Methods for Fresh Air Input

[MacLaddy]

Hello,

I'm currently working on the HVAC system of a modular building that is approximately 4,000 sqft. This building has been designed with two typical residential style furnaces (less than 2 years old), each serving approximately half of the facility. There is currently no method for the introduction of fresh outside air. However, this buildings function is that of a commercial classroom and it badly needs some fresh air.
Using ASHRAE 62-1, I have calculated that I need approximately 900 CFM of fresh air. But I have no idea how to introduce it into an existing system. Any thoughts on how to do this? I've heard some people say that it just needs to be plumbed into the return, but that seems like too simplistic an answer. If that is the typical solution, would I just find the mark by an initial balance, and then hope everything just stays the same? What type of ducting is used?

Thanks. Let me know your thoughts and ideas.
Mac

 

[willard3]

I think 900 cfm for heating/cooling is way above the capacity of the existing units depending upon your local claimate.

Get a 900 cfm unit that will meet outside air heating/cooling requirements and duct it to the spaces where required.

 

[lukaiENG]

I second Willard3's comment.

 

[berkshire]

I would inspect those furnaces carefully first, If they are less than 2 years old they may be 97% efficient condensing furnaces with a coaxial intake and exhaust.
B.E.

You are judged not by what you know, but by what you can do.

 

[trashcanman]

As per Willard3, specify a 900 cfm 100% outside air unit. to discharge into the space. It will provide conditioned air at the room design condition (75 DB/50% rh ?). It can discharge into the return or supply. Not cheap, but it is what it is. Probably will have to be a custom unit.

HVAC Engineering is not by rumor, superstition or "hope".

If the furnaces are condensing type, that does not effect the outside air supply as they would be independently ducted.

 

[MacLaddy]

Thanks everyone. Sorry for the late response.

Regarding an additional 900 CFM outside air unit. I'm thinking of specifying a 100% outside air supply with an energy recovery wheel. The main supply and return are close enough in the plenum to add a dedicated line for both. The ducting is 24/10 inches on the supply side, so it should be able to handle the CFM (450 on each).

Thoughts?

Thanks again,
Mac