Energy Recovery Wheels (ERV) and Chiller Run Hours

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Hello Bldg-Sim and eQuest Users,

I am having difficulty in eQuest V3.61 build 5653 (equivalent to 3.61e)
setting up an energy recovery wheel for an AHU with 60% min OA by design
serving typical offices/classrooms. Without energy recovery, the chiller
run-hours seem reasonable (about 4,000 hrs) and I am getting very little
coincident cooling/heating in the winter months based on the SS-C report for
the AHU. The AHU is set up with an enthalpy economizer and a min OA
schedule that is set to 0.001 for all hours to allow for economizer control
during night-cycle. The AHU runs for 6,907 annual hours including 3,749
night cycle hours. The zone thermostat type is proportional with a 2?F
throttling range. All cooling and heating load hours are met and the
chiller does not operate at outside air temperatures lower than 48?F.

Once I enable the airside energy recovery (enthalpy wheel), the chiller
run-hours jump to nearly 7,400 hours and there are a significant amount of
coincident cooling/heating hours in all winter months. The chilled water
loop control is set to demand (although I?ve tried a snap schedule resulting
in many unmet cooling hours). I am controlling the ERV with the mixed air
temperature and an exhaust bypass. I?ve tried scheduling the ERV to operate
at min OA and with a schedule matching the supply fan hours minus any
optimum start (-999) hours resulting in little change in the results. I?ve
also tried the fixed setpoint control method (using MEP data for the make-up
air temperatures) with no success.

I can reduce the chiller run-time to about 4,600 hours with the ERV enabled
if I disable the night-cycle fan control option. However, the unmet heating
and cooling hours become significant.

enabling night-cycle control also enables the ERV even with an On/Off
schedule calling for no ERV. When the ERV is enabled, the chilled water
loop also becomes available to satisfy any cooling load that the ERV is
creating and operates down to the coldest hour. I?m not sure if setting the
Min OA schedule of 0.001 (enable economizer) for all hours is affecting
whether or not the ERV is activated.

Any help with this control option or insight to the ERV calculations are
greatly appreciated.

John T. Forester, LEED AP

John Forester's picture
Joined: 2011-09-30
Reputation: 400

I have never had any trouble with the ERV module in eQUEST, I don't think
you found a bug.

You have to remember that when you use an ERV, the eQUEST default is to use
the HVAC fans (ERV integrated into the HVAC unit). The fan static pressure
is increased to compensate for the increased pressure drop across the ERV.
This extra SP increases both fan power and fan heat. The increase in
cooling power you are seeing is a result of your systems having to
compensate for this increased fan heat. The default increase in static
pressure for an ERV in eQUEST is 1.5" on both the supply and return. Adding
3" of static to any fan system will have a big impact on the performance.
If you are using packaged equipment that only has supply fans then all 3"
of SP will be applied to the supply fan which, if I had to guess, is what is
probably causing your problem.

The mixed air reset strategy with air bypass is the most efficient way to
run the ERV because it will allow the economizer to run by bypassing all the
exhaust air, that is why you don't see any change when you try different

You can however try different ERV strategies to optimize the performance.
For example using a self-contained ERV will reduce fan power and extra
cooling power (compared to an integrated ERV) because the fans are sized to
just handle the pressure drop of the ERV rather than having that pressure
drop transfered to the main HVAC fans.

One final hint: ERV defaults in eQUEST are set so the ERV's do perform
badly. This is to encourage users to discover the best way to design an ERV
for there system configuration. It's actually very easy to design ERV's
that use more energy rather than less, as you are finding out.

Good luck. I hope this helps.

Michael Tillou, PE LEED AP

Michael Tillou's picture
Joined: 2011-10-02
Reputation: 0