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Adding 2nd AC unit in attic....vent question

Posted by onedog3cats (My Page) on
Thu, Mar 15, 12 at 11:21

We are finally adding a second unit in our attic to help cool the second level of our home. I'm not all that excited about having ceiling vents and wondered what other solutions there might be.

We would like to add crown molding in the bedrooms. Is it possible to install a vent into the crown molding? Does anyone have experience with this? Any pics you could share?

Thanks in advance!

Follow-Up Postings:

RE: Adding 2nd AC unit in attic....vent question

I think it would be very difficult to install a vent in a crown molding. The molding itself would need to be large in order to accommodate the vent. In addition you would have to create an opening at the top of wall. This is where the wall plate meets the ceiling joists. The result would be a difficult installaion in my opinion.

I would install the vent in the corner above the door entering the room. This would make it less noticeable. If you need a second vent, put it another corner which is not directly viewable when you enter the room. Paint the vent the same color as the ceiling so it blends in.

RE: Adding 2nd AC unit in attic....vent question

Thanks for the response. The more I thought about it, the more I realized it probably wouldn't work.

The problem is that I want lots of vents (I'm thinking 4!). Our HVAC guy said he would do however many we wanted, but that 2 would be sufficient. I like the idea of putting one over the doorway.

Thanks for your input!

RE: Adding 2nd AC unit in attic....vent question

Put the vents far enough away from the walls to clear any crown you want to install.

Moving vents after the fact is a PITA and can upset the air flow in the system.

RE: Adding 2nd AC unit in attic....vent question

Office and conference room supply sometimes take the form of a long, narrow slit. I don't know if that can be easily adapted to a residential structure. I know that mike_home hit upon at least one difficulty with it. With new construction it would be a lot easier than renovation, that's for sure.

RE: Adding 2nd AC unit in attic....vent question

You also might find some circular vents that look better than the standard rectangular.

RE: Adding 2nd AC unit in attic....vent question

Will definitely place the vents so they clear the crown. Don't want to move vents later. I will already have enough dust in my house...don't need anymore!

I had in my mind a few long narrow slits in the crown, but realize that it won't be an easy task. They start working on it on Monday, and I'm already driving my husband crazy with the amount of vents I want. If ever we build a new home, I will have to revisit my slotted crown mold idea.

I will check out the circular vents. Never thought about those. Thanks.

I will be so happy once this is done and the upstairs is no longer 30* warmer than the first level! We're also adding blown-in insulation, so that should help a lot considering we hardly have any insulation now. So maybe we'd be okay with just two vents in each bedroom.

RE: Adding 2nd AC unit in attic....vent question

For a typcial 10X12 foot bedroom, one vent is adequte. A large bedroom of 12X15 may need a second vent.

Don't forget the conditioned air does not have to travel far to reach the vents. Also the cool air is entering at the ceiling and will drop down towards to floor. This is the ideal set up for air conditioning. If the attic is unconditioned, then make sure the duct work is insulated.

RE: Adding 2nd AC unit in attic....vent question

Duct work in unconditioned space needs insulation and the lowest mass ducting you can use.

Ductboard or flex duct works well here.

Metal duct adds a lot of thermal mass and load to the system.

The duct WILL heat up (or cool down) to the attic temperature, and it takes more energy to cool down a duct with more mass.

RE: Adding 2nd AC unit in attic....vent question

More important than more useless vents (which can upset proper air flow in the system) is the proper amount of return air vents. Each room should have one in addition to the main return.

RE: Adding 2nd AC unit in attic....vent question

Our master is about 18x22. So I think we will need 2-3 vents. There are 2 now from the main unit which seem useless.
The ducts will be insulated. And we will install blown in insulation as well after the hvac installation. Each room will also have its own return in addition to the one in the hallway.
I'm thinking (3) vents in the master and (2) in the three other bedrooms (12x15).

RE: Adding 2nd AC unit in attic....vent question


I am a licensed HVAC technician in MA and an engineer for over 30 years. In stumbling upon your post I joined to urge you to please stop and reconsider what you are doing Monday. There are many ways to provide A/C for a house and the best way to start is to do a heat load for your house. If that has not been done you are being ripped off. The question of vents and dust and moulding can easily be answered with a unit from These people and a couple of similar companies units are made for this condition and will answer all your questions. Delay the work and call the company directly. Simply put they made for historic restoration jobs, the move less air so smaller vents or different types usually hidden, less air means less dust blown around. Yes they are just as quiet or more when installed correctly and you will be happier! Cost is comparable and if you have not been given this option you need to question if the contractor is the right one for you, he should have offered this to address your particular concerns.

RE: Adding 2nd AC unit in attic....vent question

ditto that you need someone to calculate the amount of
air to be supplied and returned.

rather than focusing on the number of supply vents
per room size of the duct & supply box vs
amount of air each size duct moves is the question.

the amount of air available to be moved and distributed
is determined by the size of the hvac system
one ton of hvac is 400 cfm.
two tons...800 cfm.

there are steps to determining size and number of vents.

talk to install company.
mastic seal all ducts
seal return air
seal supply boxes to ceiling.

any air sealing to the house @ attic floor
should be done prior to insulating.

best of luck

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