System-Level Exhaust??? How?

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I need have an ERV which moves all the exhaust and supplies conditioned
OA to 2 of 3 FPVAV air handlers. I've specified the appropriate
enthalpy wheel specifics at the two affected PIU systems, maintaining
identical efficiency properties and breaking down CFM's and
fan/auxiliary power figures by weighting each by the supplied OA amount.

I understand from the help files that zonally-specified exhaust air does
not play into heat-recovery, when we specify energy recovery at the
system level (help section is copied below and highlighted for
reference).

I take it then I'm supposed to zero out all zonally entered exhaust air
flows, sum up the flows and add at the system level... but then
how/where do I specify system-level exhaust CFM rates?

I've tried to enter EXHAUST-FLOW = #### under the systems in
question in the .inp file, but it introduces the following error:

*13131* EXHAUST-FLOW = 2550

*ERROR********* ============
*****************************************************************UNKNOWN
KEYWORD or PREVIOUS .. MISSING

More errors ensue, and a long string of things appear to be
defaulted/lost when I continue to open the file... something is
obviously wrong with this approach.

NICK CATON, E.I.T.

EXHAUST-FLOW

Flow rate of the zone's exhaust fan. This entry can be omitted if there
is no exhaust from the zone, if there is only central exhaust by way of
the system return, or if the exhaust tracks the supply flow (see
AIRFLOW-TRACKING). If specified, EXHAUST-STATIC should also be entered.
The exhaust fan may be either constant flow or variable flow.

If exhaust air make-up is via the air handler (EXHAUST-SOURCE =
AIR-HANDLER), then the exhaust fan can operate only only when the system
supply and return fans operate (see SYSTEM:FAN-SCHEDULE or
SYSTEM:EXHAUST-FAN-SCH).

The SYSTEMS program simulates heat recovery from 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.
Return fan static should be adjusted, on a weighted average basis, to
reflect zone exhaust fan conditions.

The program will not allow SYSTEM:MIN-OUTSIDE-AIR to be less than the
sum of EXHAUST-FLOWs for all zones divided by the sum of supply flows
for all zones. That is, MIN-OUTSIDE-AIR will not restrict the operation
of exhaust fans. The exception is when EXHAUST-SOURCE = INFILTRATION, in
which case zonal exhaust does not affect air handler outside air
requirements.

Nick-Caton's picture
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Joined: 2011-09-30
Reputation: 605

Hi Nick,

It used to be, and looks like it still is based on the documentation, that
you would input your exhaust cfm at the system level as return air cfm.

Hope that works!

Carol

cmg750's picture
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Joined: 2010-10-05
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Nick,

DOE-2 enforces a mass balance at the system level. Whatever OA that is
not exhausted at the zone level must be returned to the central system
and exhausted through your heat recovery device. By defining the OA
volume flow rate, you are also defining the exhaust volume flow rate.

bfountain's picture
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Reputation: 201

Brian/Carol, thanks very much for your replies!

I'd like to spell out a simple example to both illustrate the concept
and verify my understanding:

Starting System Flows (CFM):

1,000 Supply Air

300 Outside Air

200 Exhaust Air, specified at a the zone(s) level.

800 Return Air

To correctly model energy recovery with this system, we need to remove
the 200 CFM zonal-specified exhaust, and add it to the return air
stream.

Energy-recovering System, corrected:

1,000 Supply Air

300 Outside Air

1000 Return Air

Am I understanding this correctly?

Thanks again!

NICK CATON, E.I.T.

Nick-Caton's picture
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Reputation: 605

Nick,
In your corrected you would have 300 cfm available for the heat recovery but I read your intent to be heat recovery on 200 cfm of exhaust.
Would you be better to specify 100 cfm of zone level exhaust to get a 200 cfm system level exhaust to recover from?
Check and/or manipulate your schedules and static pressures too.

Paul Riemer

Paul Riemer's picture
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Paul, that's very sharp of you =)!

In my example I neglected to specify or state whether I wanted to
recover energy from the 100 CFM of relief air. You're correct in
assuming I do not (relief air in the design is leaving the building via
economizer, not the ERV).

Paul's suggestion, to specify the relief air at the zones as exhasust,
does appear to be the correct solution if it's true the zonal exhaust
will not encounter the energy recovery equipment.

Here is my corrected example - does this appear correct to everyone?

Starting System Flows (CFM):

1,000 Supply Air

200 Exhaust Air, specified at a the zone(s) level

800 Return Air

300 Outside Air

100 Relief Air, exiting via economizer (no energy recovery)

To correctly model energy recovery with this system, we need to remove
the 200 CFM zonal-specified exhaust, and add it to the return air
stream. We then need to specify the 100 CFM relief air at the zone(s)
as "exhaust" to isolate that amount from the energy recovery equipment.

Energy-recovering System, corrected:

1,000 Supply Air

100 Relief Air, specified as "exhaust" at the zonal level

1000 Return Air (800+200)

300 Outside Air

The remaining balance of 200 CFM will be treated as the amount of
exhaust air for the specified energy recovery equipment.

Does this appear correct?

NICK CATON, E.I.T.

Nick-Caton's picture
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Joined: 2011-09-30
Reputation: 605

I'd leave it this way; You would be recovering heat from your exhaust and
maybe a bit from your return air, It's all good. Just be sure to add the
extra sp in at the return level to account for the ERV.

Carol

cmg750's picture
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