HI,
I am working on several school models and they generally fall into the packaged vav with reheat category. The spaces are fine for cooling, but are way off for heating. When I look at the ss-p report the heating capacity for the units shown to be zero, when I look at the ss-a report it indicates the heating load. I have entered the hot deck temp, zone entering temp, a preheat coil. Any suggestions would be appreciated.
Thanks,
Matthew J. Baron
VAV units in Equest only have heating at the zones level, not at the system level. Look at the SV-A report and you should see a heating capacity for each zone.
Kristy M. Walson, PE, LEED AP BD+C, BEMP
Hi Matthew,
I have run into this problem myself for buildings in cold climates. The problem is that the reheat coils are sized to the reheat delta T (defaulting to 30?F) which usually assumes a supply air temperature of 55?F maybe resetting to 65?F in heating mode. However, in winter, the supply air temperature can dip quite a bit lower than this (down to 30?F or lower) unless you have a central heating coil. The DOE keyword is HEAT-SET-T and it's worth checking the .inp file to make sure this keyword is set. In eQuest you need to set the ?Hot Deck Max Leaving Temperature? (maybe to 65?F) to activate HEAT-SET-T. You might also try setting COOL-CONTROL = WARMEST to reset the supply air temperature upward as loads permit and increase the ?Reheat Delta T? to 40?F.
If this doesn't work, send your .pd2 and .inp up the list and maybe someone can have a quick look at the file.
All the best,
Dan
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Daniel Knapp, PhD, P Phys, LEED? AP O+M
HI,
I have tried both your suggestions, seemed to help slightly... still very high unmet loads for heating. I have attached the .inp and .pd2 files for inspection.
Thanks,
Matthew J. Baron
Matthew,
Check to make sure the thermostats are defined as reverse acting instead of direct acting. This will allow the system to start bringing on reheat when the VAV box closes to the minimum CFM stop setting. Otherwise, the space temperature will continue to drop.
Hope that helps.
Mike Busman
Michael R. Busman, CEM
I looked at your file. When I get these types of unmet errors, the first
thing I do is set the max flow for heating (under the fan flow parameters)
to 1 or whatever ratio is allowed - I think the eQuest default is 0.5,
which probably is too less for cold climates. If this does not help, I
normally look at the min flow (reducing it if possible helps). Other is to
look at the zone design cfm - usually reducing it helps - but reducing it
too much results in cooling hours unmet. For LEED I try to get each zone
unmet below 50 -so start with the highest unmet zone. Sometimes I end up
with a combination of all the above.
Hope this helps.
-Rohini
Matthew,
You have many extraneous exterior surfaces - roofs, underground floors, and exterior walls.
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Some other issues that may or may not be contributing to your undermet heating hours:
- #/occupants seems high
- You are oversizing your boilers by 25%. If this is for LEED/Appendix G, oversize the systems only and use 0.50 capacity ratio for the two boilers.
- System sizing-ratio should be "1.00" for Appendix G baseline. If you keep it at "1.15", change the cooling sizing ratio to "1.00" and heating sizing ratio to "1.087".
- Night cycle control is set to "Stay off". This should be "Cycle on Any" for an Appendix G baseline.
Regards,
Bill
William Bishop, PE, BEMP, BEAP, LEED AP
I recollect that setting 1.15 at the main tab increases the cfm also, so
you might want to leave it at 1 and make changes at the cooling/heating tab.
-Rohini