Replies

1. 
   Bob_Walker | Apr 15, 2001 01:49pm | #1

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   Sealing the return ducts is important to prevent the basement from being depresurized by the blower which can pull flue gases into the living space.

   Sealing the distribution ducts can be important to realize an efficient system which works as designed (assuming its actually ben designed and noy just thrown in there.)

1. 
   Allan_Marsh | Apr 16, 2001 05:07am | #2

   *Bob,Your point about back-drafting is well taken.But, I was thinking about this for a new home I am planning. The only combustion devices would be the furnace with a sealed combustion system, a forced-draft water heater, and a clothes dryer. So the furnace would be no problem, the water heater has a control circuit to make sure it is drafting, and I believe the clothes drier would be able to overcome the slight de-pressurization from a reasonably constructed return.Now, we get closer to what I was too clumsy to ask with enough detail the first time. In a house such as I have described, it seems to me that it would be harder to justify. It isn't common practice here, so I expect resistance to the idea and want to hear what everyone has to say.Allan

1. 
   john_ross | Apr 21, 2001 05:45am | #3

   *How tight is this house going to be?A new house with 100% polyethylene plastic vapor barrier or foil backed wall board walls, Vinyl flanged casement windows taped & foamed, and magnetic sealed exterior doors can get real tight --tight enough for small leaks to create pressure differences.....An old house with 30+ year old double sash windows could have significant duct leaks and you'd have a hard time tracking them down if the ducts aren't all exposed.

1. 
   Rob_Susz_ | Apr 23, 2001 05:12am | #4

   *The tightness of the envelope matters a bunch because it tells us how much pressure imbalance the building can take.The bigger concern is where are the ducts that go upstairs? In exterior walls where they are behind the air barrier and compromise insulation? Kepp the ducts all on interior walls and seal the rim joist areas between the first and second floor and now it is less of a concern.Sealed ducts will also ensure that the proper quantity of air gets delivered to where it is intended. Thus you need 20% less fan flow because it is heating or cooling designated areas properly.Sealed ducts also means you will not be depressurizing the basment and sucking moisture through the very porous concrete block basement walls.-Rob

1. 
   Allan_Marsh | Apr 24, 2001 05:24am | #5

   *Rob,We are looking at about 2000 SF simple rectangle with floor trusses so will be ample opportunity to run ducts between main floor and basement ceiling. I guarantee there won't be any ducts in exterior walls (Ouch! Who would do that?).You got my attention with being able to slow the blower down 20% though. If 20% is a good number, that alone would make it worthwhile. It would be about 50% reduction of power draw of the fan and a lot less noise.Thanks,Allan

2. 
   Rob_Susz_ | May 11, 2001 06:28am | #6

   *Open web floor trusses have plenty of opportunity to leak outside at the top and bottom of the rimjoist. Seal up the rimjoist area and your on your way.-Rob

2. 
   Allan_Marsh | May 11, 2001 06:28am | #7

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   Like a house on a basement where the basement is also conditioned.

   No ducts would penetrate into attic or crawl spaces. The complete system is inside the conditioned space.

   In this case, is sealing the ducts with mastic as important?