Duct leaks

[mchild]

I have been reading this board for a while now and it seems as though a big part of the benefits of getting a real professional installation (as compared to many in the biz that are simply swapping parts and systems and charging less that the real pro’s but more than they are worth) relates to the correct installation and sealing of the duct work on a forced air system. I have also heard that many duct systems have substantial leaks. What are the areas that are most prone to leaking?

[trey r]

I wouldn't go as far to say most have substantial leaks, I will say there is NO perfect duct system. Any where there is a transition, take-off, or seam there can be some slight leakage.

The better quality installs will keep the leakage to a minimum. To have a 100% air tight duct system, the cost would be unrealistic. The materials and techniques used would not work.

[dietz h&c]

they usaly leak around the joints or around the drive cleats or s-locks are at .

[wonald]

Leaks usually are located at the seams or where the take offs are and can be sealed with caulking or tape. In one city here in Michigan the inspector requires us to either tape all of the joints and caulk all of the take offs or paint all of the connection with duct sealant. Which does cut down on leakage and make for a cold basement.

[pstu]

I have read several places that duct leakage of 10-15% is extremely common. On a classic 3-ton air handler blowing 1200 cfm, that would be 120-180 cfm leakage. Would you not call that substantial?

I am a homeowner, have been told that flex duct will generally not leak in the middle (assuming no critter chew-thru), but may well at the joints. Any joints which have not been well sealed with mastic. In my own attic I have been shown joints which have fallen partly open, hopefully you do not have that bad a leak. But if you do find one like that it is "low hanging fruit" -- easy to find and a lot of improvement when fixed.

I have been told that leakage where the supply duct comes through the ceiling sheetrock (if you have ducts there) is common. And a likely source of humidity infiltration and mold growth. Again this is something found and fixed in my own house, only one register out of maybe 20, but mold was growing there. This is one thing I would endorse as a DIY project, fixing that with caulk. I paid a pro to do that and he didn't do 3 rooms he charged me for, and my infrared thermometer went missing after his visit... but that's another story. There's good pro's, and then there's less-good pro's. If you find one of the good ones, don't let him get away, pay him what he needs to be happy.

Hope this helps -- Pstu

[classicrock4you]

my experience has shown that many leaks occur where the start collar is attached to the trunk - if you use dampers, it can be quite difficult to bend the tabs in completely

[dash]

All thwe above are true to a point.

With ceiling grilles,they leak up to 15%,where the ceiling box meets the drywall.removing the grilles to check for a tight seal there is worthwhile.Often you'll see a "durt" pattern going up to the attic.

Recent testing of flex boxes shows up to 20 cfms at the box at .1 static.

[seatonheating]

my experience has shown that many leaks occur where the start collar is attached to the trunk - if you use dampers, it can be quite difficult to bend the tabs in completely

C'mon man. You remove the damper, bend the tabs, then re-install the damper.

You can't be serious!!

[seatonheating]

Just make sure they install it like I do!!

[classicrock4you]

Sorry, Bubba, but I am serious. You would be amazed how many poorly connected start collers I see. I don't install, but am a thorough salesman that actually checks ductwork, and I find amazing leaks at start collars (assumung that they actually used them in the first place). I do know how easy it is to cut fingers, and what you install probably not the way Johnny new construction does it.

[seatonheating]

Sorry, Bubba, but I am serious. You would be amazed how many poorly connected start collers I see. I don't install, but am a thorough salesman that actually checks ductwork, and I find amazing leaks at start collars (assumung that they actually used them in the first place). I do know how easy it is to cut fingers, and what you install probably not the way Johnny new construction does it.

Then refer to them as Bubba....



In all seriousness, it is quicker to attach them properly than fight them on the wrong way. Some of the guys in this trade I just don't understand.

[deejoe]

the pics that cheatonheating posted would indicate that the duct fittings shown are so restricted (choked) that any air leakage may be actually beneficial.
Also, learn how to "crossbreak" an elbow properly

[cem-bsee]

why would anyone make starting collars nowdays?
they are so cheap at the distributors! & with gaskets & dampers !!
&
without the needs to cut tabs which cut fingers!