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Return air ducting

gravel | Posted in Energy, Heating & Insulation on March 10, 2005 11:24am

I got such a good response to my last post I thought I would try another.  I want to extend a back door landing by moving the basement stairs approximately two feet.  My problem is that the guest room return air duct must be removed for headroom clearance when I move the stairs.  The guest room spends 90% of its time unused with the heat register closed.

My question is this – will eliminating this return duct cause problems when the room is needed or reduce the efficiency of our HVAC system?  We have a new high efficiency furnace that breathes and exhausts through large PVC pipes.  Does the outside air inlet eliminate or reduce the need for return air ducts?  Thanks.

 

Farmhouse

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  1. TRice | Mar 11, 2005 12:20am | #1

    "..will eliminating this return duct cause problems when the room is needed?" yes.

    "Does the outside air inlet eliminate or reduce the need for return air ducts?" No, with the high efficiency, sealed combustion furnaces, the pvc pipes only provide combustion to the burners and remove flue gases from the house. This "system" is completely separate from the supply/return system. The air that goes from the furnace to the various rooms in your house (the supply side) has to equal, exactly without exceptions, the air going to the furnace (which is the return side, and sometimes includes some fresh air).

    The same applies to every room or enclosed area in the house that is served by the furnace. What goes in, must go out. If you have a supply and no return and no way for the air to get out of the room and back to the furnace, then air flow and comfort will suffer. Every room does not need a return (not considering whether required by code or not) but there has to be a way for the air to get out when the door is closed. So you may be able to get away without the room return, if undercut the door, for instance. What about the hole?

    1. gravel | Mar 11, 2005 08:04pm | #3

      Timbo,

      Great input, thanks.  The guest room used to have thick carpet so the door is undercut about 1 1/2 inches.  Also, air escaping under the door would dump into a hallway that does have a return.  Another thought - what if I cut out the basement stairway duct but left each side open with grills on them?  So the thought is the guest room return dumps into the basement stairwell which will have its own return.

      Your response also got me thinking about our upstairs master bedroom.  It has no return, a door with no undercut, and is the farthest run from the furnace.  It has never seemed to get much air flow from the register.  We had been thinking about installing an old floor register to allow heat from the living room below to rise into the room.  Would this floor register actually be acting as a return and improve air flow?  Other thoughts?  Thanks again.

      Farmhouse

      1. TRice | Mar 14, 2005 09:13pm | #5

        FH,

        You can do what you mentioned with the stairwell, don't see any real problems there.

        As far as the other room goes, "Would this floor register actually be acting as a return and improve air flow?" No, it would be what is usually called a transfer grille. The use of grilles between floors to heat floors above the main heated space is a practice that has been in use for along time. Primarily before ducted systems were common. 

        "Other thoughts?" Yes, first, before you hack some large holes in you floor try a few things. If you can, try opening the door during when the furnace is running and see if you don't notice better air flow. Residential furnaces operate with very little pressure and small "problems" can sometimes prevent areas from getting enough heat. Check and make sure the filter is clean, and if possible, close down the regisers closest to the furnace (where the supply pressure is the greatest) to see if you can't get more flow thru the one in question. Further options can be investigated after you have done these few things.

  2. WorkshopJon | Mar 11, 2005 01:56am | #2

    The guest room spends 90% of its time unused with the heat register closed.

    My question is this - will eliminating this return duct cause problems when the room is needed or reduce the efficiency of our HVAC system?"

    FH,

    IMO, you always have the option of adding a supplementary electric baseboard heater with it's own thermostat. Not hard at all to do, and pretty cheap, even if you call in a pro.

    I ran into the same problem when I added an upstairs bathroom in my house.  No neat and practical way to tap into the forced air HVAC system.  Think the whole schmeal (4' 220V base board heater, wall thermostat, and timer (so it's warm first thing in the morning) set me back one C note plus my time.

    Anyway, it's work considering.

    WSJ

    1. gravel | Mar 11, 2005 08:07pm | #4

      WSJ,

      Good input.  I do have a 220V curcuit for basement electric baseboard.  If the return can't be modified properly I will consider this.  Thanks.

      Farmhouse

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