What's the proper methodology for the following situation?
Let's say I'm modeling a building that qualifies for a System #5 VAV baseline system and the four different spaces have different operating hours through out the week as follows:
Space 1: Monday 8am-10am
Space 2: Wednesday 6pm-8pm
Space 3: Thursday 12pm-6pm
Space 4: Saturday 10am-4pm
None of these spaces vary anywhere close to 40 full-load hours per week from the others, so no exception can be taken to split them off from the VAV system. So what operating schedule do you now use for the spaces and to set the occupied/unoccupied periods for the HVAC controls? Do you leave the spaces with their own individual occupancy schedules and create a new schedule for the HVAC controls that is kind of a sum of all the hours any one of the spaces could possibly be occupied? Or do you set one schedule as kind of an average for the occupancies of each space and make a matching schedule for the HVAC controls?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5906 thumbs up
September 9, 2013 - 10:00 am
The only schedule requirement is that the schedules are identical to the Proposed case. So whatever schedules you need to accurately simulate the proposed will need to be reflected in the baseline. How you do this can depend on the software. Many of them only allow you to set a schedule for weekdays, weekends and holidays.
For the HVAC controls you typically set a separate schedule of occupied and unoccupied times. Again it depends on how you set up the proposed case. Typically anytime the building is occupied the HVAC controls are set to occupied.
Matt Scott
EngineerN.E. Fisher & Associates, Inc.
18 thumbs up
September 9, 2013 - 10:25 am
Ok so for the thermostat/fan schedule, it's as simple as making one schedule that encompasses all of the hours of the year that any one of the zones is to be occupied?
What about when you have a zone, such as an attic where you don't care if it gets very warm, and you want to set the thermostat for something much higher than the required "supply air temp + 20 degrees" range? If you set it to the normal value (75 degrees let's say) along with all the other zones, you may get excessive unmet load hours in the Attic zone. Is it better to do this and then explain why the unmet load hours are high, or in this case should you break away from the "supply air temp + 20 degrees" rule and set the thermostat for that particular zone to the higher designed setpoint (i.e.; 90 degrees)?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5906 thumbs up
September 9, 2013 - 10:34 am
Yes that sounds right.
Is the attic an occupied zone?
You must meet the unmet load hour requirements but you can set the temperature wherever you wish as long as the baseline and proposed settings are identical. Adjust the supply air temp to meet the +20 degree requirement as necessary.
Matt Scott
EngineerN.E. Fisher & Associates, Inc.
18 thumbs up
September 9, 2013 - 11:13 am
The attic is occasionally occupied. It's a storage room and mechanical room and the only cooling/ventilation is done with exhaust and pulling outdoor air through, either when the room is occupied or when the thermostat calls for cooling. But my point is that it's on a VAV system with a bunch of other zones. The supply air temperature is set to 55 degrees, because all of the other zones are at a 75 degree thermostat setpoint. But this Attic zone has a 90 degree thermostat setpoint in the proposed design.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5906 thumbs up
September 9, 2013 - 11:43 am
Sounds like the design is your problem! Typically these spaces have their own systems. You will need to condition this space to 75 degrees.
Matt Scott
EngineerN.E. Fisher & Associates, Inc.
18 thumbs up
September 9, 2013 - 11:56 am
The attic does have its own system (in the proposed design). Are you saying I'm allowed to model it as its own system in the baseline building as well? And under what exception is it allowed, for future reference? For reference, it's currently being modeled along with all of the other spaces on that floor, as they all are minimally occupied (0-4 hours per week). Thanks.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5906 thumbs up
September 9, 2013 - 12:11 pm
Sounds like it might qualify under G3.1.1 Exception (b). If it is heating only you can also use a system 9 or 10 from 90.1-2010. I think there is a LEED Interpretation on that one.
Matt Scott
EngineerN.E. Fisher & Associates, Inc.
18 thumbs up
September 9, 2013 - 12:12 pm
Thanks!
Matt Scott
EngineerN.E. Fisher & Associates, Inc.
18 thumbs up
September 12, 2013 - 1:51 pm
One more clarification I want to make regarding the schedules of a baseline VAV system with several zones of differing occupancy vs the schedules of several single zone units for the proposed building:
The schedules have to be identical from Proposed to Baseline: thus the occupancy schedule of a space in the proposed building must match that of the baseline building. But what about the Fan/Controls schedule? If your proposed building is a bunch of single zone systems with unique operating hours and your baseline building is a single VAV system with multiple zones you're gonna have one baseline fan schedule and several proposed fan schedules.
In other words, let's say my proposed building has two single zone RTUs serving an office area and a multipurpose area respectively. The two spaces have very different operating schedules, but are to be grouped together as a single VAV system with two zones in the baseline building. I will match the occupancy schedules of the spaces in the baseline to the proposed. But I'm only going to have one fan schedule for the baseline system (since it's a single VAV unit), while I'm going to have two unique fan schedules for the two separate proposed single zone units. Is that acceptable or do I have to create a single proposed fan schedule that accounts for all occupied hours between the two systems to match the single baseline fan schedule? I'm guessing not, but just want to be clear of how strict is "the schedules must be identical for the baseline and proposed buildings". Thanks.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5906 thumbs up
September 17, 2013 - 1:21 pm
The schedules must be identical.
If the schedules vary enough then apply exception b and this will create a separate system which can be placed on a different schedule.
If exception b does not apply then you have to choose between the identical schedules and the G3.1.1 rule about modeling a system per floor. Since the schedule issue would likely have a much larger impact on the results it would be more conservative to model the separate baseline system so that the schedules match rather than hold to the one system per floor rule.
You can lump areas with similar schedules together on one separate system.
Unfortunately I am not aware of any formal interpretations on this issue.
Matt Scott
EngineerN.E. Fisher & Associates, Inc.
18 thumbs up
September 17, 2013 - 2:10 pm
Ok, thanks for the suggestions. Seems like this would be a common issue. I'm surprised Appendix G doesn't spell it out better.
In the situation I'm describing, the hours vary by far less than 40 hours per week. It's more like one space is open on Monday in the morning for 4 hours and the other space is open on Wednesday night for 3 hours. And then both are unoccupied the rest of the week. If they were similar hours I could see adjusting them both to be the same, but in this case, that would impact solar loading and demand charges.
Is there ever application where you can have multiple VAV systems on the same floor for the baseline? Or is the "one VAV system per floor" rule an absolute?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5906 thumbs up
September 17, 2013 - 2:38 pm
It is a rule that should not be violated without a very good reason. In this case it is far worse to try and fudge the schedules or have different schedules than to create another system or two on a floor.
So if you do not meet the requirements for exception b then I would suggest that you create a separate baseline system but the same system type as the rest of the floor. So yes, if the system is a VAV then the separate system would be VAV too.
Matt Scott
EngineerN.E. Fisher & Associates, Inc.
18 thumbs up
September 17, 2013 - 2:58 pm
Understood. Thanks!
Matt Scott
EngineerN.E. Fisher & Associates, Inc.
18 thumbs up
September 23, 2013 - 4:13 pm
One more clarification, please. If I deem it necessary to model more than one VAV system per floor for the baseline, because my proposed design has some exceptional systems, Do I model the exceptional VAV systems with non-reheat VAV boxes and gas furnaces, or do I model them with reheat VAV boxes and a hot water boiler? And if I model the exceptions with the hot water heat, do I make that boiler/plant separate from that of the primary baseline VAV system?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5906 thumbs up
September 23, 2013 - 5:17 pm
Be careful how you use the word "exceptions". If using a formal exception to G3.1.1 like (b) then you would model a system 3 as it instructs. If it does not qualify for a formal exception then model the same system type, sounds like system 5 in this case, for that space as is being modeled on the rest of the floor. Use the same boiler plant for the whole building, not per system.
Matt Scott
EngineerN.E. Fisher & Associates, Inc.
18 thumbs up
September 24, 2013 - 8:21 am
What if the area I'm formally taking exception 'b' for is served by a multi-zone packaged VAV system? Do I have to model each zone of that area with a separate single zone packaged unit with gas furnace heat (system 3) or should I use a single packaged VAV system with hot water reheat (system 5) like the rest of the baseline building?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5906 thumbs up
September 24, 2013 - 9:54 am
The area applicable to exception b would likely be covered by a single system 3. Let me try an example. A school with an office suite, baseline system 5. The office suite is on a different schedule that qualifies for exception b. The entire office suite would likely be modeled as a single system 3. Each room in the suite would likely not be separate system 3s. The substitution would be at the system level, not the zone level.
Matt Scott
EngineerN.E. Fisher & Associates, Inc.
18 thumbs up
September 24, 2013 - 11:07 am
Testing.... not sure if my reply posted.