[Equest-users] Packaged VVT System Modeling Help

[Christopher Jones]

I have struggled with this on a few occasions.  From what the Carrier 
rep told me, their PVVT system will make sure heating loads are 
satisfied, then cooling loads whereas the DOE2 system makes sure the 
control zone loads are satisfied leaving other zones under heated or 
under cooled.  The only work around I have come up with is to play 
with the control zone to reduce the number of unmet hours then 
"remove" some of the heating capacity of the PVVT system and add it 
to the under heated zones in the form of reheat.  If it is a gas 
furnace system, I define a dummy gas boiler with no pumping 
power.  Not exactly reality but it does reduce the unmet hours.  This 
work around has been accepted by the CaGBC but I can't comment on 
what the USGBC would think of it.

At 03:25 PM 12/7/2009, Routh Consulting Engineers wrote:
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>I am trying to model HVAC systems using the Carrier 3V VVT zone and 
>bypass dampers with packaged gas/electric units and can't seem to 
>find a good match or dummy in enough other things to make up. Here's 
>what I've tried and, so far, failed with:
>
>1)      I've tried the PVVT system model, but the control zone 
>doesn't have a VVT damper so the equipment cycles off with no demand 
>in that zone. If other zones are still needing the same mode of 
>operation as the control zone, or worse, they need the opposite 
>mode, demands go unsatisfied and I end up with a large number of 
>hours with loads not met. I've tried using different zones on the 
>same system as the control zone, but I still end up with too many 
>unmet hours. The actual system has zone dampers for all zones and 
>controls figure out which mode to put the unit into and dampers 
>throttle accordingly. It also can change into the other mode of 
>operation if enough zone thermostats call for it.
>2)      I've also tried the RESVVT system. It's zone dampers and 
>ability to switch modes within the same hour is closer to how the 
>actual system really works, but the RESVVT system has no outside air 
>capability. The DOE-2.2 manuals note this and say to use a separate 
>air handling unit serving a dummy zone with essentially nothing but 
>the outside air equivalent to the necessary outside air of the 
>RESVVT system, which works OK for minimum outside, but doesn't tie 
>to the actual zone load to operate an economizer properly.
>3)      I've tried the CBVAV system, which bypasses extra air as 
>zone dampers throttle, similar to the actual system, but the leaving 
>air temperature control is essentially zone of greatest cooling 
>demand dominated and then relies on reheat for spaces needing a 
>little heating with zone dampers a minimum position, which I've 
>allowed to go essentially closed. The actual system has no reheat at 
>the zone dampers.
>4)      Last, I've tried the PVAVS system, but this system is also 
>controlled to zone of greatest cooling demand and relies on reheat, 
>and it slows down the fan as zone dampers throttle instead of 
>bypassing air and keeping the fan at essentially constant volume.
>
>If anyone has modeled similar actual systems and equipment or knows 
>a way around these limitations, I'd appreciate hearing from you.
>
>Rick Routh
>
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Christopher Jones, P.Eng.
Suite 1801, 1 Yonge Street
Toronto, ON M5E1W7
Tel. 416-203-7465
Fax. 416-946-1005
email cj at enersave.ca