I'm having a hard time understanding how the PVVT and PVAVS systems would
end up with dramatic differences in heating energy consumption and wanted
to know if anyone had ever compared the two before?
I'm calibrating a model for an existing building. The installed system is
one of the common Trane SWUD units with single-stage electric heat and
three-stage cooling. If I model the system as a PVVT system to capture the
staging, the energy use in June/July/August is within 5% of actual
consumption but is understated by ~25% in the shoulder months and almost
50% in Dec/Jan. If I model the system as a PVAVS system, heating months
are within 5% but cooling months are overstated by ~15%.
The only keywords I adjust between the two are the ones that eQuest
automatically adjusts when the system type changes. One possible cause of
the problem from what I can see is that the PVVT system requires a control
zone and that control zone defines temperature for all of the other zones
(resulting in a lot of unmet load hours - up to 88% depending on the
control zone selected). Any thoughts?
Jeremy R. Poling, PE, LEED AP+BDC
Jeremy,
First thing I would do is look at the BEPU reports and compare space heating, space cooling, vent fans and unmet hours. If there is a difference in unmet hours, investigate in the SS-R reports. Then compare the monthly energy breakdown for the above categories as shown in the PS-E reports. Also look at SV-A to see differences in default system sizing if you didn't enter the same values manually.
These are the differences I notice in the default settings:
PVAVS has slightly higher fan power. However, I suspect differences in fan energy will probably be more of a function of cooling control differences between PVAVS and PVVT.
Fan control is "Inlet" for PVAVS and "Variable Speed" for PVVT.
Different performance curves.
I'm not sure if PVAVS assigns zone reheat automatically, but that is a big difference between the system types.
PVVT is going to be in heating/cooling mode based on the control zone, so you should expect unmet hours in multiple zones assigned to the system.
It is not surprising that PVVT is giving you less energy than PVAVS for these reasons.
Regards,
Bill
[Senior Energy Engineer 28Jun2012]