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Baseline System 8, Supply Air temperature

asked 2017-08-01 08:37:02 -0500

Gio's avatar

updated 2017-08-15 15:49:55 -0500

Hi, I'm following appendix G standard 90.1-207 to model the baseline building.

G3.1.3.12 is "The air temperature for cooling shall be reset higher by 5°F (2.3°C) under the minimum cooling load conditions." and I found this useful question:

What if I have one floor with 50% of the zone with a setpoint manager and the other 50% with a different one? In my case I have offices with 26°C (therefore supply air temperature from 15°C to 17,3°C) and a station with 28°C setpoint (therefore 17°C to 19,3°C).

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answered 2017-08-01 16:14:28 -0500

Molly Curtz's avatar

Here is what I would consider a reasonable approach: Start with the premise that one is required by Appendix G to have a single air loop serving the entire floor. Set the design SAT for this airloop at the worst case requirement (15C in your example). Allow the SAT for this airloop to reset up by 5F from the design SAT using a SPM:Warmest Size all the zones served based on 20F supply air to room air temperature difference, as required by App. G. If I understand correctly, this can be done by setting the Zone Cooling Design Supply Air Temperature Difference field in the Sizing:Zone object.

Control your zones in the simulation to follow the desired setpoint schedules as you described (that is, some zones are allowed to maintain higher temps in cooling mode operation.) The system will supply air at design SAT of 15C, resetting up to 17.3C if connected zone loads allow, and it will operate to meet the scheduled setpoints in all of the zones served. The zones with looser temp control requirements are unlikely to set the SAT requirement when SPM:Warmest executes.

The only potential concern that I can think of is the risk that some of the loosely controlled zones might be overcooled at the zone minimum cooling flow rate.

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I think it's a good method. Yes you're right about "Zone Cooling Design Supply Air Temperature Difference field in the Sizing:Zone object.". For what I have understood, in this specific case I have to decide if to brake rule G3.1.3.12 or the rule of "every floor with one vav".

Gio's avatar Gio  ( 2017-08-03 07:14:16 -0500 )edit

I think your situation is an edge case that is not anticipated in the App.G language. But, I'm not sure that I would say you are technically "breaking the rule" of G3.1.3.12. I say this because we can make a distinction between design of the system and operation of the system. My recommendation is to design the entire system according to the SAT requirements of the most tightly controlled zone. In simulation (operation), you are then controlling the system to have looser control (allow warmer temps) in some zones. You will of course use the same T-stat setpoints in both BL and PROP models

Molly Curtz's avatar Molly Curtz  ( 2017-08-03 11:36:33 -0500 )edit

I do agree, I think it is borderline. Anyway I did as you explained: I set the design conditions related to the lower zone setpoint. The only risk is, as you highlighted, an overcooling.

Gio's avatar Gio  ( 2017-08-08 06:04:45 -0500 )edit

answered 2017-08-01 09:36:41 -0500

Have a look at G3.1.1, exception b., make sure you don't fall there.

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Also, spaces controlled to different temperatures should not be defined as the same thermal zone.

Eric Ringold's avatar Eric Ringold  ( 2017-08-01 09:42:36 -0500 )edit

@Julien Marrec I've read G3.1.1, exception b and used it for server rooms. In this case offices and station are 50% 50% so I can't decide what's "the rest of the building". Offices are not so different from the rest of the building and they do not "differ significantly from the rest of the building". Thay are far from 31,2 W/m2 of load difference.

Gio's avatar Gio  ( 2017-08-01 10:06:32 -0500 )edit

@Eric Ringold offices and stantion areas are defined as different thermal zones

Gio's avatar Gio  ( 2017-08-01 10:07:21 -0500 )edit

Oh sorry, the way you worded your question made it sound like half of a single zone is controlled to a different set point than the other half.

Eric Ringold's avatar Eric Ringold  ( 2017-08-01 10:10:45 -0500 )edit

No problem. At the moment I "splitted" the first floor, using 2 different air loops: one with every zone with 28°C setpoint and one with 26°C.

Gio's avatar Gio  ( 2017-08-01 10:14:47 -0500 )edit

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Asked: 2017-08-01 08:37:02 -0500

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Last updated: Aug 01 '17