is
appropriate, at least with respect to what is readily available in
Doe-2. The space component is assumed the same in proposed/baseline unless
the intent of the analysis is to illustrate the impacts of reducing
uncontrolled air leakage.

In addition to other noted flexibility, it is also easier to incorporate
modeling of variable flow and even HRVs.

-David

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Thanks to both of you for your answers!

David,

You say its combined in quadrature. Is there documentation somewhere about
this?

Also, I'm confused about how it's more flexible to add HRVs when using
zonal exhaust. I thought the core principle of Zonal Exhaust was that it
was directly exhausted to outside, which means not going to the central
system and thus completely bypassing any Heat recovery.
central exhaust only, not from zone exhaust. If heat is to be recovered,
zone exhaust should not be entered but rather allowed to default to the
central system."
Am I missing something?

Thanks,
Julien

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

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

2015-02-19 20:35 GMT+01:00 David Reddy :

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Hi again,

After implementing zonal exhaust, I'm still in a bit of a pickle.
For those familiar with it, I'm actually modeling a building per the ENERGY
STAR MultiFamily High-Rise Simulation Guidelines, slightly modified because
it may go for NYSERDA Multifamily Performance Program (MPP). MPP New
Construction, in its v6 version, has a different calculator that the MFHR
one for infiltration, and you have to model infiltration and local exhaust
separately.
Anyway, the calculator is telling me to put:
- 0.08 ACH in the space with hourly fraction at 0.5 (so 0.04 ACH...): it's
doing some kind of calculation to come up with 0.04 CFM/sqft of exterior
above grade surfaces (roof + ext walls)) that leads to the 0.08 ACH figure.
- Model the exhaust as about 40 CFM per apartment with a fraction of always
1.

I don't know why, but this seems fairly low to me (0.38 ACH would seems
about right for a fairly tight building when you consider that
building-wide, but here I'm only entering it for the apartments right now).
What do you think?

Ultimately, I have one major problem: the heating consumption reported is
abnormally low. I've been banging my head since yesterday trying to figure
out why. I think I've ruled out pretty much everything (but I'm confident
that missing something is possible) aside from this ventilation thing.

If I add (or increase) Zonal exhaust to a zone:
- The attached system sizing doesn't seem to change
- but suddenly I'm getting unmet heating hours...
- And I do have *some* more consumption
*This make me think eQuest somehow doesn't really take zonal exhaust into
account when autosizing *(but does for consumption)*. Do you have the same
experience with it? Any workaround to avoid this problem?*

Finally, I think I'm in dire need of a pair of experienced yet fresh
eyes... I'd be extremely grateful if someone was willing to take a look at
my model. Ping me and I'll send you the file, and hopefully return the
favor soon.

Thanks a bunch,
Julien

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

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

2015-02-20 13:04 GMT+01:00 Julien Marrec :

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Me again, (sorry for talking to myself!)

I don't know how I missed this, but for the quadrature it was right under
my nose, written under the help topic for the "EXHAUST-SOURCE" keyword:

INFILTRATION specifies that the EXHAUST-FLOW is made up via
outdoor air infiltrating directly into the zone. The exhaust fan operates
independently of the space conditioning system, and the exhaust requirement
is not included in the calculations of outdoor air flows or VAV minimum
terminal flows. The EXHAUST-FAN-SCH must be specified.

A common usage for this mode is to model residential exhaust requirements
for bathrooms and kitchens. When the exhaust fan operates, zonal air
infiltration is modified by adding the infiltration and exhaust flows in
quadrature:

Net infiltration = [ (Infiltration Flow)2 + (Exhaust Flow)2 ]1/2

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

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

2015-02-20 13:04 GMT+01:00 Julien Marrec :

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Julien,

I don?t have time to dig into your questions at this moment (deadlines!), but just a quick check for you, are you modeling electric resistance heat via the system-> baseboards entries? If so, I don?t believe eQUEST ever autosizes baseboard heat, you always have to manually input (or use a formula based on space square footage?).

To get around this I often use the PTAC system, zero-out the fanpower, and then select electric resistance heat as the primary system level heat source. eQUEST should autosize that for you, and it would seem to be a thermodynamically similar system to inputting via-base-board (though I?m sure somebody can point out why it is better to use baseboards).

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

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Nathan,

Thanks for the answer. Your baseboard workaround is interesting, I'm
curious whether it produces differences or not.

I'm not using baseboards. I have both UVT systems (gas furnace) for my
heated-only spaces as well as PTAC (hot water coil / DX cooling coil) for
the apartments, and both are behaving the same way.

I've played around a little bit, switching between Zonal Exhaust and
Outside Air Flow. It's not taking the airflow into account when it's set as
Zonal Exhaust, see attached pictures (Tenant Storage is a UVT with 400 CFM
exhaust, 2Sys is a PTAC with 41.6 CFM exhaust)

I don't know what to do...
Aside from running the model with it as Outside Air Flow, writing down the
sizing, going back to the model to put it back as zonal exhaust and
manually setting the sizing... I'd do that if I had only a couple of
zones... But I don't, so it seems highly unpractical.

Anyone has ideas?

Thanks,
Julien

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

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

2015-02-20 16:33 GMT+01:00 Nathan Miller :

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Hi Julien,
Forgot to mention this, I've noticed this too... We've looked a number
of work-arounds but unfortunately I am also under a deadline and don't
have time to go track down what we have been doing lately...

Here are a a few work-arounds that I recall looking at.

1. Use larger HEAT-SIZING-RATIs to get capacities up
2. Specify equivalent infiltration at the Space, but set the
infiltration schedule to 0 on all days/hours other than the design
days. This requires some coordination with the design day windspeed
if you are modeling the wind-speed proportional infiltration that I
mentioned, otherwise, pretty straight-forward if you are using the
constant infiltration input.

Will try to dig up more definitive information later today when I have time.

David

PS: That info on the HRV modeling is also included in the help for
EXHAUST-SOURCE = BALANCED-INFIL. This approach seems like a reasonable
work-around for unit-by-unit HRV/ERVs.

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David,

Your Solution N?2 is smart!
I have my space infiltration defined as being Air Changes per hour, so I
can compute the appropriate design day infiltration multiplier pretty
easily (using quadrature). I end up with some very high multipliers for
some of my spaces but it seems to do the job correctly as far as sizing
goes.

It's tedious if you have a lot of different rooms with zonal exhausts, but
I don't see anything else that would do the job in a better fashion anyways.

Thanks for the advice,
Julien

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

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

2015-02-20 17:09 GMT+01:00 David Reddy :

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Julien,
Glad to hear you got it working. I looked back over some of our past
projects, and wanted to follow-up on a few key inputs that I'd recommend
reviewing and adjusting to see how they impact your model:

ZONE:SIZING-OPTION: In my opinion, ADJUST-LOADS is the correct setting.
The eQ Wizard sets this to ADJUST-LOADS, but if you have restored BDL
defaults, the DOE-2 default is FROM-LOADS, so you'll want to create a
user default to change this.
ZONE:DESIGN-HEAT-T and ZONE:DESIGN-COOL-T: These are important inputs
for sizing zone air flows, and then subsequently the size of
heating/cooling coils. Make sure they are properly coordinated with the
system deisgn supply air temperatures.

In addition to the space infiltration option I mentioned, you could
follow a similar approach by setting a zone OA rate (OUTSIDE-AIR-FLOW,
OA-FLOW/AREA, etc) equal to the design exhaust rate, and using the
SYSTEM:MIN-AIR-SCH to set the system OA to 1 on design days, but 0 on
all other days. However, one thing to keep in mind is that if you
specify OA at the zone, the zone system fans will run continuously even
if SYSTEM:FAN-CONTROL = CYCLING. I think setting the FAN-SCHEDULE to a
schedule of 0s and NIGHT-CYCLE-CTRL = CYCLE-ON-ANY, is another way to
make the system operate only on calls for heat/cool, but I'd recommend
testing it to confirm. In any case, a continuously running fan is not
really an issue if the fan power is 0, so if you've set up your model
that way, this other approach might be OK and easier to implement.

David

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