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Placement of Forced Air Room Vent

RKReges's picture

I have a gas-fired forced air heating system.  I removed an interior wall and found a 7" round duct which is now in the middle of the room.  I want to move the duct which will require 3 new 90 degree turns.  To eliminate 1 or 2 old turns, I'd like to cut a new floor vent into the upstairs bedroom.  As expected, the old vent is under a window.  The new vent would not be near any window.  Will code allow placement of the new vent away from the window and, if so, what are the pros and cons of such placement?

(post #115857, reply #1 of 7)

Code doesn't require vent placement to my knowledge. System design and loads dictate placement of supplys and returns. Standard placement has been beneath windows because that was usually the most drafty point in a room. I have even seen old homes with returns beneath the windows and supplies located on interior walls. Same logic of a sort was used in that design.

Put them where you want them in a remodel, but keep them as far from the return air vents as you can in each room.

Adding additional nineties to a run will reduce the velocity of the discharge air. Expect a cooler room durring heating season and warmer durring cooling.

(post #115857, reply #2 of 7)

More knowledgeable will chime in soon.

I have seen forced air vents in all sorts of locations, under windows and not, all seemed to work.

Two possible explanations for installations under windows:

1) HVAC installer can pretty much assume that the HO will not be burying the vent under furniture if it is located under a window.

2) May be hold over tradition from the days of cast iron radiators which were deliberately placed under windows to take advantage of the cold air currents flowing down the wall from the windows. Sort of a convection air circulation system without a forced air fan.

3) Or, there may be an advantage to placing the vents under windows.


Never underestimate the value of a sharp pencil or good light.
Never underestimate the value of a sharp pencil or good light.

(post #115857, reply #3 of 7)

First, I'm not an HVAC installer, contractor or expert. Just been around awhile.

In the days before double/triple glazing, condensate was a common problem. Having  the supply/return air under the window provided air movement that eliminated/prevented condensate on  the glass.

Since an exterior wall (especially if it has a window) is one of the main sources of heat gain/loss, it is my understanding. that it is best to get the conditioned air to these areas in order to have a more evenly conditioned space. Otherwise, you may find that it will be cold/hot near the exterior walls.

If your register could be located in the ceiling, you could simply direct the airflow in the direction needed. However, in the floor, this is a little harder to do satisfactorily

Since you have not filled in you profile and did not say where you are, it makes it a little harder to be specific. If you are living in Hawaii, I doubt it make much difference where you locate it.

(post #115857, reply #4 of 7)

A disadvantage of locating a vent under a window is that it blows the drapes around, and heat can be lost running up between a drape and a cold window.

However, somewhere on an outside wall is generally preferred.

But, if your walls are very well insulated, it makes little difference where in the room the vent is, so long as it turns over the bulk of the air in the room.

(post #115857, reply #5 of 7)

You are getting reasonable responses. To clarify ... your supply register is under a window, but the duct serving that is soemhow located in the interior wall you moved. Correct?

Modern house HVAC design has steered away from supply registers under windows or at exterior walls ... because there is a higher interior surface temp due to better insulation values. This means less disparity of temperature between the interior wall and the exterior ... so it means a lot less where you locate the registers. This may be less true if you have an unusually LARGE window area in the space.

Locating away from the return air is a good idea as Wayne I think pointed out.

Increasing duct length will decrease air flow. Ditto more elbows.

You need to think about duct layout and furniture layout and think about the path between supply and return. May be no easy task.

Your location and the energy construction of your house ... would be helpful information. Feed us so we can help a little more. Is this the only register for the room? how big is the room? Does the added space next to it have a register/exterior walls.

There ain't NO free lunch. Not no how, not no where!

(post #115857, reply #6 of 7)

If you can reduce the length and/or number of turns in the supply branch by locating the register somewhere else, by all means do so.

What really counts in the air delivery is the velocity of the air as it throws out from the register. Increases in length or turns will just reduce the velocity at the register.

As air leaves the register, it gathers and starts moving ("entrains", in HVAC lingo) room air. The entrained air can be 10 to 20 times greater than the volume of air from the register. Higher velocity = better entrainment, or mixing of room air.

The proximity of the return air to the supply air is not important compared to the velocity of the supply air. To understand this, think of the hotel rooms you've been in: the thru-the-wall unit that heats and cools the room has its return air directly under the supply air, but it still heats and cools the room just fine (although noisily, but that's another problem.)

Keep in mind that returns don't drive the performance, the supply does. The supply pressurizes the room, and the return simply relieves that pressure. Having too small a return is a much greater problem than where it's located. Returns should be at least 30% to 50% larger than the supply.

As others have said, the practice of locating registers under windows was from the days of single-pane windows and uninsulated walls/floors.

(post #115857, reply #7 of 7)

Also (remembered from the dark ancient days when I was taught mechanical design theory) windows are the least insulated part of the wall assembly.  Applying conditioned air at thet location helps 'average' out the assembly, which also reduces drafts, which is a primary source for occupant discomfort.

Occupational hazard of my occupation not being around (sorry Bubba)

I may not be able to help you Occupational hazard of my occupation not being around (sorry Bubba)