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Duct through rim joist

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  • #16
    Yikes, this is getting more complicated than I expected.


    I've got two furnaces that supply different parts of the house. Thermostat in basement and ground floor, no t-stat upstairs. Only one ac condenser.



    ​​​I assume my ground floor furnace is the one that would need to be tapped into.

    I tried a similar load-calc site but I just punched in data for just that room (instead of entire house) and it said I needed 25-30,000btu and about 800cfm.


    I assume I'd be on the lower end of that range since this room is attached at one side openly to the rest of the house.


    I thought a rough calculation would give me a close approximation of what I need. Also, if my supply was undersized, the ac would just run longer until it satisfied the thermostat.


    Hate to admit it but I might have to call a pro to figure this one out.


    Yes, its an ECM variable blower.

    The bozo that originally set-up the room had one tiny vent feed from 4" flex hose. Even I know that wouldn't cut it.




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    • #17
      The link I gave is an ACCA approved site - not sure if the other site you went to is ACCA approved which means the results may not be accurate.

      How detailed was the analysis - did you have to put in things like R values and glass type etc - or was it a generic room type?

      You can do a block load calculation for the room - ie treat it like a stand alone house - and then derate the result to take into account the conditioned air and adjacent wall from the house on one side. There actually may be a special type of wall to take this into account during a block load calc.

      Your overall setup would make it hard to get the correct temperature in that room since the thermostat is elsewhere - getting to the setpoint on the thermostat does not mean the room will be comfortable. Also if the heat load is greater than the cooling capacity of the ac the setpoint in the room may well never be satisfied on a hot day assuming the thermostat can sense the temperature in the room - in this case it cannot.

      Sounds like a case for tapping as much heating/cooling from the main system as you can and then look at a supplemental heat pump if necessary as we had discussed earlier.

      To get a central system to work well in this kind of situation you would probably need a zoned system with multiple thermostats and dampers in the registers and ducts to get a more even heat distribution.

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      • #18
        I'm sure its a great site but don't really feel like signing up just to use their calc.
        The site I used was not overly detailed, but truth be told, I'm not sure if I could give all the answers they would ask. They asked for insulation type, location etc but only asked "few,average or many" windows instead of details like window size and low-e glass.

        Exactly, I just entered room as a stand alone house.

        Yes, thermostats seem to be in odd places. I'd expect the ground floor t-stat in a centrally located room instead of a distant bedroom, and a thermostat upstairs as opposed to one in the basement.
        I have no problem moving the t-stats to a better loaction to better ballance the temps if needed.

        ​​​​​​​Problem with zoning is its usually done by adding a second huge stack which would never fit where I want it directed.

        Or, do they have something like miniature 8" dampers that could close off say, three or four other ducts while opening my two or three new added ducts to the room? Sounds like that'd be expensive.

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        • #19
          I'm sure you can get dampers for 8" duct and in theory you can close off some of the ducts/registers in the main house and open up the ducts to this room to concentrate more heating and cooling there - but again you would need a suitable controller (unless you plan to keep them set at fixed positions which is also possible after an attempt at air balancing) and also there is a limit before duct noise becomes an issue and also in a ECM blower system there is an upper limit to the amount of static it can develop.

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          • #20
            Ok thanks, I think I'm going to pull the trigger and add the two 8" ducts and a return and see how it works. I'm hoping a 8" return will also do the trick.
            My crude estimation is that I can get roughly 30kbtu/hr into the room, and I can play with the vents to see where dampers should be added and how much they need to be restricted in order to optimize it. It may be slightly less than required for the space, but I'm hoping it'll make it adequate to make it a comfortable space.
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            I really appreciate having your guidance on this blue_can.


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            • #21
              Originally posted by BHD View Post
              I do not seen a problem,
              If the rim joist is sitting on a wall there is no problem.
              I have done so much with so little for so long, I can do almost anything with nothing now!!!!!!!

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              • #22
                Glad to help - sounds like what you are planning to do would be the best course of action. Not sure if your local utility has a tool lending library like ours - but if they do and you can borrow a flow hood you can use that and check how much cfm you are getting via the 8" ducts and then estimate the btu/hr input to the room from the existing system based on the total cfm and btu/hr rating of the furnace/ac. You can then estimate the btu/hr rating of a supplemental system based on the difference between the load calc and the input btu/hr from the existing system (if any). You can also see how it performs and cross check the results with the calcs - dampers in the existing ductwork to boost airflow and btu/hr capacity into the room by restricting the supply air elsewhere would also be an option as discussed earlier.

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