LEED System Nr 8, Ventilation Requirements VS Sensible Load

asked 2015-10-28 12:46:34 -0500

Gio
1148 ●1 ●13

Hi! I'm modeling system nr 8 for a baseline building. I set the same ventilation as the proposed building (G3.1.2.5). In the Air Loop, I modelled the same minimum air flow with a recirculation. I set also a supply air temperature based on the outdoor air temperature (I'm not so sure also about this, is it right?). So, if the supply temperature is blocked, how can it control both ventilation and sensible load if we have recirculation? The system should modulate the air flow to meet the sensible load, what if the ventilation requirements are higher or lower? In winter, if the minimum air flow is not enough is not a problem because of the reheat boxes. But in summer?
I can do it without recirculation, but it would consume too much energy. It's a little bit tricky to understand, I hope I explained it well. Thanks.

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answered 2015-10-29 12:33:36 -0500

Annie Marston
4297 ●4 ●31 http://www.resustain.com

Here are the rules from Appendix G as I understand them:

For a baseline system the supply air temperature from the AHU to the space should be a delta T of 20F between the setpoint in the space and the air temperature

The outdoor air should be the same between baseline and design case

The minimum air provided to each space by the baseline system 8 should be 30% of peak flow rate or outdoor air whichever is larger.

So the coils modulate (from 0 - 100%) in order to meet the load at minimum flow. If more cooling is required in the summer then the flow rate increases to its maximum for that space to meet the load.

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I have some difficulties to understand some chapters of Appendix G.

About "For a baseline system the supply air temperature from the AHU to the space should be a delta T of 20F between the setpoint in the space and the air temperature". I think you are talking about G3.1.3.12... but it is talking about the air temperature for cooling. What about heating?

"The outdoor air should be the same between baseline and design case", perfect as it's written in G3.1.2.5.

"The minimum air provided to each space by the baseline system 8 should be 30% of peak flow rate or outdoor air whichever is larger.

Gio ( 2015-11-04 06:00:56 -0500 )edit

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answered 2015-11-04 06:00:41 -0500

Gio
1148 ●1 ●13

I have some difficulties to understand some chapters of Appendix G.

About "For a baseline system the supply air temperature from the AHU to the space should be a delta T of 20F between the setpoint in the space and the air temperature". I think you are talking about G3.1.3.12... but it is talking about the air temperature for cooling. What about heating?

"The outdoor air should be the same between baseline and design case", perfect as it's written in G3.1.2.5.

"The minimum air provided to each space by the baseline system 8 should be 30% of peak flow rate or outdoor air whichever is larger.", in this case I think you are referring to G3.1.3.14... but it's talking about fan-powered boxes (reheat), not about the AHU in my opinion.

Therefore, if more cooling is required in a zone I need to use recirculation, but this cause a recirculated air also in the zone that do not demand more air: the result would be a low fresh air supply. This is because, if I have a fixed supply temperature I can't provide both ventilation and sensible load with a VAV system.

I do think it's impossible to meet both those requirements with a VAV system. The solutions could be: - use 100% outdoor air; this could make the baseline system use more energy (that's dangerous for baseline) - neglect the decrease of fresh air when sensible load increases - use 2 different VAV systems, one for ventilation and one for sensible load

I hope I've been clear enough... I have been thinking about this quite a lot (and I didn't even talk about dehumidification and humidification) but I can't find out where I am wrong. Thanks!