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  1. #1
    Join Date
    Jan 2013
    Posts
    6

    Re-Routing Warm air

    I hope this is an easy question. I have a wood burning stove as primary heat and most of the heat rises into this high vaulted ceiling on the 2nd floor. I've put a vent fan in the ceiling there and hooked it , via the attic, w/ 25 ft fiberglass insulated 6" flex to an oval in wall ( not insultated but not in the attic ) in an attempt to move some very warm air into the ceiling of a cold room on the first floor that just doesn't warm up from the stove. Project completed and I was very disappointed. Seemed like low air flow and colder than expected air. The fan is a 110 CFM bath vent fan recommended to me by the local HVAC place. It's 4" outlet concerned me ,and was thinking 110 CFM isn't enough flow. I thought about upping to a 340 cfm fan , but perhaps that's not my problem, perhaps the type of fan is or something else. Any suggestions ? This simple project is starting to get out of hand.

  2. #2
    Join Date
    Jun 2011
    Location
    New Braunfels, TX
    Posts
    87
    Without pictures its going to be hard to help. The length and size of duct are big factors of heat loss.
    Also a bath vent fan is NOT going to be able to push air that far at a speed great enough to compensate for heat loss.
    A better way to re-use this heat is via a ceiling fan on low (doesn't have to be in reverse) this will combat stratification (hot thermal layer up high, cold thermal layer down low).

  3. #3
    Join Date
    Jan 2013
    Posts
    6
    Thanks for getting back to me. Here is a drawing of what I've done. I can take picts of the specifics but this might give you a big picture that unless I had a camera with xray lense I couldn't show. Perhaps you can see a fan wont do it at this point. In that 2 story void is a giant light fixture. It was a construction friend who suggested I do this when he visited, up on the 2nd floor it can be upper 70's and in the cold room upper 50's Name:  aircirc.png
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  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,288
    110 CFM through 25' of 6" flex duct transitioning into oval duct descending (I'm guessing) another 8 to 12 feet...the resistance of that much duct run coupled to a woefully underpowered fan explains your disappointment.

    Not to mention the heat loss the air in the attic portion of the duct is experiencing.

    You would probably be better off placing a ceiling fan where your fart fan is right now and blowing the warmer air toward the first floor. That and remove your project, as it is now a heat and air bleeding circuit into the attic. You can try a stronger fart fan, but I'm not confident you'll see much better results.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  5. #5
    Join Date
    Mar 2009
    Location
    Mount Holly, NC
    Posts
    3,010
    you might try reversing the flow... pump the cold air into the hot room... less heat loss into the unconditioned space, and SOME heat will migrate into the cold room this way...
    in other words, put the bath fan in the cold room, not in the upstairs hallway...
    but I agree, you'd be better off, with ventilation pumping the cold room air into the hot room, not into the upstairs ceiling.
    The TRUE highest cost system is the system not installed properly...
    The three big summer hearththrobs...
    Mel Gibson
    Dwane Johnson
    The A/C repairman

  6. #6
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    They make much larger inline axial fans. An 8" model I think can move around 300 CFM. You just need to make sure hte duct is very, very well sealed and insulated. Plus if mounted in the attic with a descent length of duct on each end, it will be nearly silent.

    I would be concerned that you would accelerate the warm air movement upstairs. I argee that you might be better darwing cool air fro mthe cold room and blowing it into the upstairs that over conditioned.

    You could also just install a small air handler and do the same thing on a larger scale with more ductwork. Open up a few walls and you can distribute the air throughout the home, and if desired add an AC or heat pump in the future.

  7. #7
    Join Date
    Jan 2013
    Posts
    6
    The ceiling fan is not an option. What I couldn't depict in the drawing was that there is a walkway on the 2nd floor where the duct fan is installed that would block the fan's flow and there is a giant light fixture in the open area that my wife isn't going to allow me to move. I see panasonic has http://www.kitchensource.com/bathroom-fans/fv-nlf1.htm a 340 cfm quiet inline fan that I was hoping would be the fix. Accelerating warm air upstairs and eventually partialy downstairs would be ideal, IMHO. The fireplace room is in the 80's , unbearable and at times even upstairs upper 70's is not too pleasant , but the downstairs cold room is our main hang out so we'd like to figure a way to make this work. Blowing cool air upstairs wouldn't create a flow back downstairs just by theromodynamics ? Not sure how that would force the warm air into the cold room either, since there is a 2 1/2 foot header ( structure ) blocking off that room in the entries ceiling. You actually can see the flow on the stair's floor and ceiling with incense is quite dramatic. Hi it goes up , low down to the point you can feel quite a draft. .I was hoping this would create a new flow dynamic. We do have a.c duct running downstairs but that is a closed system that I'd prefer not messing with any more than I have. All and all the stove distributes heat almost perfectly in the house, just one room I'm trying to make bareable over the winter. We actually used to move out of the room every winter because it was too hard to heat. So..... Is 340 the ticket ? 400 ? 600 ? I don't mind stealing heat from the other spaces with some loss, nor noise as the stove's fan is pleanty and other spaces have more heat than needed, just not sure what type of fan and the configuration to make it work. Obviously 4 inch 110 fan I was reccomended is inadequate. Thanks guys for all your advice !!!!

  8. #8
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    The airflow rating of hte bath fan is with almost no ductwork attached to it. It drops off fast if you stick 30' of flex on the end of it. Could go as low at 100CFM.

  9. #9
    Join Date
    Mar 2009
    Location
    Mount Holly, NC
    Posts
    3,010
    Quote Originally Posted by motoguy128 View Post
    The airflow rating of hte bath fan is with almost no ductwork attached to it. It drops off fast if you stick 30' of flex on the end of it. Could go as low at 100CFM.
    I bet the airflow with 30' of ductwork attempting to blow warm air down against gravity would be more like 35cfm from a bath fan...
    The TRUE highest cost system is the system not installed properly...
    The three big summer hearththrobs...
    Mel Gibson
    Dwane Johnson
    The A/C repairman

  10. #10
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,288
    Your wood burning stove may be part of the problem if it does not draw combustion air from outdoors. If the stove takes air from the house, air from outside must come into the house to replace air lost up the stovepipe. That makes it harder for the house to stay warm, and can lead to other problems as well.

    The ceiling fan idea was mentioned as a way to stir up stratified air (layers of air with different temperatures). In winter it can run in reverse to prevent a direct downblast of air but still stir things up and decrease thermal stratification. But if your wife values a light fixture over a possible solution to discomfort, well.... <shrug>
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  11. #11
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    You could install another wood stove in the cold room. OF course now the upstairs will probably be 85F.

    You can't evenly heat a semi-open 2 story space with convection without recirulating a large volume of air downward, or otherwise mixing the air in some manner (such as central durctork)

  12. #12
    Join Date
    Jan 2013
    Posts
    6
    Many thanks for the advice !!
    I believe the wood stove does draw air from the outside via the outer layer of the multi-walled pipe that was put in and creates a large flow in the house.

    So is there a fan than might be adequate ? Right now we're at 110, and get minimal flow. I was hoping 340, would be 3x what we have, they come larger... It just needs to bring a few degrees into the room, seemed like a sound idea. Do you guys think it's not worth following through ?

  13. #13
    Join Date
    Jan 2013
    Posts
    6
    Can anyone reccomend a fan that might help ?

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