placement of vents under windows

[michelyn8]

Help me understand the logic here.  In probably 99% of the houses I've lived in, the heating/AC vents are placed under windows - why?  All that does is cause your heat or AC to blow up into the curtains unless you use those special covers to re-direct the air flow into the room.  In my bathroom, its not only under the window but when the door is open, that blocks the airflow even more because the door stops right over the vent (home built in early 1900's with baseboard heat/no AC and central air added within last five years) and its a narrow vent to boot.  I was in the shower this morning which is on the opposite side of the rooom from the window and vent and suddenly got hit with a draft because of the route it circulated.  Nothing like breaking out in chill bumps while trying to shave your legs. 


This is one of those make me wonder things I've never figured out.  Can anyone explain this to me?

[Deleted]

Found this at "merchantcircle.com":

The original warm air furnaces did not have blower (fans). The systems operated on a gravity principle - warm air rises and cold air (more dense) falls. It has always been a know fact that cold drafts could be felt dropping down from the windows. Hence, the original designs placed the cold air returns under the windows, by exterior doors or exterior walls. The hot air vents were located on the inside walls. The returns grilles were usually large. Warm air leaving the furnace was expanded (less dense) and required large ducts to allow the air to flow away from the furnace. As the warm air flowed away it left a low pressure area in the heating chamber. This pulled the cold, dense air down to the furnace to be heated. Because the cold air was more dense (took up less space), the return air ducts were smaller.
Shortly after World War 2, manufacturers started putting fans in the warm air furnaces. Returns were still placed under the windows. Better circulation was achieved because of the fan, however, the temperatures in the home were not even.
In the mid 1950's, Carrier Corporation did a study on the cold air drafts near the windows. Tests showed that the cold air velocity (speed) exceed 40 miles per hour. The high velocity would cause the cold air to flow over the return air grille and travel across the floor. The hot air was flowing out of the vents on the inside walls. If a person was in the middle of the room, one side would feel the cold draft and the other side would feel the warm air.
So, Carrier determined that the cold air velocity could not be stopped; but if the warm air vents were placed under the windows, they would heat the cold air up before it hit the floor. The design was then changed to place the warm air registers on the exterior walls, windows and doors, with the return air on the inside walls. By the mid 1960's this concept became more common place - they had to teach old timers new tricks.
And as Paul Harvey used to say - "Now you know the rest of the story".

[Deleted]

They usually place the vents on the perimeter because that is where most of the heating/cooling load is.

[Formerly SK]

Awesome info Beer! 

I always assumed it was because furniture is less likely to be under windows (don't want them blocking windows), and you don't want furniture over heat vents so you put the vents near the windows so the furniture could be everywhere else. 

We have a two story house and in the upstairs the vents are in the ceilings.  I don't know how efficient it is, but man it's sure nice to not have to think about the vents when placing furniture.

[alabamagal]

It is also where there is no traffic.  Do you really want a vent in the middle of your bedroom (might be under the bed) or in the middle of your living room.

One I don't like in my house is in the main bathroom the vent is right next to the toilet.  When I had young boys it seemed like I could never get rid of a bad smell in the room, then I realized that their aim may have not been so good.  YUKKK.

[Deleted]

I grew up in a house that was built right around 1900. The vents were in the floor behind the bedroom doors. I remember sitting on them on cold winter days. 

[michelyn8]

Thanks for the info beerwench.  I knew there was a reason that involved the engineering/mechanics of HVAC.

To be honest, placement of vents doesn't affect my furniture place all that much.  If I want to put something under a window where a vent is, I just close off the vent.  My sofa is under the front window and the vents there are just far enough apart not to be covered.  But the baseboard heat registers are a big annoyance for arranging furniture. The only register with a breaker still on is the one in my bathroom.  I use that to heat up my bathroom a little faster in the morning but I don't really care for it.  I turned it on the other night before giving DGD a bath and had to be extra vigilent getting her in and out of the tub so she wouldn't touch it. 

[Abby Normal]

In our house they placed them right under the window and right in front of the opening to the sliding glass door (which was carpeted so you could never clean it well).  When we replaced the flooring, we moved them both.  Now they are in front of the closed section of the sliding door and just to the left of the window.  Looking at the duct work below, we think they cut the flooring in the wrong spot and stretched the ducts to make it work.

[Chocolate Lover]

I'm still getting over vents in the floors. I'm wracking my brain trying to recall if I've ever seen it. All the vents down here are in the ceiling. MAYBE high up on the walls but I don't recall any in the floors. 

[sesfw]

use those special covers to re-direct the air flow into the room

When I moved into DHs home (built in 1998) in Michigan with a basement I bought a bunch of the 'redirectors) for the vents.  Homes with basements usually have floor vents but homes built on slabs have ceiling vents.

Never could understand why the windows and curtain/drapes needed heating instead of the center of room.  The redirectors were great and worth every penny.

[Tiny]

Beer, that's an interesting explanation!

I know at my friend's newer house (1980's?) the A/C and Heat vents are under windows in the floor - so when the air is blowing --  the curtains/Drapes move (which causes their cats to think it's play time - which sometimes results in shredded curtains/drapes )


My house, built in 1947, has the cold air returns under the windows and heat vents on the inside walls.
I have a hard time keeping a consistant tempurature across the house but I don't think it's due to the placement of the vents (at this point it's poor insulation that is the #1 culprit). My house is pretty much in it's 'original' state - but with a new 2 stage furnace (which made a big difference in overall comfort in both winter and summer!)

I've got some vents behind furniture - but for the most part it all works.

[tskeeter]

Nov 13, 2013 12:59:17 GMT -5 Chocolate Lover said:

I'm still getting over vents in the floors. I'm wracking my brain trying to recall if I've ever seen it. All the vents down here are in the ceiling. MAYBE high up on the walls but I don't recall any in the floors. 

A lot depends on how houses are built in the area where you live. 

Where I live, houses are usually built on concrete slabs.  No basements or crawl spaces.  So, furnaces are usually in the attic and all of the duct work runs through the attic.  Vents are in the ceiling.  But above cold areas, such as windows and doors.  Rather than several cold air returns throughout the house, there is often a single large (ours are 20 X 30), cold air return for each furnace.

When I lived in an area where almost every home had a basement, vents were in the floors.  Furnaces were in the basement, and floor mounted vents means you didn't have to run as much duct work.  And you could put insulation in all of the exterior wall cavities instead of having duct work in the wall so you didn't have room to insulate some of the stud bays.

[Chocolate Lover]

Nov 13, 2013 14:29:28 GMT -5 tskeeter said:

Nov 13, 2013 12:59:17 GMT -5 Chocolate Lover said:

I'm still getting over vents in the floors. I'm wracking my brain trying to recall if I've ever seen it. All the vents down here are in the ceiling. MAYBE high up on the walls but I don't recall any in the floors. 

A lot depends on how houses are built in the area where you live. 

Where I live, houses are usually built on concrete slabs.  No basements or crawl spaces.  So all of the duct work runs through the attic.  Vents are in the ceiling.  But above cold areas, such as windows and doors.

When I lived in an area where almost every home had a basement, vents were in the floors.  Furnaces were in the basement, and floor mounted vents means you didn't have to run as much duct work.  And you could put insulation in all of the exterior wall cavities instead of having duct work in the wall so you didn't have room to insulate some of the stud bays.

Probably explains it, I live in no basement land

[alabamagal]

Then you probably live in "A/C land" vs "heater land".  If you are putting in cold air, you get better circulation from above, hot air from below.

[Value Buy]

My house is a two story, and is schizophrenic in heat register installments.
Rec room, dining room, den, half bath, registers on the floor.
Living room, kitchen seven feet up on interior walls.
Upstairs, all four bedrooms, and bathroom all registers in the ceiling, where the hot air never really gets down to ground level in the winter.
Now in summer, same thing-cold air blows out of the ceiling, but travels lower due to fact cold air drops.
As kids have moved out, we also realize we should leave the bedroom doors open for maximum air flow through the house.
It seems with bedroom doors closed, the furnace does not work as efficiently, although there are cold returns in the rooms too.
House was built in late seventies.
After we moved in, we noticed on the air ducts coming from the furnace, were all sorts of black felt marker notations on which vents to close and open for summer and winter seasons.
I guess the original owners had problems with the air flow for maximum comfort, too.
If we look to buy another used house, that will be the first thing I look for, before making an offer.

[Chocolate Lover]

Nov 13, 2013 15:38:12 GMT -5 alabamagal said:

Then you probably live in "A/C land" vs "heater land".  If you are putting in cold air, you get better circulation from above, hot air from below.

  ok, that's not what I went looking for but I was so happy to still see the zombie there. Now I want to dance...........


And yes, to A/C land