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My current house is a single story built over a crawlspace and all the HVAC registers are in the floor. In all of the larger rooms, at least one (and usually more) register was placed near an exterior wall and almost always near a window. Looking throughout the house, it seems intentional. I've noticed this before in other houses, and I've talked a few friends and the informal poll tells me that for houses with floor registers built in different states built decades apart, it appears the registers were intentionally placed near exterior windows. In my case, given the layout of the crawl space, it seems like it would have been easy to place them them in other locations.

My amateur intuition tells me that for both heating and cooling, having registers near a window is going to lead to a lot of waste. Is this layout actually common? If so, why?

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You are correct that it leads to some waste because the closer the vents are to windows, the higher the temperature difference across the windows. Higher temperature differences cause faster heat transfer. However, the energy you lose because of this is probably much less than the amount lost in the ducts themselves through air leaks and thin (if any) insulation. –  Evan Johnson Apr 16 '13 at 21:30
    
Windows condense moisture, heat evaporates it and takes it away. Window rot prevention. –  Fiasco Labs Mar 18 at 4:38

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up vote 10 down vote accepted

Yes, placing registers (or other heat sources such as radiators) near exterior windows and doors is the usual practice. This is done in order to combat cold drafts and ensure a more even temperature throughout the room. Here's a Q&A on the subject from Ask This Old House:

Window glass is the coldest part of a wall. When warm room air hits it, the air cools, and cool air sinks. The movement of cool air creates floor drafts that most people find uncomfortable. The placement of forced-air heat registers or baseboard heating units under the windows counteracts this process by sending up warm air to mix with the cool. The end result is that the room feels more comfortable.

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It's also convenient for room usage. you are unlikely to want to put furniture underneath a window - so having a vent/radiator there doesn't inconvenience anyone. –  mgb Jun 25 '12 at 0:58
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It also helps dry out a major water source, rot and mold haven. –  Fiasco Labs Mar 18 at 4:40

This DOE reference indicates, with better double glazed windows and better insulation, that ducts should not be run in outside walls anymore.

HVAC Ducts Shall Not Be Run within Exterior Walls

In the past, it was common practice to run ducts inside a wall cavity of an exterior frame wall. It is sometimes done today. However, we have learned that this practice will create an energy penalty and can cause durability issues within the wall cavity ... In older homes with poorly insulated walls and single-pane windows, supply registers are often located at or on exterior walls to condition the cold walls and cold air that would leak in at the windows. In new homes with better air sealing and insulation and higher performance double-pane windows, there is less heat transfer through exterior walls and less air leakage in and around windows. Exterior supply air throws are no longer necessary to maintain comfort; shorter duct runs with interior throws are preferred for improved energy efficiency and better HVAC performance.

https://basc.pnnl.gov/resource-guides/hvac-ducts-shall-not-be-run-within-exterior-walls#block-views-guide-static-blocks-block-2

Of course, this would not rule out floor registers below windows.

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Nice find! I had not given that any thought. –  wallyk Mar 26 at 1:12

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