Return Air Venting Question

Discussion in 'HVAC Heating & Cooling' started by 5shot, Apr 16, 2010.

  1. 5shot

    5shot New Member

    Messages:
    7
    Location:
    spokane, wa
    In our home, the previous homeowners finished the basement, but only put one cold air return in (on the ceiling no less).

    I can get to the return air duct, for the upstairs bedrooms, in the ceiling of one of the downstairs closets. I can tie into this duct in the closet, and run it down between the studs in the finished walls, and add 2 more cold air returns in the 2 downstairs bedrooms.

    If I do this, will it cause the returns upstairs to draw too little air? I know they all end up in the same place, but would like to be sure before I start cutting holes.

    Should the downstairs returns be run in their own ductwork?

    Getting some of the cold air off the downstairs floor should surely help in cooling the rest of the house.

    Thanks
    Last edited: Apr 16, 2010
  2. Lightwave

    Lightwave New Member

    Messages:
    98
    Location:
    Vancouver, BC
    Return air ducts need to be sized in proportion to their respective supply ducts. If you tie in new return registers to existing return ducting you stand a very good chance of unbalancing your system and making inter-room temperature differentials worse. In other words, yes, adding return registers to existing ducts could cause the upstairs registers to lose suction.

    Return air imbalance issues involving closed rooms tend to confuse the heck out of HVAC contractors because they're not familiar with the problem.

    Because you've implied that you're having problems cooling the entire house, your best option is to do a full Manual J and Manual D workup for the entire house before cutting or installing anything. Compare your Manual D duct sizing results with what's actually installed and correct as necessary. Installing returns blindly is unlikely to fix your problem.

    If you're not comfortable putting in the time and money to do the Manual J and D work yourself (it's not easy), then hire an engineer to do it for you. Engineers are usually cheaper than trial and error alterations.

    Finally, be advised that even temperature control in a multi-story house is a hard problem. Expect to need to re-balance your HVAC system (using dampers) when you switch between heating and cooling seasons.
  3. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
    Not sure about your system(s) but do you have 2 AHUs or one (Two T-stats) ? Is your house two story plus a basement? If so you really need two units. On a two story you loads peak at different times and that is what causes the problems.
    But adding a return or two (w/ balance dampers or return grill dampers) certinley does not hurt, especially if you have a furnace.. You will have two check the space between the studs to assure no fire breaks and cut the bottom plate on the stud wall, etc. Also check and make sure you have enough free area between the studs. I would extend the return duct to the bedroom and not rely on the closet to draw thru or under the doors. You could possible run an exposed duct in the corner of the closet if it lines up with a good space below.
  4. 5shot

    5shot New Member

    Messages:
    7
    Location:
    spokane, wa
    I know that I will have to adjust registers in the dieffernt seasons, but I know that there is not enough return air from the basement.

    I will look into the analysis that you recommend.

    Thanks
  5. 5shot

    5shot New Member

    Messages:
    7
    Location:
    spokane, wa
    We have one thermostat upstairs. It is a split level with an upstairs and a basement. I was going to duct from the ceiling of the closet into the stud wall, then put the register on the wall of the bedroom, adjacent to the closet, not in the closet itself. The closet would have given me access to two bedrooms, but would add two additional registers to a duct that already has two on it.

    I can access one bedroom interior wall and a hallway interior wall directly and duct them directly to the main cold air return duct, but it would have been cleaner to do it the other way.

    I can also move the existing cold air return in the basement to the floor without too much trouble.

    Getting the whole thing balanced is my main concern, although I doubt it could be much worse than it is now.

    The downstairs is a good 15 degrees colder than the upstairs, regardless of whether the heat or a/c is running.
  6. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
    As long as you are talking about return ducts (not supply ducts) it can only improve your system. You will have to adjust dampers for the different seasons. Try and get the dampers in the return grills otherwise you will have to put dampers in the return ductwork. Is the downstairs return ducted back to the return on the air handler or does it just dump into the basement room. Not knowing how your system is ducted but there is a good chance that when you put the returns in the bedrooms you may can install a return duct directly from the return grill opening to the AHU if it is not already done. How many tons or BTUH is your system?
    Do you have a furnace (gas) or all electric? Just curious.
  7. 5shot

    5shot New Member

    Messages:
    7
    Location:
    spokane, wa
    The one return in the basement is ducted into the main return air ductwork.

    I can run ductwork from one bedroom and a hallway directly into the main return duct on the AHU, and I can tie one of the downstairs bedrooms into the upstairs return duct.

    I don't mind having to adjust registers as the seasons change at all, so that isn't an issue for me at all.

    I hate wasting all that cold air in the summer, and without circulation down there, it brings the temp down in the winter.

    The A/C unit is 2.5 ton, but I am not sure how many BTU's the furnace is (Gas). I know it was sized for the total square footage for the home, but the homes were originally sold with the basements unfinished.
    Last edited: Apr 17, 2010
  8. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
    Furnace with a cooling coil I would guess, don't be stingy on you return duct sizing but don't go overboard either. It can only improve you system.
    Good luck
  9. 5shot

    5shot New Member

    Messages:
    7
    Location:
    spokane, wa
    Yes, the cooling coil is right above the furnace.
  10. Lightwave

    Lightwave New Member

    Messages:
    98
    Location:
    Vancouver, BC
    In that case, you need to do the Manual J/D workup for the entire house. You might need to reduct both floors to get acceptable performance.

    You should definitely install in-duct dampers to allow for season changeover. Register louvers are too loose to be effective for balancing.
  11. 5shot

    5shot New Member

    Messages:
    7
    Location:
    spokane, wa
    I can't access most of the ducting due to all the ceilings being finished (unless I demo a bunch of the basement ceilings). Are there dampers that can be dropped into the ducting from the register openings?

    Are you refering to just the return air registers? Both upstairs and down?

    For the calculations - how is the CFM for the supply and return lines determined (right off the AHU or should they be measured)?

    Thanks
    Last edited: Apr 18, 2010
  12. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
    I can't access most of the ducting due to all the ceilings being finished (unless I demo a bunch of the basement ceilings). Are there dampers that can be dropped into the ducting from the register openings? GET OPPOSED BLADE DAMPERS AT THE GRILL. THE OPTION IS MULTISHUTTER DAMPERS WHICH ARE CHEAPER AND CAN MAKE NOISE.

    Are you refering to just the return air registers? Both upstairs and down? IF YOU CAN PUT THEM ON ALL YOUR RETURNS THAT WOULD BE GOOD, BECAUSE YOU MAY HAVE TO SLIGHTLY CLOSE THOSE GRILLS THAT ARE GETTING TO MUCH RETURN. IF IN SOME AREAS YOU DO NOT HAVE A RETURN (THOSE AREAS THAT ARE GETTING TO MUCH AIR YOU CAN PUT AN OPPOSED BLADE DAMPER AT THE THE SUPPLY GRILL) UNLESS YOU HAVE A SUPPLIER YOU MAY HAVE TO PURCHASE NEW SUPPLY GRILLS WITH THE OB DAMPER ALREADY INSTALLED. BDFORE YOU PURCHASE ANYTHING YOU NEED TO VERIFY THAT THE GRILL CANS ARE DEEP ENOUGH TO ACCEPT A GRILL WITH THE DAMPER MOUNTED ON THE BACKSIDE.

    For the calculations - how is the CFM for the supply and return lines determined (right off the AHU or should they be measured)? YOU MAY HAVE TO ESTIMATE. NOMINAL CFM FOR A 2.5 TON UNIT IS 1000 CFM (RULE OF THUMB IS 400 CFM PER TON) IF YOU CAN DETERMINE YOUR SUPPLY "DUCT" SIZES. I CAN PROBABLY TELL YOU WHAT IT SHOULD BE IN A PERFECT WORLD. YOU WILL HAVE TO PULL THE SUPPLY GRILLS OFF TO GET THE INFORMATION AND DO A LITTLE SKETCH. YOU CAN SEND IT TO MY EMAIL.

    Thanks
  13. 5shot

    5shot New Member

    Messages:
    7
    Location:
    spokane, wa
    Thanks - much appreciated.
  14. Lightwave

    Lightwave New Member

    Messages:
    98
    Location:
    Vancouver, BC
    No, they need to be built into the duct work.

    http://www.spiramir.com/product_12.cfm

    For a 15 degree temperature spread, you need to be looking at all your ductwork, return and supply, for both floors. Getting things right might require reducting most or all of the house.

    The Manual J and D calculations will tell you how many CFM you need for each room. Measuring CFM requires special tools.
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