6inch to small?

[brianshouse]

The beach house is 40x28 and is on pilings. The floor consists of webbed trusses and only allow for 6in ducts to be run through them. Their is no attic space in the house, it is vaulted ceilings. Will the velocity of a 3.5 ton system be to much on 6in ducts with 16 registers?

Thanks, Brian.

[Irishmist]

Wondering what the basis of your question is. Is this a new 3.5 ton system or a new air distribution system? Are you having problems? If so, what are those problems? Now knowing what type of physical arrangement you have, it is extremely difficult to say how your system will operate. To say it is 'okay' and not know how it is laid out would be doing you a great disservice. As always, more info would be helpful. All the best, John.

[dash]

Brian,
As John stated above ,much more info. is needed ,to answer you question.


That said,16 branch runs ,each 6" round ,can uaually handle the 1225 to 1400 cfms (airflow) that you have.

Now ,if the sizing of the trunk/plenum for supply and return,grilles ,etc. is not be correct,you'll still have a major problem.

You need Manual J,S and D calculations to size it,select equipment,and design the duct system.It's not rocket science ,but it is a science.

Rules of Dumb(thumb),regardless of experience ,should be avoided!!

[Carnak]

If it is on pilings then is it a lattice surrounded crawlspace below?

[Irishmist]

so Brian, I'm wondering, we solve this problem for you, do we get a week at the beach house? lol

[brianshouse]

Haha, sure John!

This is a new construction and their will not be a crawlspace, OSB will be nailed under the trusses. The web in the trusses are 8 1/4 inches in the middle, so I might be able to run 8inch flexible duct? although I don't know if the gang nails will tear the outside of the duct when pulling it through. I can run my supply trunk in the middle of the house going beside a truss. The trusses are on 2' centers so I will have around 20-22" wide x 14" tall area to run the supply trunk in. I have positioned my air handler (down flow) to be in the center of the house so each trunk will be 12' on each side of the air handler.

Thanks for any input!
Brian.

[peytonc2682]

Do not put flex under a beach house!!!!! Also 8" pipe will not fit into an 8 1/4" space. 7" will barely make it. If the house is 40x28, why would you need a 3.5 ton system? Do you have lots of windows?

[Irishmist]

Now you've switched gears on us and are talking about 8" flex pipe. Flex runs are not a good idea under the best of circumstances. Also, you should maintain 1" clearance between any warm air supply duct and combustible material, i.e. wooden trusses. You may want to spring for the very reasonable investment of the HVAC Calc as indicated at the top of this page, and be SURE that you have the right size system. If not, you are making a bad situation worse, because if the basic size of the system is wrong, then you are sizing duct based on an incorrectly sized system. So far, all I can see that is worth commenting on is the 'central' location of your air handling unit. Be doggone sure your supply (and return if it is located in non-conditioned space) is insulated and airtight. I am hoping you are having a licensed, insured, and qualified contractor perform this installation for you, so that being said, why are you asking us and not him these same questions. Even better, he should have proposed a design for you as to how the ductwork would be laid out. What you are attempting is not something a lay person should try to undertake. Seriously. It's not rocket science, but you're gonna wish you had someone to call on when this thing doesn't work right. All the best, John.

[brianshouse]

All the beach houses here use flex duct but they do not pull flex through the webs, they have enough room to run the supply trunk in the center of the truss, and then the flew is run beside the trusses. Somehow my trusses where engineered different and my space is too small so I will have to run the supply trunk differently. I have seen the trunk under the trusses and it was boxed in later. I don't plan on spending thousands of dollars on a job that wouldn't take hours to do. I want plenty of tape and mastic with no leaks, It is hard to not find a jackleg around here. My parents have the same stle house and their 3 ton system just doesn't do it with the cathedral ceilings, thats why I'm going with 3.5 ton.

[comfortdoc]

The A/H will be sitting on the floor?

[brianshouse]

Yes like my parents house it is setup vertical with a downflow. A door with a grill cut out is the air return.

[papa_jo]

have you thought of exposed duct hanging from the ceiling in a conditioned space no insulation needed or spial duct like you see in some dinner places??

[comfortdoc]

Interesting. Can you post pictures of your parents system?

[dan sw fl]

Originally posted by brianshouse
All the beach houses here use flex duct but they do not pull flex through the webs, they have enough room to run the supply trunk in the center of the truss, and then the flew is run beside the trusses.

Somehow my trusses where engineered different and my space is too small so I will have to run the supply trunk differently.

My parents have the same stle house and their 3 ton system just doesn't do it with the cathedral ceilings, thats why I'm going with 3.5 ton.

What is the total AHU air flow in your parents house?
What is the deltaT across the coil?
i.e. 75'F room temp, 56'F at coil outlet ?

What is the S.A. temp at the AHI outlet and at the diffuser?

++++++++++++++++++++
Use R-20 spray foam, THEN 2.5-Ton unit mounted in conditioned space may be sufficient.
+++++++++++++++++

[dash]

Originally posted by brianshouse
All the beach houses here use flex duct but they do not pull flex through the webs, they have enough room to run the supply trunk in the center of the truss, and then the flew is run beside the trusses. Somehow my trusses where engineered different and my space is too small so I will have to run the supply trunk differently. I have seen the trunk under the trusses and it was boxed in later. I don't plan on spending thousands of dollars on a job that wouldn't take hours to do. I want plenty of tape and mastic with no leaks, It is hard to not find a jackleg around here. My parents have the same stle house and their 3 ton system just doesn't do it with the cathedral ceilings, thats why I'm going with 3.5 ton.

With supply grilles in the floor .Manual d is critical,the need to push the cold air well above the occupants,ie >7'.

An undersized duct system won't do this and adding a half a ton won't correct the problem,in fact it could make it worse.


How's the air flow at your parents house??

Manual J,S and D,and you'll be fine.

[pecmsg]

Brian

How old is your parent’s house, what size, How many windows, what direction is it facing? Where is it located? These are just some of the questions we need to correctly size this system.
Around here the mice Love flex duct to gain access in the winter!

You’ve haven’t mentioned heating, so I assume it will be shut down in the winter.

Check out http://www.mrslim.com and look at there MXZ unit. 3 indoor units, 1 outdoor. Heat / Cool & Dehumidification.