HVAC question

Discussion in 'HVAC Heating & Cooling' started by idoc4u, Jan 18, 2010.

  1. idoc4u

    idoc4u New Member

    Messages:
    83
    Location:
    Indiana
    I would like to cut into my 6" round branch supply duct and attach a boot and register for heat and a/c to a basement bathroom I am finishing.

    Can anyone tell me how to go about finding a fitting with a flange that allows me to cut into the duct, attach a round piece of duct and bring it down to the ceiling where I can attach a register?

    Also, when I look at registers at local hardware stores, the package appears to have the register, but no louvers to control flow volume. Is this a separate purchase?

    Thanks to all.

    Attached Files:

  2. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    If you do that it will seriously effect the air flow to the original register. Anyhoo. nobody makes such a fitting.
  3. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
    If you use round duct you can dove tail the perimeter of one end of the new round duct and turn up every other dove tail 90 degrees to the new duct. Cut a round hole in the existing duct than fit the new one in. Than reach into the new duct and bend each dove tail (that you have not already bent) till it is 90 degrees on the inside of the existing duct. They make a dove tail tool but you can also do the same thing with tin snips.
    I would but in at least 4 self tapping screws (for metal) equally spaced around the perimeter or the new duct keep you from accidently pulling out the new duct. Get some duct sealer (paste type) and seal the perimeter of the interface. I you use round duct you will have to externally insulate it with 1-1/2" to 2" thick insulation. Assuming you do not have an insulation stapler you can seal the duct with an approved duct tape (typically a foil back tape).
    I reference to the grill you should purchase a grill (something for the grill to fit and attach to) can with that you can route the new duct into. I you have to make another connect and the grill can does not have an interface round you will have to so similar to what I indicated for the main tap. Dove tail etc. If you have a round interface the duct fitting into the round interface should be crimped to fit into the round grill can interface. The grill can will have to be insulated also.
    In ref. to the grill/diffuser you can typically purchase one with a multi-shutter damper (damper blades move parallel to each other when operated) or if you want to step it up a notch you can purchase a grill with no damper and than purchase an opposed blade damper seperate that fit it ti the back of the grill. Before you do this you need to ensure you have enough room to fit the grill can grill etc.
    There are other ways for doing this what I have discribed is cheap but works. See attached web site for more ideas
  4. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
  5. idoc4u

    idoc4u New Member

    Messages:
    83
    Location:
    Indiana
    Would it make sense in this case to use a 6" T-Saddle, screw it into the horizontal 6" round duct, extend from the saddle down to the ceiling and connect to the damper and register? I would use self tapping sheet metal screws to connect all the above and then foil tape around contact points in the duct. Thanks again for replys/opinions.
  6. idoc4u

    idoc4u New Member

    Messages:
    83
    Location:
    Indiana
    Would tapping into the existing 6" branch round duct with a saddle and short run to a new register vs running a new duct from the main trunk make a significant or insignificant difference with regard to air flow?

    I'm trying to determine if it would be more effective to run an entirely new branch off of the main trunk to supply heat to a 10X10 basement bathroom or if I could simply tap into an existing 6" round branch running over the area of interest with a 6" saddle and drop down 8" to a new damper and register. The latter would be much less work, but I'm not sure if it would limit the air supply to the new area as well as limit the supply to the register it currently serves as compared to running a completely new duct to the new area.

    In other words, am I stealing away to much air from both registers and not serving either well by tapping into the one branch, or would it be a wash between having two registers attached to the one branch vs two separate branches.

    Hope that makes sense based on the description and photo above.

    Thanks again for replies.
  7. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    It would be much much better to run a separate duct from the main. You only get about 640cfm through a 6" round at normal static (depending on length of run and other factors) Besides that if you don't WYE off the 6" the straight through part of the run will get 90% of the air flow.

    Using a straight starting collar doesn't work either because they are not curved too fit the pipe radius.
  8. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
    No matter what you decide your HVAC system only puts out so much air for the unit size. You will be robbing peter to pay paul with your air system unless you have a selective fan speed on you AHU or Furnace.
    One thing to keep in mind is if you have an exhaust fan in the toilet area you need to ensure the exhaust exceeds the supply air to the toilet or you will get some odor back into the surrounding area. So if you toilet exhaust is 100 CFM you would want to put say 90 CFM of supply air so you get somewhat a negative pressure in the toilet room with the door closed.
    Normally an 8" duct would be designed for around 200 CFM. If you have access to a Trane Ductulator which may be on the web you would normally size for .08 " loss per 100 ft or duct. For a furnace you may want to size your duct for .05" per 100'.
    Not sure what code you use but IMC says about 50 CFM EXHAUST for a toilet or urinal. You mighe want to start with that. The other approach is to rule of thumb it with a max of 1 CFM per SF but make sure that your exhaust fan exceeds the supply air.
    You are really talking about a small duct here when you look at a Ductulator. If it is a small toilet and lav (not knowing how much area) I would guess you are talking about a 4 to 6" duct.
    How much area and how many toilets are you thinking ....is there a shower also?
  9. idoc4u

    idoc4u New Member

    Messages:
    83
    Location:
    Indiana
    Thanks for the advice! I will run a 6" from the main trunk to the 10X10 bathroom and connect to a 6" damper and register. I would like to then install a bathroom exhaust vent and light combo and run a 4" duct to the outside.

    Should the exhaust duct be 4" sheet metal or does the flexible 4" duct make more sense, since it is insulated and I'm trying to pull humid air out of a bathroom that has a toilet, vanity and shower?

    Thanks again. You are all quite helpful.
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,139
    Location:
    New England
    For your exhaust, insulated only makes a lot of sense if it is going through unconditioned space. Otherwise, the smooth surface of a metal duct will improve the cfm output of the fan.
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