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

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@xxxxxxxxxx]
Sent: Wednesday, October 03, 2007 3:14 PM
To: BLDG-SIM@xxxxxxxx
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@xxxxxxxx [mailto:BLDG-SIM@xxxxxxxx] On Behalf Of Andrew Craig
Sent: Wednesday, October 03, 2007 1:35 PM
To: BLDG-SIM@xxxxxxxx
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...

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?
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@xxxxxxxxx
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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