Why are heating vents always located on outside walls?

I find heating vents on outside walls in every house I’ve looked at that has forced air heating. This seems wasteful to me since that means the ducts must be running through outside walls, which means heat from the ducts would be escaping to the outside through the insulation. Wouldn’t it make more sense to locate the ducts in inside walls?

Answer from Green Energy Efficient Homes

While it’s true that in most houses you’ll find the heating vents on outside walls (or rather, coming out of the floor near an outside wall), this doesn’t mean that the actual ductwork is necessarily running through the outside walls. In many cases the ductwork comes up an inside wall cavity, then travels between joists to the heating register opening. The two main reasons for heating vents on outside walls are to reduce condensation on windows, which can prematurely deteriorate if water or ice builds up on the inside of the glass due to cold outdoor temperatures, and to prevent cold pockets from forming in the room near outside walls.

I’m grateful to Ryan from Anchorage, Alaska, for the following more detailed explanation – which he sent me to correct my original answer, in which I stated that locating heating vents on outside walls was merely a factor of the whim of builders or a question of poor design!

There are several reasons to put vents at exterior walls, or more specifically, at exterior windows. Any architect will tell you that they do this intentionally, even in brand new LEED certified buildings. When it is cold out, condensation builds up on windows if they do not have air moving past them. This moisture can prematurely destroy window sills and infiltrate the exterior wall, destroying its insulation and growing mold. Those of us in northern climates are also familiar with seeing ice on windows which don’t have proper ventilation. Other reasons stem from the creation of cold pockets. This creates chilly drafts, which people react to by turning the heat up. If the cold air near the window is heated, the whole room stays comfortable with less energy. You don’t need to heat the warm air, just the cold air. This all should be done in a way that maintains insulation between heated space and non-heated space. I’m sure that there probably are some retrofits that run ducts through exterior walls in ways they shouldn’t, but that’s a design problem, not a problem with the concept of venting at exterior walls.


It is indeed true that there are houses where the ducts run up outside walls – this is even the case in my own home. We have been undergoing some major remodeling lately – removing the center load bearing wall from the back portion of our house – and the builder routed three ducts that used to go up the center wall, up outside walls. While this is less desirable because those ducts are more exposed to the outside and therefore some of the hottest air in the house is close to the outdoors, the builder did try to put as thick a layer of insulation as possible between the ducts and the outside walls. Since we had decided to go with 6″ insulation (up from 4″ in the original ground floor), and since the ducts he used were 2″ deep, 14″ wide ducts instead of round ducts (designed to fit inside a stud hollow), we still managed to get 4″ of Roxul insulation behind two of the three ducts in question.

8 replies
  1. Christine
    Christine says:

    The very last paragraph of this article is more in line with thinking I learned in doing home heating calculations in my architectural classes. The idea was not to put vents in outside walls because those outside walls are all ready cooler than interior walls in the winter, so why run your warm air there from the heating unit? It will only compromise the heat which will affect the temperature of the room making it less efficient and comfortable. Please comment.

    Reply
    • Robin
      Robin says:

      You are right, running ducts on outside walls will reduce overall efficiency due to heat losses through those outside walls. The only situation where you would want to run heating ducts on outside walls is where no other option is available, as was the case in my renovation. The middle wall was removed from the downstairs, so only the outside walls remained as an option. However the actual vents were routed through the downstairs ceiling to come up in the center of the house (where they had always been) since there were no windows on the outside walls in question.

      Reply
  2. Brenda
    Brenda says:

    I am going to be put a bottled gas heating stove in a room that does not need to be vented. so would it still need to be on an outside wall?

    Reply
    • Robin
      Robin says:

      Putting the heating stove on an outside wall will help reduce condensation on windows, but it is not necessary to put it there.

      Reply
    • Robin
      Robin says:

      If there is no heat at all, is there any air coming out when the furnace fan is running? If the furnace or duct work were not properly sized for the number of vents you have, you would feel a faint current coming from the vents, but perhaps not much heat. If the vents upstairs near the thermostat let out lots of heat, and warm the area around the thermostat, that can cause the thermostat to shut down the furnace early, so you only get heat out the basement vents for a short time. If the vents are shut off (there are often louvers near the furnace to shut off different duct mains) or part of the ductwork has gotten knocked out of whack in the basement ceiling, then you would have no air at all coming out of the vents even with the furnace fan running full blast.

      Reply
  3. Kim
    Kim says:

    I want to renovate my kitchen and take out a supporting wall that contains the heating duct to my daughter’s bedroom. The only thing I can see to do is to have it make two turns (which I am assuming will decrease the force of the warm air) and run it up an exterior wall. Is this a terrible idea? Will her room forever be cold in the winter? Otherwise I would have to put a pillar so as not to change the duct work at all.

    Reply
    • Robin
      Robin says:

      I did more or less what you are describing when I took out the central supporting wall in my downstairs. I had heating vents to a bedroom and a bathroom that went up the center. Both vents got routed up the north wall and then back to where the old vents were. I didn’t notice any change in the amount of air going to either room and even on the coldest days (and it goes below -30F here) we’ve been comfortable. We switched from a 6″ circular duct to a 2×12 duct that fit in between two studs. There would have been one extra bend. You can compensate for extra bends by having the cross section be wider than the original duct (circular ducts have lower air resistance than rectangular ones, so you need extra cross section area for that reason too). If the vent has a partly closed baffle near the furnace, you can also open it more. If your daughter’s bedroom gets comfortably warm now I expect you will be fine.

      Reply

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