VAV ERU w/ CV bathroom exhaust

[cdxx139]

Hello, I have a high-rise art school project that I am designing using a four pipe Fan Coil Unit with primary air system from a Dedicated Outdoor Air System (100% outdoor air unit w Energy Recovery wheel)

To meet LEED Silver, a point requires CO2 sensors in high density occupant spaces. The plan would be for the CO2 sensor to control a modulating damper (like VAV box). The supply fan VFD shall be controlled on the Static Pressure of the supply ductwork. As dampers close, the SP increases, the Supply Fan (SF) VFD slows, and fan energy is saved. (My goal is maximum efficiency, for max LEED points).

The Return Fan (RF) of the ERU would track the SF, so as the SF speed reduces so will the RF. The RF exhausts the bathroom that have a constant volume exhaust of 50 cfm per ASHRAE 62-2007. So as the RF speed slows, it will reduce less than code required exhaust from the bathrooms.

How can I resolve this?

1. Provide exhaust boxes, modulating dampers on the RF systems, and let the RF be controlled on RF SP, instead of tracking the SF. I would prefer not to do this, as it will require VAV boxes on the return of the art classroom etc. This would work, but now I have to add return boxes to every space in a high rise? That can get expensive and require space that is at a premium.

2. Double the exhaust rates required, so when the RF goes to 50% speed, the code required cfm is still exhausted.
This may work, but doesn't feel very energy efficient. Unless you come from the school of providing 10 Air Changes per Hour of exhaust air from the bathrooms is good practice, because that would require over 100 cfm of exhaust, so this system would work well with this control sequence.

So I was wondering if there was another way, other than additional boxes, or guess engineering.

Thank you

knowledge is power

 

[ChrisConley]

Dedicated washroom exhaust fan feeding into your ERU? Our building code (currently) requires us to duct washroom exhaust separately to an ERU, so infrequently we use a separate fan as well. Depends on the relative volumes involved.

 

[cdxx139]

Yes, I may have to have a dedicated wash room exhaust fan at each floor blowing into the ERU return shaft. If I use an Air Flow Measuring Station, and have an ECM motor on the fan, it will ramp up or down as the down pressure changes, and allow to exhaust the code required exhaust.

The washroom exhaust is about a 1/3 of the total supply air. Ill check the code, but if I provide purge air to the ERU, pressurizing the supply side, I should not get much cross-contamination. I believe less than 10% of Class 2 air is acceptable.

knowledge is power

 

[Ziggypump]

If you are ducting the return air from the spaces, a return VAV box isn't that big. It doesn't take up too much space, check out Price RxV series. The only thing that takes up space is the control box, otherwise the VAV is the same size as the duct.

Plus, are you already going for the OA oversize credit? Might help with your idea 2.

 

[walkes]

Perhaps using a constant volume box for the exhaust at each floor or riser (depending on your configuration), such that the exhaust airflow is independant of pressure and a variable volume unit to modulate the remaining 2/3 relief airflow. With airflow monitoring on the relief as you had noted.

 

[cdxx139]

Thanks Ziggy, buts its less the size and more the additional cost, multiply that by 100 spaces and it could get expensive.

And walkes, a constant volume box at each riser would not allow me to reduce the airflow returning from a CO2 controlled room, while exhausting constant flow from the bathrooms.

I think I went the wrong way and didnt think it out early enough in the projects. They are reducing the fan coil count to save cost. So I am going to make the whole system constant volume. The building is either occupied or not. Lousy energy savings, but first cost seams more important for now....or until they realize we cant get LEED certified, then maybe they will "find" some money.

knowledge is power

 

[Ziggypump]

Good luck

 

[tys90]

Welcome to 90% of the projects that start out as "LEED"

 

[cdxx139]

Problem solved. The school will be mostly full when in occupied mode (or so the client says), so the variable volume energy savings will be diminished. I went constant volume for the whole system. Thanks all.

knowledge is power

 

[cry22]

Seems late but, here are my two cents any way.

Looks like you are trying to kill 2 birds with one stone (trying to handle RA adn EA with one ERU). Except that toilet room air and RA are not supposed to mix by code.

Well, why don't you use 2 stones for your 2 birds? use a dedicated ERU for the toilet exhaust and connect to your DOAS system.

I take it you ar not in the US.
Here, toilet air is not THAT stale, it is not contaminated to a point where it needs purge because our plumbing fixtures are vented individually as opposed to just plumbing stack venting around the world, reason most European and middle east bathrooms SMELL like bathrooms. In the US, one can almost sleep in the bathroom.