Energy Recovery Wheel on VAV System - zonal exhaust?

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I am modeling a laboratory air handler system with 100% OA, ERV at the air handler and a separate laboratory exhaust fan. I modeled the laboratory exhaust fan as zonal exhaust and thought I would only get energy recovery from the remaining exhaust at the air handler but this is not the case, as described previously below and as I confirmed using a custom hourly report.
How then to model energy recovery only for the central exhaust and not the zonal exhaust?
The one thing that comes to mind is to derate the design ERV effectiveness based on the % exhaust that returns to the ERV. The figure below (from Trane?s Ronnie Moffitt, in the ASHRAE Handbook S26.11) shows the reduction in energy recovery as a function of bypassed exhaust.
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Yes the RETURN-CAP-RATIO limits flow to the ERV, as long as zone exhaust AIRFLOW-TRACKING is not set to TRACK-SUPPLY or TRACK-BOTH. (I just tried modeling it.) If the zone exhaust tracks supply, there is no return to the air handler, and if there is an ERV it sees the total volume of zone exhaust.
The problem with specifying RETURN-CAP-RATIO or a system RETURN-FLOW that is less than the SUPPLY-FLOW is that fan energy for the return fan is only registered for hours when the system supply flow rate exceeds (SUPPLY-FLOW ? RETURN-FLOW). For example, my 100% OA system has 23,000 cfm supply and 13,000 cfm return with the rest going to a lab exhaust fan and some exfiltration. There is no return fan energy reported until the hourly supply flow exceeds 10,000 cfm. Interestingly, the ERV fan power penalty associated with the system supply/return fans (when selecting HVAC-SUPPLY/RETURN for ERV-FANS) does not seem to be influenced by the zonal exhaust fan power even though the energy recovered is influenced by zonal exhaust flow.

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OK pour a fresh cup of coffee everyone?

Some of this is a continuation & update on the thread hooked in from last year, so I?m opting to more comprehensively addressing the issues in lieu of a more focused response. Apologies for length, but I am hoping a few others find this worthwhile.

Following is a diagram that reflects my present understanding of actual zonal/system air stream relationships in doe2/eQUEST models. It?s probably still wrong somewhere (and I know the doe2 reference entries contradict in a spot or two) but I believe it?s accurate for the topics under active discussion. I advise printing this off to sketch on for some of the examples to follow along (a few copies wouldn?t hurt for future reference):
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Bill?s query is really complex and has a few layers I am guessing are easy to miss, so I?m going to deconstruct and build on a few issues in isolation. I will eventually conclude with some constructive advice (promise!).

Issue #1: Getting a grip on exfiltration in eQUEST (this is something we really should lock down first)

The specified situation is:
+ 23,000 Supply Air (100% OA)
- 10,000 Zonal Exhaust
- 13,000 Return Air
(Assume no infiltration for the example)

How much exfiltration is happening (I?m providing a hint with the +/- signs)? Adding these up from the zones? perspective: The CFM available for exfiltration occurring in this design scenario is zero.

Let?s presume your mechanical designer wants to design some degree of pressurization with all building openings closed, and so intends 2,300 CFM of exfiltration @ full flows (10% of Supply). In the previous example we have no supply air available to exfiltrate, so to account for this we must reduce the operating return airflow by that quantity.

Adjusted setup is:
+ 23,000 OA / SA
- 10,000 ZE
- 10,700 RA
(Assuming no infiltration)

The net effects here are

1. 2,300 of the supplied air into the zones exfiltrates from the building due to fan pressurization.

2. The ERV does not see that exfiltrated air, so you can only recover heat from (at most): 10,700 + 10,000 CFM = 20,700 CFM.

An aside: the doe2 reference entries suggest you can only achieve exfiltration either by RETURN-CAP-RATIO or by specifying duct losses. You can more simply specify a return CFM less than the supply CFM directly. Return airflow inputs take precedence over return capacity ratio inputs in any case.

Another sidebar: About that ERV Summary report?
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? The ?exhaust? and other air flows reported at the top of the ERV Summary SIM report are regurgitations of the corresponding inputs for ?design? ERV airflows. Those inputs are ONLY used to indirectly determine ERV fan/wheel power (when you don?t tell it the answer more directly with the kW inputs), and they do not reflect actual ERV airstream rates.

? AFAIK you have to create a custom hourly report including ?ERV Outdoor airflow (CFM)? and ?ERV Exhaust airflow (CFM)? for the system to observe the behaviors I?m claiming above. Here are the variables I have noted for generally evaluating system Airflows:
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? ? and here?s what that looks like in an INP file, for a system named ?AHU1.? You can copy/paste this into the last section of your INP and it?ll work so long as you change the second line to name a system actually in your model (you?ll probably want to rename the report block while you?re at it).

"Assess Airflows Block - AHU1" = REPORT-BLOCK
VARIABLE-LIST = ( 17, 20, 21, 22, 292, 293, 222 )
"Exfiltration Report" = HOURLY-REPORT
REPORT-SCHEDULE = "Hourly Report Schedule"
REPORT-BLOCK = ( "Assess Airflows Block - AHU1" )

? The rest of the ERV Summary report is relatively useful / intuitive:

o The middle of the report is great for quantifying net annual ERV effects (short of a parametric run to isolate the energy impact by end-use breakdown), and also visualizing when & why you may be seeing counter-productive (?excess?) heat recovery and pre-conditioning

o The very bottom of the report (2nd page) gives a more detailed accounting of how often freezing/condensing conditions are occurring with the ERV equipment, which paints a little more informative picture than associated ATTN messages for when your model is advising you to address that in design.

Issue #2: ?My zonal exhaust shouldn?t get seen by ERV equipment!?

If your zonal EXHAUST-SOURCE is ?Air Handler,? Zonal Exhaust quantities cannot be directed ?around? the ERV (that is, pushed directly to the exterior as some related doe2 reference entries suggest). Zonal exhaust joins the relief airstream and will be seen at the ERV when ERV is included for the system.

If you instead choose either ?Infiltration? or ?Balanced Infiltration? for EXHAUST-SOURCE, the zonal exhaust stream is totally divorced from the central system & ERV equipment, and goes straight outside.

I totally agree. A toggle/checkbox next to the zonal EXHAUST-SOURCE would be a really nice feature. Something like ?this exhaust goes straight outside? vs. ?this exhaust is seen at parent system energy recovery equipment.?

For clarity: None of the zonal AIRFLOW-TRACKING options prevent Zonal Exhaust from reaching the ERV? but we?re circle back to that issue later.

Solution #2a: ?So what about David?s suggestion??

David?s advice to turn zonal exhaust into exfiltration (doe2 terms) is a great idea under a few conditions:

1. I need to choose zonal EXHAUST-SOURCE = ?Air Handler?


Fully addressing the ?why:?

Returning to the round numbers in the starting example, what happens if we force return to zero (i.e. don?t enter a return airflow and specify RETURN-CAP-RATIO as 0.0?

Recap for those keeping score:
+ 23,000 Supply Air (100% OA)
- 10,000 Zonal Exhaust
0 Return Air (don?t specify a return airflow & specify RETURN-CAP-RATIO = 0.0)
(Still locking out infiltration for the example)

+ 23,000 Supply
- 10,000 ERV Exhaust
- 13,000 Exfiltration (great! As expected.)

Say you instead wanted 13,000 CFM seen at the ERV and only 10,000 CFM of zonal exhaust to be excluded from ERV. Simply specify a smaller amount of return:
+ 23,000 Supply Air (100% OA)
- 10,000 Zonal Exhaust
+ 3,000 Return Air

+ 23,000 Supply
+ 3,000 Return
- 13,000 ERV Exhaust (10,000 Zonal Exhaust + 3,000 Return Air)
- 10,000 Exfiltration

Notice for each of these cases, the results from the zones? perspective are a zero sum, so long as we don?t bring infiltration into the party.

Special considerations for this approach:

1. Return fan energies are no longer easily accounted for with the usual inputs, because the return fan isn?t engaged for exfiltration quantities

a. A relatively simple solution: add RA fan power to the supply fan?s kW/CFM input

b. Acknowledging: a little fan heat contribution is shifted around in doing so (from the return air stream to the supply air stream), but in the macro level I expect this is an acceptably minor concession for most projects.

2. Might not be a problem per se, but Zonal Exhaust fan energy merits a little extra attention:

a. SV-A reports match your zonal inputs for exhaust CFM quantities (10,000 CFM in the above 2 examples)

b. Custom hourly reports however go a little bonkers: zonal exhaust bumps up to 23,000 / 20,000 respectively (appears to be a sum of zonal exhaust + exfiltration).

c. Resulting zonal exhaust power doesn?t shift with these edits in the SV-A reports, but may merit further investigation to confirm zonal exhaust power is accurate if you?re going down this path.
Solution(s) #2b: ?? but I gotta have TRACK-SUPPLY or TRACK-BOTH!?

David?s suggestion works until you specify exhaust tracking supply or exhaust/supply tracking each other (?both?).

Either tracking selection results in zonal exhaust that will bump upward (to 23,000 in the prior examples) to match the supply airstream. This removes all potential at all hours for exfiltration. This by extension means all exhaust will pass through the ERV equipment.

3 parting thoughts:

1. Bill?s proposal in his original query isn?t that crazy? (or at least it?s crazy in a good way!):

a. Assume the conditions of the zonal exhaust airstream and the conditions of the ?central? return airstream are effectively the same

b. Derate the efficiency of the ERV so that it returns an amount of heat transfer commensurate with a reduced ERV exhaust airflow, as you anticipate.

c. Share those ?derate curve? coefficients with your friends on the lists once you?ve worked it into an expression (please!)

2. [Speculative territory] You might alternatively sidestep the above limitations with zonal exhaust having to hit the ERV by pooling all zones requiring TRACK-SUPPLY or TRACK-BOTH under a separate (linked) system.

a. Remove ERV inputs in the linked system serving TRACK-SUPPLY / TRACK-BOTH spaces. I?m calling this the ?lab? system.

b. Retain ERV inputs in the linked system serving all other zones. I?ll call this the ?corridors? system.

c. Establish an OA-FROM SYSTEM tie ? lab system sources OA from corridors system.

d. Adjust central airflow inputs (Supply / Return) between both systems to appropriate the correct supply/return/exfiltration to each pool of zones (per the above instruction).

e. Adjust central fan power inputs between both systems to ensure fan energy is not over/under represented

f. The link will ensure all current/future edits to the associated schedules, controls, unitary efficiencies, etc? remain consistent (hopefully!)
The usual problem with OA-FROM-SYSTEM ?DOAS? applications with dummy zones is that you have to jump through extra hoops to ensure heat in the return/relief aistream gets seen at the DOAS? in this case we make that ?problem? work for us: The ?corridors? system will only see internal loads from the child spaces.

3. Consider runaround loops for heat recovery in lab general/fume exhaust airstreams are not an unheard of (if not super-duper efficient in transfer) ? you might actually need/want to weight the total effectiveness for more than one ERV system.

Whichever path you choose, you?ll want to continue using custom hourly outputs to ensure the intended behaviors are modeled. Draw up your expectations for SYSTEM vs. ZONAL airflow streams under different conditions (before and after ECM measures), and verify the expected behaviors using those hourly outputs.


#ERV, #VAV, #exfiltration, #variable-exhaust, #required-reading-before-your-next-laboratory-model

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Nick Caton, P.E., BEMP
Senior Energy Engineer
Regional Energy Engineering Manager
Energy and Sustainability Services
Schneider Electric

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