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Old 01-03-2009, 09:47 PM   #16
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HVAC - Airflow Issue


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Do you actually set your stat to 68 in the summer.

Its a code violation to have the garage on the same forsed or gravity air system that serves an occupied area.
CO poisoning potential.
Yes, we like to have a cool house, so we run between 68-70 in the summer. In the winter we run 73-75.

Good to know on the code issue if I was to put a vent into the garage, I didn't know about that, but it makes complete sense.

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Old 01-04-2009, 05:20 AM   #17
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HVAC - Airflow Issue


Would you by chance, happen to know what humidity, your house was at last summer when you had it 68 in the house.

I keep my place about 72F and 48%RH.
Which feels about the same as 68 and 54% RH.
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Old 01-04-2009, 10:23 AM   #18
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I dont as that was a different house and I didn't have monitoring setup. We never had a problem with moisture on windows, the tile on slab floor, or other areas either.
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Old 01-07-2009, 11:56 PM   #19
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HVAC - Airflow Issue


Ok, I spent some time mapping out the duct paths as they are today and scanned them in.

It looks like to the one side of the A/H there are 12 6" ducts. So 12x85 cfm = 1025.
On the other side there are 15 6"" ducts. So 15x85 cfm = 1275.

If an 8"x24" metal duct is rated at 930 CFM, that would explain part of my AirFlow problem.

I think I should replace the supply and return with 10"x24" which is rated at 1320 CFM and not worry about stepping down sizes from the A/H unless the cost justifies doing so. The airflow would be close on the side that has 15 6" take offs, but still within specs I believe.

I'll also have a new box built for the airhandler to sit on to allow more headroom at the top and reduce turbulance. I would also have a 32"x12" drop down and at least a 20x25 filter slot made.

I further think my return system needs a total revamp. While each room has its own return in it, they are limited by the size of the interior walls. My walls are 2x4 on 16" centers. I need at a minimum 180 CFM per room for return, right?

For the 1st floor, that will be an "easy" fix, for the 2nd floor I think I'll have to utilize the chase way that had the fuel oil furnace doubleinsulated exhaust stack. If I remove that (already planned to) I'll have a clear 20"x20" chase up to the 2nd floor where I could put a return vent on it....this would be in the central hallway upstairs and would provide plenty of additional return capacity for the 2nd floor. There is no attic to utilize for running additional vents to each room.

Does my logic/math seem correct?
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HVAC - Airflow Issue-hvac-supply.jpg   HVAC - Airflow Issue-hvac-return.jpg  
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Old 01-08-2009, 12:01 AM   #20
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HVAC - Airflow Issue


Can't read it....not dark enough.
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Old 01-08-2009, 04:17 AM   #21
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You should use a reducing trunk line. By reducing you maintain relatively the same velocity for the air that is coming out the vents closest to the the furnace as the ones the furtherest from the furnace.

Using the former chimney space for additional return, will be a big help.
A room that has a 1-6" supply, doesn't ned 185 CFM of return.
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Old 01-08-2009, 09:02 AM   #22
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Ok Thanks.

The only rooms that have a single vent, are the bathrooms, kitchen and utility room. These rooms dont have returns in them already. The kitchen is pretty open to the living room.

Since you suggest I do the reduction, based on the flow, I think I can do it at the same point as the existing ductwork. SO I'll go from a 10x24 to a 10x16.
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Old 01-08-2009, 01:01 PM   #23
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I picked up the duct from the supplier today. I ended up with 10x24 and 10x20 (for the reduced sizes)

They didnt have any drop downs that would work, so now to get a sheet metal shop to fabricate me something up.
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Old 01-12-2009, 10:01 PM   #24
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What is the best way to seal each take off from the trunk line? Most of them are 6"x10" rectangle to 6" round duct.

Do I just use a bead of silicone on each joint? On the old ductwork, it wasn't sealed and I can see dust around each takeoff to show it was leaking pretty good.
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Old 01-13-2009, 05:30 AM   #25
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You can use silicone caulk.
Mastic can do a better job.
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Old 01-13-2009, 10:23 AM   #26
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CONFLICT Of INTEREST......CONFLICT OF INTEREST!!!!
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Old 01-13-2009, 04:50 PM   #27
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CONFLICT Of INTEREST......CONFLICT OF INTEREST!!!!
Must be an "inside" joke.
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Old 01-13-2009, 08:59 PM   #28
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Re: duct design here is a very good step by step 4 CD set from ACCA.
kinda expencive maby for a homeowner 1 time use. But I have it and refer back to it all the time for proper duct design. http://www.acca.org/store/product.php?pid=212
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Old 01-13-2009, 09:01 PM   #29
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Thanks. I am only replacing the trunk lines. According to the manual J I did, the takeoffs for each room are acceptable. If after I replace the trunk lines, I am still not "happy" with the flow after new tests, I can easily upgrade/replace the first floor service as it is accessible in the basement. The 2nd floor as far as I am concerned, isn't changing.
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Old 01-13-2009, 09:29 PM   #30
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Must be an "inside" joke.
On another site I'm a member of.
Its a NO DIY site.
I'm a new mod there. And would close this thread, because it would violate the rules of that site.

So thats the joke.

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