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How to dertermine return air vent sizes ?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Runs With Scissors, Jan 5, 2006.

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  1. Runs With Scissors

    Runs With Scissors New Member

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    Hi again folks.

    So our stove (quad 5100I) been set up now for a month or so and we are very pleased with it so far, nice heat, warm floors, makes me very sleepy when I sit near it etc. and we are ready to move on to the next step, moving some air around.

    My wife and I have reasearched floor vents and have found many attractive styles to choose from and even ones that close when (god forbid) there is a fire, but we never realized there were so many different sizes available.

    So heres the situation, we have a centrally located masonary chimney (20'L X 4'd X30'h block with stone front/sides) extending from the basement floor to about the middle of the attic where it gets norrower and exits the roof (there are 3 chimineys in the structure, B ment FP, LR FP,oil burner). At the base of the chimney in the basement is our insert and directly behind the back wall of the chimney structure is the stairwell leading up or down depending on wich way you are going.

    As things are right now we have a 8-12deg difference (depending) in temp between the upstairs(groung floor) and basement and want to install some return vents in the floors of 3 rooms furthest from the stairwell to help bring up some of the warm air from the basement.



    So the question of the day is....... How do you dertermine how large of a vent (vents) to use?

    I would like to have this setup with two things in mind, not using vents that are larger in size than needed and letting natrual convection do the work of moving the air rather than fans.

    I have given thought to ducting the vents together and using a duct booster fan to move air but I would think with the right size vents gravity will do the work. I'm planning on installing three vents, one of witch will be in the room with the thermostat for this end of the house witch is another reason I dont want to use a duct fan over convection in the event the basement gets cooler than the upstairs.




    Any ideas or formulas to help figure this out ?

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  2. elkimmeg

    elkimmeg Guest

    Basic formular one sq inch for every sq ft of floor area. 12/12 room 144 sq ft 12/12 vent 144 sq in.
    But there is a friction factor, the vent fins themselves reduce the actual area 25% in a metal vent
    Wood vent components are larger and reduce athe actual free passage air flow even more than the 25%

    This applies to rooms with flat ceilings no more than 8' in height, There are actuall vollume formulars, but this simple formular is close enough. I also add that fusible link danpers are the way to go in the returns. The lowest possible melting point are the best.
  3. vgrund

    vgrund Feeling the Heat

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    I had to google that topic. What an ingenious idea! Fascinating.
  4. elkimmeg

    elkimmeg Guest

    As much as the vollume is important also is their locations. It is a great idea to remove the cold air which makes room for the warmer air and starts a convection flow, making locations all the more important
  5. Runs With Scissors

    Runs With Scissors New Member

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    Hi Elk

    Vent sizeis determined by room size?

    I was thinking along the lines of vent size should somehow be related to the size of the stairwell eg. X sqft (cubic ft ?) of rising warm air vs. X sqin of return. If using these formulas do I need to take into account hallways and other rooms the air will pass through before it will cool and find the vents or just the rooms the vents are located in?

    As far as location I agree this is an important consideration as our master bedroom is 15 x 23 with the master bath off to the side. Taking this into account I may use two vents in this room. Ive ruled out putting a vent in the M. bath as I can see more potential for problems than benefits.
  6. Todd

    Todd Minister of Fire

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    As far as a formula to figure what size vent? I don't know. I have a couple of 10"x14" floor vents. One right over the stove with a fan inside, and another on the opposite side of the room. My stairwell is located in the middle and does both supply and return. All in all they do a pretty descent job, and the temp difference between upstairs and downstairs is roughly 5-8 deg. On a normal winters day in the teens or 20's my upstairs is in the mid 70's.
  7. Runs With Scissors

    Runs With Scissors New Member

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    Todd, I thought about a vent directly over the stove (cieling temp directly over the stove at 110deg) but we have nice hardwood floors and couldnt stomach the thought of cutting a hole in it. plus with the stairwell so close the benifits would be neglible in our situation.
  8. elkimmeg

    elkimmeg Guest

    the hall iand stairs are so large an area that it can easilly supply air drawn out by the return vents its not even a factor
    If you do go this way check out ceiling damper with temp activated fusible links and also manual dampers on the grills so you can open and particall close off rooms and direct the flow
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