ERV on OA-From-System Comments from Jeff Hirsch

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Just wanted to share an e-mail from Jeff. I did not know that an ERV on a
OA-From system was not supported by eQuest. That explains why I could never get
the correct airflows on this type of design.

Unfortunately almost every project I work on does specify a VAV DOAS with an ERV
to supply ventilation air to some type of HVAC system, such as VRF indoor coils,
chilled beams or PTHP units. I will have to try the work around Jeff suggested.

Paul Diglio, CEM, CBCP

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Reputation: 400

Stormy:

In the past when I have modeled just an DOAS that did not have any heating or
cooling capacity with and ERV was to add an ERV to each zone system that the
DOAS fed. I used the same kw/cfm that the DOAS system had. So I chopped up the
DOAS with it's ERV into smaller ERV's on each system. This seemed to work good.

The problem with the project that I am working on now, is that the DOAS feeds
chilled beams. The DOAS chilled water coil and heating coil is sized in the
design with the assumption that and ERV is conditioning the outside air. The
DOAS has to dehumidify the air because the chilled beams cannot remove moisture
as there is no provision for condensate removal. If I remove the ERV from the
DOAS, the chilled and hot water coils are not large enough to condition the OA
before it reaches the ERV on the chilled beams and I can't add a chilled or hot
water coil to the ERV's on the chilled beams. So I loose the additional
heat/cool capacity designed into the DOAS.

Confusing.

Regards,

Paul Diglio, CEM, CBCP

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Reputation: 400

I have specified a DOAS as a single zone unit before and then designated it as the OSA From System in all of the other HVAC systems. If I leave the Design Outdoor Air and Exhaust flow inputs empty under the heat recovery tab, the outside air from the other HVAC systems does not show up in the ERV. But if I total the design outside air values and then input these into the Design Outdoor Air and Exhaust flows, the ERV works and seems to be representative.

Rick Goeres, PE, LEED AP

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Rick:

Thanks for the response. I have tried what you are suggesting and have not been
successful in modeling the same energy recovery results that manual calculations
indicate should be achieved. It seems to me that the return airflow is from the
zone that the DOAS is attached to and not the other zones, as indicated by the
return air temperature. In other words, it lloks like the only heat availabe
for recovery is from the zone or dummy zone served by the DOAS.

I would be interested in looking at one of your projects if you care to share
one.

If the eQuest developer, Jeff Hirsh tells me that an OA-From with an ERV is not
supported, I will have to take him at his word. He did not say what aspect is
not supported. Like I said, in the past I have chopped up the ERV and attached
a smaller one to each system served and achieved the results that my manual
calculations indicated would be achieved. I am just at a loss with a DOAS that
has chilled and hot water coils as well as an ERV, since the coils have been
sized assuming that the ERV conditions the air previous to it entering the
coils.

Without the ERV on the DOAS my discharge air temperature in the winter
(Westfield, MA) is as low as 30 degrees and the chilled beams, modeled as IU,
cannot supply the required discharge air temperature. If I try to remove the
coils from the DOAS and chop them up and increase the heating/cooling capacity
of each chilled beam system, my discharge air off the DOAS drops as low as -5 in
the winter and again the chilled beams with their mini ERV's cannot condition
this air.

Any ideas?

Thank you,

Paul Diglio, CEM, CBCP

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Reputation: 400

I will jump into this thread late. What the OA-FROM-SYSTEM keyword does in DOE-2 is to set the incoming outdoor air temperature of the system using the keyword (the borrowing system) equal to the return air temperature of the system it specifies it is getting its OA from (the lending system). That is all. Thus there is no real transfer of air using this keyword.

Regarding using an ERV with the OA-FROM-SYSTEM keyword, the ?air? is not returned to the lending system (see above) so there is no or little heat to recover in the exhaust airstream. There are 2 possible workarounds for this. I have used the frost control ?preheat exhaust? in the ERV keywords to set the exhaust temperature from which heat can be recovered. I then use electric heating for this exhaust preheat and put that preheat on a separate, zero cost, electrical meter. I then compare the heat added on the dummy meter to the internal gains in the actual space to ensure they are comparable. Alternately, you could add a dummy internal load in the lending system space to increase the exhaust air stream temperature to give you your heat for heat recovery.

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Brian:

Thanks for the comments. I was thinking that I might use the preheat option to
simulate heat returned from the borrowing systems. Any way to cool the exhaust
air so that the ERV can precondition the outside air in the summer?

Regards,

Paul Diglio, CEM, CBCP

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Reputation: 400