New Duct Velocity and CFM questions

[beachwalker]

Hello again. My two credos for this summer:

Experience: What you get when you don't get what you thought you were going to get in the first place.

Research: It's what I am doing when I don't know what I am doing. (Werner Von Braun) and I've done lots of it on this forum this summer.. thanks for the help.

After installing two Lennox 18 SEER units (beginning of june) and foaming the roof with closed cell foam (last week), we are now replacing the supply ducts.

I purchased and used the HVAC calc program and found my cooling load is 1/2 ton less than what is installed. I need to determine the supply trunk and branch sizes and as far as I can see, it comes down to CFM per branch and velocity. (each branch has to carry cfms of the rooms it is feeding)

3 ton air handler currently set for 1275 cfm plus Ultra-Aire dehumidifier at 300 cfm, so I am going to size the main trunk for 1575 cfm total.... then calculate the branches.

On dehumidify, the lennox drops to 70% of cool cfm (892) plus 300 from Ultra-Aire= 1192.

Questions:

Is it correct to size main trunk for 1575?

What velocity is correct for supply tree branches? I have seen min 500 fpm to max 750 fpm

Is it important to downsize the branches in straight runs as the cfms decrease along the way? I take it this maintains velocity?

What velocity is correct for flex runouts? I have seen max 500 fpm.

What velocity is correct for returns? I have seen 550 fpm min.

I know that is a lot of questions, but it is important to me to get this right.

Thanks very much. I will not give up until this system is working like it is supposed to. FYI after new units were installed, we had heavy duct sweating in the attic and poor humidity control, probably due to oversized unit, but that is why we foamed the roof and added the dehumidifier.

[Carnak]

FYI this is not DIY beach. Either find a good contractor or maybe hire an engineer or someone to layout the system for you.

[beachwalker]

I am dealing with the premier contractor in the area. Their manual D program was on one computer that crashed, so they no longer have the capability to do it. I am not doing this installation myself, but I have been forced to purchase a program to enable me to have a certain level of confidence that the system will work properly.

The answers to these questions will help me to calculate the duct sizes for the contractor to install.

If you can help provide the answers to the questions, I would appreciate it. I thought this forum was for the exchange of information. I have responded to other threads where I thought I could contribute.

[dash]

So,what's the problem after the foaming of the attic???

[dan sw fl]

Originally posted by beachwalker
Research: It's what I am doing when I don't know what I am doing. (Werner Von Braun)

... two Lennox 18 SEER units (beginning of june) and foaming the roof with closed cell foam (last week),
we are now replacing the supply ducts.

3 ton air handler currently set for 1275 cfm plus Ultra-Aire dehumidifier at 300 cfm, so I am going to size the main trunk for 1575 cfm total.... then calculate the branches.

On dehumidify, the lennox drops to 70% of cool cfm (892) plus 300 from Ultra-Aire= 1192.

Questions: Is it correct to size main trunk for 1575?

we had heavy duct sweating in the attic and poor humidity control, probably due to oversized unit, but that is why we foamed the roof and added the dehumidifier.

RE- SEARCH:
____Searching Again, Not knowing where one has been.

What are the ACTUAL air flows and E.S.P.?

[Carnak]

Originally posted by beachwalker
I am dealing with the premier contractor in the area. Their manual D program was on one computer that crashed, so they no longer have the capability to do it. I am not doing this installation myself, but I have been forced to purchase a program to enable me to have a certain level of confidence that the system will work properly.

The answers to these questions will help me to calculate the duct sizes for the contractor to install.

If you can help provide the answers to the questions, I would appreciate it. I thought this forum was for the exchange of information. I have responded to other threads where I thought I could contribute.

Beach, you are not the one who should be sizing ducts for the contractor, this is way over your head. For one thing if your air handler and dehumidifier are going to share the same duct work, their airflows are not accumulative.

This is not a DIY site, with lessons on duct sizing for amatures, bad enough letting you guys size your own equipment.

This is a place where pros do give some advice, and those who seek advice sometimes become the experts and turn around telling others looking for advice how to do things. It does not matter if you slept at a Holiday Inn Express.

In my opinion, you ignored THE best advice you were given and that was to seal your damn ducts in the first place.

[Edited by Carnak on 09-06-2005 at 07:58 AM]

[docholiday]

I'm not going to address the duct issue, but maybe help understand the 1/2 ton oversized issue. Is that 18 SEER system offered in 1/2 ton sizes? I would suspect it is not and since it is two stage you would round it up.

I am a little concerned with a "premier" contractor who isnt addressing his lost computer program and letting the HO fend for himself.

[Carnak]

I missed the premier's computer crash. Pretty sad if you have to rely on a computer. Hire someone else.