Hello,
I am trying to model a building with FCUs and a DOAS to deliver outdoor air
to the individual zones. I have put in the dummy zones for the DOAS system,
modeled as a single air handler with chilled and hot water coils. I have
changed the walls of the dummy zone to air walls, specified the DOAS to get
OA flow as sum of all zones and for the FCUs to get OA from the DOAS. When
I run the model, I get a lot of unmet hours corresponding to the dummy zones
and the DOAS system. I have tried but cannot seem to turn off the return
air path for the DOAS (since it is 100% OA) and have specified the minimum
OA fraction for the DOAS as 1. I am not sure if the unmet hours means that
my DOAS is not adequately cooling the OA load or if it something else.
Also, my "pump & aux" energy in the model seems very high for a building
that only has 3 pumps. Can someone familiar with modeling DOAS shed any
light on what might be causing some of these problems. I have attached the
.inp and .pd2 files for reference.
Thanks,
Marcus Eliason
I haven't had a chance to look at your .inp file, but when you are creating a DOAS system, make sure that you at least do the following:
1) Make sure the fan schedule is forced off at night and setback times.
2) Make sure the throttling range for the dummy zone thermostat is at least 4 degrees (I usually use 6).
3) Make sure that the heating/cooling schedules that the dummy zone follows are set to something like 120 degrees cooling and 20 degrees heating after hours, just to make sure that a call for heating/cooling doesn't show up in your report, and mess up the unmet load hours.
Just looked at your file: you need to specify some sort of minimum OA control or minimum air schedule for the FCUs (I think). Right now nothing shows up.
Also, why is the DOAS system serving so many "dummy" zones? I *think* your DOAS system is EL1 Sys1 (SZRH), yet it is serving 20+ zones. I'm confused...
James Hansen, PE, LEED AP
One further question that has been bothering me. How would you set up ERV with the DOAS? I've seen some posts from the archives but none are too clear. The return air temperature of the DOAS appears to be too low to use an ERV on. Any suggestions?
John Grando LEED AP
All,
I believe it is the RA temperature of the Dummy Zone that is passed to
the units with OA-From specified, so you are right John that the RA
would be too cold to return to an ERV, assuming your dummy space is
something around 55F.
I have spent many a late night trying to work around this problem and
can recommend the following approaches:
1. Model any Energy Recovery as an exceptional calculation (in a
separate model) for LEED
2. Incorporate the Energy Recovery into the FCUs or AHUs (although you
have to be really careful to properly account for your fan energy and
flow rates).
Regards,
Jonathan M. Curtin, EIT, LEED(r) AP
Thanks, what do you think about using preheating the exhaust (i'm doing OA cooling only) to a temp that i'm fairly certain is the zone max exhaust temp (75F) and then just subtracting out the energy used to preheat the exhaust temp after the model runs? Any comments?
John Grando LEED AP
I would agree with Jonathan's second approach, include the fan power from the DOAS into the energy recovery of the FCUs. If the DOAS has heating I suppose it could be accounted for under preheating, though I have not tried that one yet. I have been told that using dummy zones is not necessary good practice with equest though it has been awhile since then. Good luck.
Kevin