Air return system

Bezalel Posted By Bezalel, Jul 19, 2006 at 10:24 AM

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  1. Bezalel

    New Member

    Nov 19, 2005
    I'm thinking of adding a simple air return system using one of these in-line Panasonic whisper fans:

    My house is 2100 sqft, 1 story, with the stove in one corner and the master bedroom on the opposite corner. I'm planning to install the fan and ducts in the attic, cutting vent holes in the ceilings of the master bedroom and the living room (where the stove is). I need help in determine the following:

    1. Will a 120 cfm be enough? I like it for its low power and low noise (most likely this fan will be continuously on through out the winter).

    2. I'm eyeing those flexible ducts with insulation from Home Depot. They seem to be the most easy installation, compared to the rigid pipe. Plus with the pipe, I'd have to install some kind of insulation around it. I want to minimize the time I need spend in the attic for installation. Do you see any problem with this?

    Basically the 2 fans I'm looking at:

    120 cfm = 36W, 1.0 sones, 4" diameter duct

    240 cfm = 57W, 1.4 sones, 6" diameter duct

    I'd appreciate any help I can get. Thanks!
  2. elkimmeg


    Someone mentioned code issues. Can you tell me that the flexible duct is designed to transfere heat from a wood stove? Nowhere have I ever found that this use is an exceptable listing? Please educate me if you find one. The listings for flexible bducts are to be connected to a sealed conbustion system. No wood stove is a sealed combustion system. With non user secondayr air feeds, is not the deffinition of sealed. Remenber that heat from a furnace is from an exchanger, completely sealed from the fire chamber.
    Flexible duct carry only a class A fire rating for the outer wrap. The iner wrap is plastic poly that is very combustiable.
    What about containment? Containment of a fire or smoke saves lives The research supporting containment is overwelling.
    In single familly dewllings, natural containment is your floors and ceilings. That 1/2" sheetrock has 1/2 hour containment value. This is especially important facillitating a fast safe exit. Studies also have proven you are safer sleeping with your bedroom doors closed.

    The intent here is to distrobute heat to a bedroon from a wood stove. Again we are most vunerable while sleeping. That same heat passage, can also be an excellerated pasage for smoke co and co/2's, in your most vunerbable state. During rough frame inspections, we focus of draft stopping every wire duct should be draft stopped. Its code, But why? Containment.
    Residential codes are more relaxed then comercial applications. If this were a commercial application, a smoke damper would be required in that line.. Tied into the general alarm system, to shut down, to contain the spread of smoke, again containment. The comercial air distrobution system may also be tied into the alarm system, triggering a system wide shut down, again containment.

    Everything you are planning to do is counter to containment, even the very materials purposed, flexible ducts. I have challenged every mechanical contractor to prove me wrong. If I put a coke 20oz coke bottle under a fawcet and run the water 24 hours, how many oz will fill the coke bottle? Supply without equally engineered returns is useless. Especiall trying to introduce lighter warmer air into a cooler air enviorment . Common sense tells you its not going to happen. Truth is, you would be better served placing your in ducts fans in a return system removing the cooler heavier air, to make room for the warmer air to enter. Your return locatiions should be in the floor or close to them to take out the coolest air and to also draw the warm air down.

    Then there is the issue of mechanically removing air in close proximity of your fuel burning appliance.. That appliance needs x amount of combustion air to opperate correctly and draft correctly. By removing air mechanically, you produce a negative pressure situation near that appliance. This negative pressure situation can cause your not so closed wood stove combustion chamber, to back draft spillage.. You just have increased the risk of drawing co and co/2 into your living space and now provided the means to transport them into your bedroom.

    I purposely did not quote codes but explained how dangerous this practice can be, by using common sense.
    If I went threw all the NFPA codes, mechanical codes, and manufactures listing,my scanner light bulb would burn out first. All wood stoves manufacture listings specs, are required to note the wood stove cannot be connected to your air delivery system. It is not designed to do so. IF you need convincing I read this yesterday in VC resolute Acclaim manual, when answering a chimney height issue. I can cut and paste that language or wording. So someone has never seen it in code? The installation manual is code. I suggest re reading it. There is also code concerning cutting unprotected holes in ceilings and floors.

    Right now, I am working with my state and IBC, international codes, to adopt alarming in conjunction with the smoke alarm systems in residential codes. Doing this is not all that expensive.
  3. begreen

    Staff Member

    Nov 18, 2005
    South Puget Sound, WA
    Glad to hear that you are working on this Elk. That was my first thought. I wouldn't consider putting in such a system without it being tied to a smoke detector that would break the circuit to the fan when it senses smoke and sounds the alarm.

    But as Gideon pointed out, I also expect that the temp of the air delivered to the bedroom might be lukewarm at best. Is there a possibility of putting in a small muffin fan to blow about 90-100 cfm towards the bedroom? It can be installed in the upper corner of a door jamb. If there is a straight hallway to the BR that may be enough. Otherwise, use a safe portable or baseboard heater for the bedroom.
  4. elkimmeg


    The key is to remove the heavier cool air. the fan location should be on the floor, air flow directed in the direction of the stove
    the warmer air will enter at the top of th doorway. As yo draw the cooler air out the fan will also act to draw in warmer air and draw it down. Many make the mistake of trying to force lighter warmer into colder heavier air . Really that does not work well.
  5. velvetfoot

    Minister of Fire

    Dec 5, 2005
    Sand Lake, NY
    I am fooling around with a 14 w fantech radon fan, model HP 2133, 134 CFM at 0 gage pressure. It is incredibly quiet.
  6. Bezalel

    New Member

    Nov 19, 2005
    Thank you all for your valuable inputs.

    I want to clarify something: I intend to draw cold air from the Master Bedroom into the Living (Stove) room with this fan & duct system. I hope the effect is drawing the warm air through the hallway into the Master Bedroom. I can install more smoke alarms in the hallway.

    It makes more sense to have the vents near the floor rather than the ceiling, but it much easier for me to install this from the attic.

    Elk raised a great question about negative pressure. So I should use the lowest power fan to reduce the negative pressure? The whole house has a pretty "open" structure. Even the entrance to the Master Bedroom is double door which will be opened, so I'm not concerned about passage obstruction.

    By the way, the (cold) return air vent will be about 15 feet from the stove (it's a large living room).

    If anything, this system should help circulate the warm air through the hallway, not through the duct system. Do you still see a problem with it? Will I violate any code?

    I read elsewhere in this forum that a cold air return system is a great addition to keep the house warm. How else do people do it so that it meets codes?

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