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How do you model exhaust fans in eQuest?

asked 2015-02-19 10:45:02 -0500

updated 2015-07-11 17:00:19 -0500

I'm modeling an apartment building that has toilet and bathroom exhaust and no mechanical supply for the apartments. Make-up air comes from trickle vents.

I have been thinking about the best way to do this, that is whether:

  • I should assign this to a zonal exhaust fan (EXHAUST-FLOW), or
  • 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

  • Will eQuest actually take into account that 50 CFM of outside air should be added (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?
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answered 2015-02-27 03:24:50 -0500

updated 2015-02-27 03:45:48 -0500

Modeling exhaust fans in eQuest

Prelimininary Findings

I've read all relevant eQuest-user threads, and started my own here, and got extremely useful feedback.

It seems that the consensus is indeed to model bathroom and kitchen exhaust as a zonal exhaust, such as the following:

Zonal exhaust

As you can see, I'm setting a flow (CFM) and a corresponding kW/CFM for the fan power. I'm setting the control as Constant Volume in my case, with a flow schedule that is always one.

The EXHAUST-SOURCE is set to "Infiltration", meaning that the make-up air comes from infiltration through the facade (which is the case for apartments without mechanical supply of air, such as apartments with trickle vents).

From eQuest help 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

How does eQuest account for this: further problems

As seen previously, it seems that the actual net infiltration through the wall is combined in quadrature (square root of the squares) between infiltration flow defined under "SPACE" and the Exhaust Flow.

Yet, I found out that I had low heating consumption and a bunch of unmet hours after adding the zonal exhaust.

To investigate, I played on the exhaust air:

  • I increased the outside air on certain zones and compared the SIM file
  • I moved the outside air from Zonal Exhaust to Outside air flow and compared the SIM file once again

On the following screenshot: On the left, "Exhaust Air" column, I set it as a zonal exhaust fan (like the picture above, except it's 400 CFM). On the right, I've set it as Outside air flow (on the central system).

Comparison between exhaust air and outside air flow

Full size picture: Click here.

I've got a good news and a bad news:

  • eQuest does take into account the exhaust air when calculating the consumption (that's the good news)
  • eQuest doesn't seem to take into account the exhaust air when autosizing the system! That's the bad news.

Workaround to get eQuest to autosize correctly with zonal exhaust

Since eQuest doesn't understand that it's supposed to size the system to handle the added infiltration, you need a workaround. One enlightened user on the previously mentioned mailing list suggested a smart one.

It's a little bit tedious if you have a lot of different zones with different zonal exhausts, but hopefully you won't have to repeat those steps more than 5-10 times.

The basic idea is to use a different infiltration ... (more)

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answered 2015-02-25 13:24:03 -0500

lisang gravatar image

Hello, based on my understanding of energy modeling, eQuest included, infiltration is an input, not a calculated output. Therefore, you use the built-in infiltration models to specify how you want infiltration "modeled". No matter the balance of air in the building/zone, infiltration remains as you input it. There will be no change because you have exhausted an extra 50 cfm of air. The infiltration you input will impact the thermal loads only, not the balance of air. Balance of air is done by the user.

To overcome, you may use a method developed at NIST (Ng et al.). It takes into account building characteristics, central system flow (including exhaust), and weather. The report can be found here You can also find the method in the ASHRAE Journal July 2014 Issue.

Good luck and let me know if you have further questions.

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Thanks for the answer and the report!

I understand this is your area of expertise, and I see your point about balance or air being separate from infiltration.

But I think your statement that there will be no change in infiltration because I have exhausted an extra 50 cfm is ambiguous in this specific case. Indeed, if you define zonal exhaust with make up air coming from infiltration, both the space calculated infiltration and the zonal exhaust equivalent infiltration are combined in quadrature (see my answer).

Julien Marrec gravatar imageJulien Marrec ( 2015-02-27 03:42:24 -0500 )edit

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Asked: 2015-02-19 10:45:02 -0500

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Last updated: Feb 27 '15