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Old 01-08-2017, 02:05 PM   #1
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Dual problem - Frost in attic / condensation in range hood pipe Read more: http://ww


I will try to keep this as simple and brief as possible.
BACKGROUND:
Central KY
Daytime temps in 20's - F
Nighttime tems in single digits - F
5:12 Roof asphalt shiingles - osb roof deck
Vented ridgecap (confirmed open and properly installed)
Full vented soffit with baffles to allow insulation
18" fiberglass blown in attic
all ceiling penetrations are airsealed with foam/caulk
2yr old house 2000square ft
RH = stays around 48%
Use bathroom vents on timer (soffit vented) religously
*Use RANGEHOOD VENT religiously when cooking (broan 634 roof vent)
This morning wife noticed water dripping from the bottom of the range vent. Further examination revealed that water was dripping from bottom of blower housing (about 1cup total at this point) onto the range top. In a panic I went to the attic and noticed that there did not appear to be any water leaking on the OUTSIDE of the metal pipe... assume it is condensing inside the pipe and dripping down. We have not used the fan today, but have used it a lot over the last few days while cooking. I also noticed a lot of frost (thin layer) in an approx 2' diameter area surrounding the vent pipe in the attic - on the osb. I also noticed frozen ice crystals in the attic covering about 1/2 of the house. The ice crystals were not a "solid frost" layer but a small crystal every 2" or so.
What have I done wrong here???
The metal vent pipe is placed as follows:
-exits fan through cabinet
-enters attic (sealed around drywall penetration with closed cell foam)
-goes through approx 18" fiberglass insulation
-exits insulation exposed through airspace in attic above insulation for another 2'
-"dumps" into roofvent (basically just a metal pipe with the roof vent sitting on top of it
**when I had my wife turn the fan on with me in the attic, I could feel some air blowing back down into to the attic. I am assuming the air is being forced up through the pipe, then hitting the damper on the vent and some of it is being deflected back down around where the pipe is just "stuck" into the hole in the roof vent.
How do I "fix" this?
Insulate pipe?
eliminate roof vent?
vent to soffit?
is it possible that I have attic ventilation issues as well, or is the frost coming from the warm moist air that I have been pumping into my attic?
Summary: TWO issues here --
1. water dripping from fan motor
2. light frost in attic
Please let me know what other info is needed. I am almost in panic mode here. I spent 2 years designing the house that I plan to live in the rest of my life...
Thanks so much.


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Old 01-08-2017, 05:21 PM   #2
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Re: Dual problem - Frost in attic / condensation in range hood pipe Read more: http:


Do you have dampers on all your vent caps to prevent cold air from being drawn or blown back into vent pipes? Are they all insulated? Is your house or attic under a negative pressure? The pipe may be too small for duct length. Pictures?
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Old 01-08-2017, 06:16 PM   #3
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Re: Dual problem - Frost in attic / condensation in range hood pipe Read more: http:


Yes, you are dumping warm air into the attic and warming up that space.
That's why there is frost on the OSB. Mold is not far off.

You need the exhaust going thru the roof deck properly, not fed to a roof vent.

Also, you need to insulate the duct all the way up.
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Old 01-08-2017, 06:28 PM   #4
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Old 01-08-2017, 07:36 PM   #5
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Re: Dual problem - Frost in attic / condensation in range hood pipe Read more: http:


Quote:
Originally Posted by Pern View Post
Do you have dampers on all your vent caps to prevent cold air from being drawn or blown back into vent pipes?

YES - DAMPER ON BROAN ROOF JACK AND INCLUDED IN THE BROAN FAN ASSEMBLY

Are they all insulated?

THERE IS NO INSULATION ON THE PIPE IN THE ATTIC

Is your house or attic under a negative pressure? The pipe may be too small for duct length. Pictures?
I AM UNSURE OF THE NEGATIVE PRESSURE. PIPE IS 8" PER INSTRUCTIONS AND APPROX. 5' TALL - ALL VERTICAL


Suggestions on insulation? Thanks
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Dual problem - Frost in attic / condensation in range hood pipe  Read more: http://ww-vent-pipe.jpg   Dual problem - Frost in attic / condensation in range hood pipe  Read more: http://ww-vent.jpg  
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Old 01-08-2017, 07:45 PM   #6
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Re: Dual problem - Frost in attic / condensation in range hood pipe Read more: http:


Quote:
Originally Posted by 123pugsy View Post
Yes, you are dumping warm air into the attic and warming up that space.
That's why there is frost on the OSB. Mold is not far off.

You need the exhaust going thru the roof deck properly, not fed to a roof vent.

Also, you need to insulate the duct all the way up.
Previous post shows installation instructions that were followed and a view from the inside. Please inform of proper way to exhaust.

The 8" metal pipe is running through a 9" square (approx) hole that was cut into the roof deck. The Broan 634 roof vent is basically sitting on top of the 8" pipe... per instruction manual. Obviously, this design isn't working too well since there is not a clamp of any sort between the roof jack and pipe. However, the instructions did not specify this. I made sure the roofer followed the instructions.

Suggestions on type of insulation for pipe?

Suggestions on how to seal the pipe to the square hole in the roof jack?

Thanks again -- whatever valid advice I receive, I plan on following. I am not going to ignore this problem an open up a can of worms for mold.

Side note: I went back into the attic about 4 hrs after the water was dripping from the motor housing and most of the white frost had disappeared and there was only light frost particles on the underside of the deck. maybe one small (less than 1mm) ice crystal every 3-5 inches
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Old 01-08-2017, 07:47 PM   #7
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Re: Dual problem - Frost in attic / condensation in range hood pipe Read more: http:


Quote:
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? dont understand
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Old 01-08-2017, 08:26 PM   #8
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Re: Dual problem - Frost in attic / condensation in range hood pipe Read more: http:


That roof vent does look like the right type, but I am surprised there is no method to secure the duct to the hood. I'm also surprised they say to use "duct tape" rather than foil tape. I realize it is not a heating duct, regular duct tape just dries out after a while and looses its seal.

You can buy insulated ducting which is basically a duct wrapped in insulation and poly. You could probably accomplish the same thing with your current duct with some fibreglas and poly so long as you seal it well with tuck tape.
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Old 01-08-2017, 09:19 PM   #9
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Re: Dual problem - Frost in attic / condensation in range hood pipe Read more: http:


Quote:
Originally Posted by lenaitch View Post
That roof vent does look like the right type, but I am surprised there is no method to secure the duct to the hood. I'm also surprised they say to use "duct tape" rather than foil tape. I realize it is not a heating duct, regular duct tape just dries out after a while and looses its seal.

You can buy insulated ducting which is basically a duct wrapped in insulation and poly. You could probably accomplish the same thing with your current duct with some fibreglas and poly so long as you seal it well with tuck tape.
My HVAC guy did all of the connections and used the foil tape on the joints. I wonder if I should seal between the square hole in the roof deck and the round pipe with spray foam before insulating? The air deflecting back down into the attic is unsettling.
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Old 01-08-2017, 10:30 PM   #10
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Re: Dual problem - Frost in attic / condensation in range hood pipe Read more: http:


Not an HVAC guy... maybe your back-draft damper is frozen due to excessive moisture because the pipe wasn't installed per minimum code (insulate when passing through unconditioned areas (attic)- no referral for him). Are you sure they didn't enclose an adapter plate for different shapes of ducting- that seems pretty lame- otherwise.

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17,000 dryer fires a year, when did you last clean the inside of the dryer near motor or the exhaust ducting?
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Old 01-08-2017, 10:52 PM   #11
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Gary,

I went to the roof to verify the damper was free. Also, I can hear the damper of the exhaust fan (in the cabinet, conditioned portion of the house) operating. The Broan roof vent did not come with any adapters. I picked this up myself in the box at the store. The vent simply sits on top of the pipe as the instructions suggest... which I see now as poor design.

At this point I am probably going to consider the following:

1. insulate the pipe in the attic and foam seal the penetration through the roof - I am comfortable doing this myself.

or

2. install a completely new roof vent that is firmly secured to the exhaust pipe and of course insulate the attic portion. This choice will be the most worrisome since it will require removing shingles. I will most likely have my roofer come back to make the connections.
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Old 01-09-2017, 12:31 PM   #12
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Re: Dual problem - Frost in attic / condensation in range hood pipe Read more: http:


Considering Broan uses that #634 as an all-purpose one, for most round and the flat application, that is lame; http://www.broan.com/products/filter...b-3fd4adadc400 IMHO, canned foam would work but be a bear to remove. Perhaps the pro HVAC guys here could recommend a better product, or maybe just HVAC foil tape- not the cheaper plastic based junk.

You may always get back pressure with that design if the hood doesn't fit the pipe exactly. I know there is a big difference in dryer termination hoods/adapters; http://dryerbox.com/ratings/dryerfittingschart.htm

I'd pick behind door #2 above me....

Gary
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