bathroom exhaust/ventilation

[swoosh172]

I have a 25,000 SF office building with a large restroom. I am locally exhausting it at 10ac about 500 cfm, but am confused as to whether or not I have to make that 10ac (500cfm) up with outside air ventilation through the main air handler coil. It is a VAV w/ reheat application. The bathroom requires only 100 cfm to maintain interior cooling loads. Can I just exhaust the air locally and make it up with transfer grilles from the adjacent conditioned corridor and just ignore the outside air makeup at the coil level? If I don't make this up at the main coil via OA ventilation, then the area would be under a high negative pressure. What do ya'll think??

 

[tys90]

You are describing two different problems. If you transfer the air but don't make it up at the coil the bathroom won't be under a high negative pressure. It doesn't know if the air it's transferring is OA or not, it's just air.

However, you need to make sure your building doesn't go negative as a whole because of the bathroom (and other exhaust).

 

[swoosh172]

Thanks tys. Is it acceptable engineering practice to just increase the supply diffuser cfm to the bathroom to meet the 10 air changes. This would increase the size of the supply fan, but not change the main coil load much at all.

 

[swoosh172]

Sorry, one last thought. Would it be safe to say that the 500 cfm difference between supply and exhaust does not create a big pressure differntial in the building. The building is new construction and 25,000 sf with a few entrances, there may be enough infiltration to makeup the air difference. The reason this matters so much is because if I have to bring in 500 cfm more Outside Air through the main coil it is such a large increase in load due to having to meet the ashrae 62.1 vent calculation to be leed certified, ensuring that enough make-up air makes it to the space with a vav box system.

 

[willard3]

You should introduce the OA near the bathroom and let the bathroom stay negative. You want to capture any odors getting into the building and this will ensure that.

You're going to have to condition all the OA or make the occupants uncomfortable.

Depends upon your codes as to whether you can use transfer grilles, especially in a corridor used as a means of egress.

 

[tys90]

swoosh172, in a VAV/reheat scenario you will probably end up reheating a lot more if you increase the supply air flow to 500. If you do that, I wouldn't increase it to 500 either, you want air to flow into the space from other areas to keep odors contained.

Think of the building like a control volume. Look at the air entering (ventilation), the air leaving (exhaust and relief) and make sure you don't have more air leaving than entering. Never count on infiltration to make up air. Sometimes you need to provide more OA than 62.1 requirements because there is too much exhaust in the building.

 

[yusef814]

You may utilize air from nearby storage rooms or any other exhaust points (with higher air quality than toilet air as per ASHRAE 62.1) for air transfer to the toilet. But, make sure that it remains negative as this is the primary purpose of providing local exhaust in this area. The rest of it, I suggest should be made up at the coil,to maintain positive pressure in the building.

Don't rely on infiltration as this is unfiltered and difficult to account as heat load.

 

[walkes]

Swoosh172, you have a 25,000 sq ft building the ventilation requirement for the building, not including occupants will probably be 1500 CFM (0.06 CFM per sq ft), that is more than enough to cover the 500 cfm exhaust. So in essence you are already bringing in the make-up air for that and more.

Just remember to look at your ventilation requirements for the building as a whole and compare the exhaust requirements. If your ventilation outweighs your exhaust, you have enough make-up air and will need to relieve some air. If your exhaust exceeds your ventilation then you need to bring in more make-up air.

 

[swoosh172]

Thank you everyone! I was confused about the ventilation requirements but Walkes has cleared it up. I thought that eventhough I have close to 2,000 cfm ventilation for the building I still have to add 500 cfm ventilation air to make-up for the local exhaust, making the total OA 2,500 cfm and increasing the size of the ahu, coils, etc. This appears to be incorrect. Thanks again.