[BLDG-SIM] Modeling packaged VAV with gas heat,

[Fred Porter]

All;

Or trying to in DOE-2.2. 

 

As Andrew points out; many or most of these systems operate differently
than the simple constant setpoint, or lack of central heating coil, John
describes. These are the DOE-2 defaults, and will show little or no
central heating unless OA is high and airflow is turn downed
considerably. But with the very common VAV AHU with gas furnace and
electric zone reheat, there is incentive to maximize the use of the
furnace, for a variety of reasons. Also the reference to preheating the
OA is misleading with respect to DOE-2 and most actual packaged VAV
systems, as the preheat is usually in the mixed air. 

 

Note that some VAV RTU furnaces can only be used for warmup, they
actually cannot turndown low enough to allow tempering during normal
occupied operation, and if the mixed air is cold, it will just be sent
down to the electric coils. 

 

For warmup periods, in typical practice the supply air temp is set up to
85 or 95F for a while in the morning until some control (return air, max
or average of zones) signals that this warmup period has been completed.
Numerous variations of box damper control can go along with this (all
stay at minimum; open if below setpoint/close if above). In general the
reheat coils are kept off until the warmup period is over. The warmup
described in the DOE2 documentation merely indicates closing the
economizer, and turning the cooling coil off. As far as I can tell;
DOE-2 cannot directly model the control of the zones and furnace now
typical; and the various workarounds we've tried to mimic it are
brittle, error-prone, kludgy, and/or neglect significant energy terms.
They include:

*  Putting the electric reheat on a loop to elec boiler and turning off
   that loop during warmup. Raising the zone setpoint to occupied during
   the warmup, and then apparently if ALL zones are calling for heat,
   AND if the COOL-CONTROL = WARMEST, the SAT will float up to
   HEAT-SET-T or some temp between HEAT-SET-T and COOL-SET-T. The zone
   min-air in the core zone could be scheduled down to avoid
   overheating…
*  Again putting the elec reheat on a loop with elec and gas boilers
   scheduled via LOAD-MGMT & EQUIP-CTRL. Various things can be tried
   with AHU fans and SAT control during this period.
*  Something similar but with virtual baseboards manually sized for each
   zone, and creating a virtual gas boiler on a virtual baseboard loop,
   only enabled during warmup, while the reheat loop is scheduled off.
   Again, various things can be tried with AHU fans and SAT control
   during this period.
*  Making COOL-CONTROL = RESET, and using dual reset schedules, with the
   warmup ramping from SAT from 60F @ 60F ambient to 95F @ 20F ambient,
   or whatever.

 

If this typical practice can actually be modeled as practiced, it would
be nice to have detailed examples in some documentation or somewhere on
the DOE2.com website. If this common mode on a common system can't be
modeled as practiced, one of the utilities or states that accept DOE2
simulation results for incentive calculations needs to contract the DOE2
authors to implement more robust and realistic morning warmup routines.
Variations include using the central AHU furnace and supply air for
heating during all unoccupied periods. In some cases the simulation
needs to allow a scheduled variation in THERMOSTAT-TYPE from
PROPORTIONAL to REVERSE-ACTION during warmup or unoccupied central
heating.  Does anyone know if E+ can model these sequences
realistically?

 

 

As far as the LEED/App G question; App G meant for mixed/"hybrid" heat
source systems/buildings to be compared to all gas baselines (i.e. the
electric coils would be replaced by HW coils from a gas boiler). USGBC
reviewers reinterpreted this in an EA Cr1 CIR to allow the baseline to
be a similar hybrid as the proposed. 

 

Oh yeah the gas consumption shown under pumps/aux is the default pilot
light. Set FURNACE-AUX to zero.

 

--

Fred W. Porter, B.S., LEED A.P.
Senior Engineer
Architectural Energy Corp., Boulder CO

 

  _____  

From: Aulbach, John [mailto:jaulbach at nexant.com] 
Sent: Wednesday, October 03, 2007 3:14 PM
To: BLDG-SIM at gard.com
Cc: Andy Frichtl
Subject: [BLDG-SIM] Appendix G Baseline HVAC system and modeling
packaged VAV with gas heat

 

Andrew:

 

I don't know what your climatic area is, but why should ANY heating show
up at the central unit? A VAV w/reheat, in my meager experience, is
controlling the central cooling coil to 55 DegF and adjusting reheat
coils (and VAV airflows) from there. When winter shows up, your cooling
coil is REALLY backed off (or off) and the VAV still flies, with heat
added at the space (terminal) level.

 

The only purpose you may have for a central heating coil is to preheat
outside air to, say, 40 DegF. Depending on where your climate is, this
may RARELY happen?

 

Does this sound right, folks?

  _____  

From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Andrew
Craig
Sent: Wednesday, October 03, 2007 1:35 PM
To: BLDG-SIM at gard.com
Cc: Andy Frichtl
Subject: [BLDG-SIM] Appendix G Baseline HVAC system and modeling
packaged VAV with gas heat

I have two questions that have come up recently…

 

1. If the a proposed building pursuing LEED is going to use a Packaged
   VAV system with a gas furnace in the unit and electric reheat in the
   boxes, would the baseline building (according to ASHRAE Appendix G)
   fall under the Fossil/Electric Hybrid category or the Electric
   category since that is the predominant heating source? 
2. Secondly, what is the best way to model this type of system in
   eQUEST?  I have experimented with first defining a packaged VAV unit
   in the wizard with hot water coils as the heating source.  Then in
   the detailed edit mode, I changed the system “Heat Source” to furnace
   and “Zone Heat Source” to electric and deleted the boiler and hot
   water loop.  When I simulated this model, no natural gas usage showed
   up in the BEPS report under Space Heating, however, there was a tiny
   amount (~2% of total heating energy) that appeared in the Pumps & Aux
   category.  Can anyone explain what’s happening and/or suggest the
   best way to model this kind of system?  As a side note, from projects
   we have monitored here in Oregon, we would expect to see about 30% of
   the total heating energy use go to natural gas for this type of
   system, primarily from morning warm-up. 

 

Thanks for your input.

 

Regards,

 

Andrew Craig, EIT, LEED® AP | Mechanical Designer

INTERFACE ENGINEERING 
708 SW Third Avenue | Suite 400 | Portland, OR 97204 

direct: 503.382.2696
office: 503.382.2266
fax: 503.382.2262
email: Andrew_C at ieice.com
web: www.ieice.com 

Consultants of Choice to the Built Environment for over 35 years
Kirkland, WA | Portland, OR | Sacramento, CA | Salem, OR | SanFrancisco,
CA

 

 
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