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Air handling unit, Leed certification

asked 2017-07-31 10:51:59 -0500

Ricky gravatar image

Good afternoon,

I am modeling the baseline building hvac system for a Leed certification. There is a hvac system No.8 which serves two different zones on the same plane (a single system is required), providing primary air and cooling the environments. In the proposed building, an area has a fancoil while the other is directly connected to the AHU. Moving to the baseline, if both zones require cooling but only one requires air exchange, the second zone will receive an unnecessary air replacement, contradicting G3.1.2.5.

How can I fix the problem? Do I have to create more AHU?

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Good question. Same problem for zones in the same floor (that should have the same VAV) but with different setpoint temperature. For G3.1.3.12 the supply air temperature should be different (?)

Gio gravatar image Gio  ( 2017-07-31 11:14:37 -0500 )edit

See if this post answers your question

David Goldwasser gravatar image David Goldwasser  ( 2017-07-31 11:30:36 -0500 )edit
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@David Goldwasser Thank you, but I still don't understand. Immagine: two zones in the same floor. Both demand cooling but only one ventilation.

Ricky gravatar image Ricky  ( 2017-07-31 11:41:16 -0500 )edit

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answered 2017-07-31 15:00:53 -0500

updated 2017-07-31 15:06:54 -0500

I don't see the contradiction with 90.1-2007 G3.1.2.5 (Note: different versions of the standard have different numbered sections, so it's helpful to specify the standard version when citing a specific section).

In your proposed case, say Zone 1 (served by AHU) receives 50 CFM of outside air, and Zone 2 (served by FCU) receives 0 CFM. In the baseline model, you would model both zones attached to a single AHU, which you model delivering a minimum 50 CFM of outside air. Thus the minimum outside air ventilation rates are the same (50 CFM) in the proposed building and baseline building designs, which meets the language of 90.1-2007 G3.1.2.5.

Edit: your FCU zone might fall under the G3.1.1 exceptions, in which case it would not need to be attached to the AHU in your baseline, and the System 4 serving it would be modeled with 0 CFM of outside air.

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I confirm that this was the standard 90.1-207. If I understood correctly, do you mean that in the baseline building I have to respect the minimum external air ventilation rate globally but not at the level of a single thermal zone?

Ricky gravatar image Ricky  ( 2017-08-01 03:43:32 -0500 )edit
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@Eric Ringold If I use a single AHU with two zones with different setpoint, how can I set the supply air temperature with G3.1.3.12?

"G3.1.3.12 Supply Air Temperature Reset (Systems 5 through 8). The air temperature for cooling shall be reset higher by 5°F (2.3°C) under the minimum cooling load conditions."

I read this "https://unmethours.com/question/17852/how-to-model-supply-air-temperature-reset/" but if I have different setpoint it does not work

Gio gravatar image Gio  ( 2017-08-01 04:07:43 -0500 )edit
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@Ricky yes, the language of that section specifies matching ventilation rates between the two _buildings_, not individual zones.

@Gio you are asking a separate question for which you should create your own post. But I'll say a zone with a different set point will probably meet the exceptions of G3.1.1 and get it's own single-zone system.

Eric Ringold gravatar image Eric Ringold  ( 2017-08-01 08:06:07 -0500 )edit

Thank you Eric, I thought this could be connected as topic. I'll create another question.

Gio gravatar image Gio  ( 2017-08-01 08:30:16 -0500 )edit

Thank you @Eric.

Ricky gravatar image Ricky  ( 2017-08-01 08:55:17 -0500 )edit

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Asked: 2017-07-31 10:51:59 -0500

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Last updated: Jul 31 '17