Building excess air


Hello, I’ve been studying HVAC for about a month now reading several documents, standards & guides. One thing that it’s not clear to me is this:

Where I work we have subcontracted de HVAC design of some projects because here we don’t have HVAC experts(I am a electrical engineer but don’t have all the knowledge necessary to design by myself an HVAC system). So here is my question:

I was looking at two different HVAC projects that were subcontracted to see how designers design their HVAC system. After all my readings of the subject and after analyzing that I thought I was somewhat clear on all the concepts of the HVAC design until I saw the following:

Both Projects are office hvac designs. They have a Fan Coil Unit or some type of air handler that I don’t remember for each zone. They have a series of ducts supplying fresh air to every Air Unit. Taking a random zone for example, they have 8 diffusers each supplying 750 CFM for a total of 6000CFM. A return duct specified for returning 6000 CFM. The fresh air duct is specified for supplying 200 CFM of outside air to meet ventilation requirements. The return air goes directly to the air handler that is on the false ceiling and mixes 6000 CFM with the 200CFM of fresh air, and then supplies it back to the space at a specified 6000 CFM. My questions are the following:

1- I thought that if you were supplying 200CFM of fresh air, you need to exhaust 200CFM coming from the return duct, so when balancing the air 200CFM Fresh air + 6000CFM return air – 200CFM exhausted return air = 6000CFM supply air. But in this case they are not exhausting any air outdoors, so apparently they are mixing 6000CFM(Return air) + 200CFM(Fresh Air) = 6200CFM which doesn’t seem logical to me because the supply is 6000CFM. In a system like this, how can you exhaust the air outdoors? do you need to install a duct connecting the air handler return to the outside, or how can I achieve this?

2- Even if I design the return duct at 5800 CFM so that 5800 CFM(return air) + 200 CFM(Fresh air) = 6000 supply air, do I still need to exhaust air outdoors so I wont have pressure problems or what do I do in this case? This bothers me a lot because I saw this same design in 2 different projects, and I am really confused as I’m still learning. If someone can give me some explanation on this specific subject of air balance it would be much appreciated, as learning by my own has been one difficult task, considering also that most good documentation are not in my dominant/main/mother language that is Spanish.

Thanks in Advance.

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Posted by Jose
Asked On February 18, 2020 2:48 PM
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