new ducts have drafts

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Old 09-24-10, 01:37 PM
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new ducts have drafts

Sorry if this is a silly question. I recently had new ductwork installed in the attic of my single story rancher. With the system off, I feel a slight draft coming from the ceiling registers. Is that normal, or is it an indication that there is leakage in the system somewhere?
 
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Old 09-24-10, 02:44 PM
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I've seen cold attics create a significant draft with cold air dropping out of the ceiling vents and warmer air being pulled in others and rising into the attic. So look for any hot/cold convective loops. Hot attics are normally creating a low pressure in the attic as they try to pull in cooler air from the soffits. This negative pressure would normally be stealing air from those attic ducts rather than forcing air into them.

As far as sealing, every seam and connection should have been 100% sealed before large amounts of insulation were applied. This is an area where I strongly disagree with the HVAC standard of modest levels of insulation. In the presence of moving air, think wind chill, the heat transfer is amplified. Burying them in R-40 should be a minimum IMO. But all I see typically is around R-6.

What were the temperatures when you noticed the draft and was the wind blowing?

Bud
 
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Old 09-24-10, 03:36 PM
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Thanks, Bud. Today it was around 93 outside, with a nice breeze. The metal ducts are insulated (probably R-6 or so, and I believe the flex ducts are R-8. I don't think that the contractors went to great trouble sealing the ducts where they meet the boots (I see only plastic zip ties holding the duct onto the boot), and several of the registers are in really tight, cramped places. I guess I may have to get some duct sealing goop and try to seal them properly, huh?

I don't know how I could check for a convective loop, exactly. The thing I found odd was that the draft from the register was definitely cooler air than what must have been in the attic on such a hot day. I just checked that same register, and don't feel a draft from it now.
 
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Old 09-24-10, 04:46 PM
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If you can generate some smoke, toy train or incense sticks, when you feel that draft you can go looking for where the air is being pulled in. Somewhere in between those two points would be the temperature differential for warm air to rise and force the draft you are feeling to fall out of that duct.

Hard to guess from a distance .

Bud
 
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Old 09-24-10, 07:11 PM
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I'll give the smoke test a try next time I feel the draft. Thanks for the help!
 
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Old 12-08-10, 06:28 AM
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Winter update to original post

We're in the 20's today, and I noticed two things:

1. There is a noticeable light flow of cold air falling from most of the registers when the system is off. It's between 50 and 55 degrees.
2. I had the installers cut into a header in the living room, and they used the wall space to add a low return (I thought that might help draw more cold air out in the winter). There is a strong draft of cold air pouring out of that return (I actually sealed it off this morning). I compared that with the original 12" return in the ceiling, and that one isn't drafty at all.

So, does this sound like normal draft from attic ductwork, or might there be some serious leakage in the system? From what I can see up there, no mastic was used around any of the connections.
 
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Old 12-08-10, 07:00 AM
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Yes all duct work should be insulated with r-6 and all seams should be sealed with duct seal and flex should be conected with panduit straps
 
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Old 12-08-10, 07:05 AM
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The flex ducts are insulated with R-6/R-8. The rigid ducts have the sealing tape on joints, but I don't recall seeing any mastic used anyplace. How should the connections between the flex ducts and the register boots be sealed, exactly? It looks like they just slid the duct over the boot and secured it with a plastic tie wrap.
 
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Old 12-08-10, 11:08 AM
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Inner sleeve needs to strapped then outer insulation needs to be strapped. Is metal duct insulated on inside?
 
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Old 12-08-10, 07:37 PM
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Okay, so they skimped on the flex duct connections for sure. I don't understand your question about the metal duct being insulated on the inside. Do you mean the duct leaving the furnace and heading up into the attic? That is wrapped and taped. I do notice some tiny air leaks around the furnace seams and the return sheet metal joints, but I'm guessing that's normal. What's troubling is how much cold draft is coming into the house from the registers and that lower return. It's not a good thing, and I don't know if it's because of the installation, or if it's expected. We have had high winds for the past few days, so I'm sure that's not helping much. Just trying to figure out if there's anything I can do to lessen or eliminate it.
 
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