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  1. #1
    Join Date
    Dec 2011
    Location
    Minneapolis, MN
    Posts
    5

    Value of using old (small) ducts as cold air returns

    Background:
    I have a 1900's house that the previous owner upgraded (from gravity?) to a 90% 130k BTU furnace. It was tied into some existing in-wall ducts to supply heat to the 2nd and 3rd floors, and floor vents are used to heat the 1st floor. The only cold air returns in the house are three rather large floor registers on the first floor, all more or less in the same room. All heat supplies are in the middle of the house or near middle walls.
    I have an ecotec motor in the furnace and it is set to run continuously on low speed to give some balance to the house. All walls and the attic are well insulated, but the windows and cold air return placement make the upper floors colder than the main floor much faster than the thermostat knows... hence the need for a continuous low running fan to distribute heat a bit better.

    Question:
    The house has in-wall mini-ducts which connect to baseboard medallions that are not currently hooked up to anything. They are located opposite to all of the heat supplies on the 2nd floor (none on the 3rd floor). When I say mini duct, most measure at 2.5" and one at 3.5". They run inside exterior walls and are not plugged. I hooked a 5hp vacuum cleaned up to them and got good flow down from them down to the basement where the furnace is. Being that I have no 2nd floor returns in any rooms, would it be worthwhile to add additional ductwork and tie them into the cold air return of the system? I am hoping they could be used as returns to get a bit better overall air circulation in the house. The rooms they are located in are not that cold, but if any doors to the rooms are closed, they do get warm due to limited circulation air flow. I realize they are small and will provide minimal air flow, but I thought hey, better than nothing.

    At worst, they could be plugged with insulation to provide a very slight improvement in exterior R value insulation, but I think there would be a bigger bang for the buck comfort-wise by making them returns.

    Thoughts?

  2. #2
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,060
    Are they small pvc pipes? Your return needs to carry a large volume of air at a vary low pressure I do not think you will accomplish to any noticeable degree much volume being moved

  3. #3
    Join Date
    Dec 2011
    Location
    Minneapolis, MN
    Posts
    5
    All of the ducts are metal. I had a spare chunk of it in the basement rafters which turned into a good rain down-spout extender in a pinch

    I'm not expecting miracles from them, but since the basement is in the process of getting finished walls and a proper ceiling, I wanted to get others opinions on whether it would do me any good to get them connected (or sealed?) prior to making them inaccessible.

  4. #4
    Join Date
    Sep 2009
    Location
    Arnold mo
    Posts
    3,955
    Can you send me a private message with your address so I can see your house on zillow & google maps? Your symptoms of a colder upstairs is the opposite of what usually gets complained about.
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

  5. #5
    Join Date
    Dec 2011
    Location
    Minneapolis, MN
    Posts
    5
    Quote Originally Posted by tipsrfine View Post
    Can you send me a private message with your address so I can see your house on zillow & google maps? Your symptoms of a colder upstairs is the opposite of what usually gets complained about.
    Sorry, no PM yet as I am a newly registered user with no privileges yet. Here is a pic that sums up the house from the north west corner:



    Prior to using 'continuous low', the 2nd floor was only a degree or two lower than the 1st, but the 3rd floor was consistently 5+ degrees off because it only has one supply with no returns.

    Adding to the original background a bit, we also have one room that due to a previous owners remodel has no heat supply. This south-east facing (not pictured) room has lots of windows and is overly hot in the summer and cold in the winter. The only hope for it at this time is the 3" in-wall metal ductwork under discussion. I might be able to sneak a line up a doubled interior wall for this particular room, but the intent was to use that for another supply and return up to the 3rd floor.

  6. #6
    Join Date
    Jan 2001
    Location
    Pamnyra VA.
    Posts
    710
    You might have a balloon type constucted house.This is where all exterior wall cavities are fully open to the basement.(no plates )

  7. #7
    Join Date
    Dec 2011
    Location
    Minneapolis, MN
    Posts
    5
    Quote Originally Posted by skibme View Post
    You might have a balloon type constucted house.This is where all exterior wall cavities are fully open to the basement.(no plates )
    The house was fully re-insulated from the exterior about 10 years ago. The neighbor told me this. Whoever blew in the new insulation was not very good at fixing the holes they left. Maybe the previous owner told them it was going to be resided? In any case, for the size of the house, my winter energy bills are not any more than I would expect from a house this size/age. My previous house was much smaller and the bills were similar.

    I had a home energy audit with a blower door test and came out 30% less drafty than similar houses in the area. There is still room for improvement (ie: windows), but that is a big $$$ project.

    If, by balloon construction, you were implying an exterior cavity could be used for HVAC purposes, the foundation dictates otherwise. 22" of solid rock. They cut slots for the 1st floor joists. I am guessing they put these tiny ducts in when they built the house because they go into the rock and somehow magically turn 90 degrees and go up into the walls above.

  8. #8
    Join Date
    Sep 2009
    Location
    Arnold mo
    Posts
    3,955
    Here is my advice. Before doing anything else, I would google your own address in Google maps and then hit the "search nearby" button and type in BPI ENERGY AUDIT, and also RESNET ENERGY AUDIT. You should see a list come up of energy auditors in your area. Do a little research & find one you like and have them come out and perform an energy audit of your home. I would love to see a blower door test done with an infrared camera scan of your home. I am sure you would find some major issues with your homes thermal envelope that need to be addressed before you even start considering the hvac issues.
    I know you said the insulation was good, but you'd be surprised what an energy audit could uncover.
    EDIT: Just missed your last post. I'd still like to see some infrared with a blower door running. Did they seal off the floors from the wall when they blew in that insulation? I thought I saw those small holes on the side, but I didn't notice anything around the band joist area, or where the floor separates the 1st & second floor.
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

  9. #9
    Join Date
    Dec 2011
    Location
    Minneapolis, MN
    Posts
    5
    I have not had an infrared test performed yet, but plan on having one done someday. I am sure it would be interesting and show more areas for improvement. The results of the blower door test are around here somewhere... Unfortunately things like that are easy to misplace in the stacks of paper that mysteriously accumulate over time.

  10. #10
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    His house sounds a little like mine in that stack effect and the normal convection in a home is minimal.

    My suggestion might be to first see if you can find some better quality supply registers for the upstairs rooms to increase airflow a little and provide better throw and air mixing.

    Using space or duct in exterior walls for a return should be avoided IMO. Any leaks will suck in cold air.

    Another thing to consider is increasing the blower speed if the duct size can handle it. If for example you have a 60F temp rise now, you could try for a 50-55F temp rise up to a point where you feel it's getting noisy. You could also try balancing the system to get more airflow upstairs. Just a couple dampers on 1 or 2 duct downstairs in the warmest rooms can make a huge difference. The higher air velocity will give you more throw and mixing.

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