why do builders put radiators under windows?
 

hi i have always wondered about this one. am i missing something here? why do builders put radiators under windows? i would thought it best to put the heater as far from windows as possible, but often i see houses with radiators positioned just below a window with long curtains covering them. wouldn't the heat just go straight out the window?

It is suppose to keep frost/condensation off the windows to keep them clear.

I remember learning that heaters should always be on the outside walls of a house whenever possible, but I don't remember why. I guess they're probably more efficient when the heat has to fight the cold right away, rather than waiting for the heat to travel all the way across the room to cancel out the cold air.

Hiya,

Here's why: the air near the window (or walls) is chilled due to heat loss out through to the outside. The chilled air drops towards the floor.; and left unchecked, this causes drafts throughout the whole room.

By putting the heater under the dropping chilled air, it can best counteract the air flow -- the heated air mixes with the chilled air and then moves throughout the room better.

If the heater/radiator is located near the inside of the room, it would warm the air and it would move in the same direction as the chilled drafts, and this would not be as effective at warming the whole room.

The window is the coldest spot in the room, if the radiator was in some other part of the room then you would have a cold spot and a hot spot in the room, it's also the leas likely place to want to put furniture, and should help keep the window frost free.

I asked a guy about this and he mentioned the airflow issue and draughts. He also said with todays well insulated windows, it's not a problem. You could put radiators wherever you wanted but out of habit, builders put them under windows. I'm always having to tuck the curtains under the rads and it looks unsightly but it's better than a cold house.
Whenever i get to build, i'll use underfloor heating. Solves that issue!

ollie

i think it would save a lot of energy if the rads were put away from the windows. my gut instinct says that hot spots would not be an issue (for me) -i could live with that for cheaper bills. draughts only occur if you have leaky windows. if builder took this to extreams they would be heating their houses with (outside) patio heaters!!!

from a chemical engineering point of view a room is like a mixed flow reactor. if you put the input and output too close together you get feed short circuiting the reactor.

Hi,

The windows are always going to be the coldest part of the wall, because they cannot be as well insulated as the wall itself. So, the air inside the window will always be chilled (relative to the average air temp in the room) and so the natural air motion will always be there. Sure, it's not as extreme with a good, tight window -- but it would probably be less efficient to move the radiator somewhere else in the room.

I help design buildings for a living, and the laws of physics and thermodynamics have not changed. The best place for a conventional radiator is under the coldest part of the exterior wall.

the natural convection will be even more improved by having the rads away from the window. on one side of the room the hot air will be moving up from the radiator, on the other side the room cold air will be moving down from the window. the two flows will be working together mixing the air in the entire room in a circular motion.

if you have the radiator under the window the falling cold air and the rising hot air will have to fight each other thus slowing the speed of the resulting air flow and reducing the mixing. also the other parts of the room will be dead zones because they are too far from any source of heat or cold. cirtians will trap the rising air flow and make the mixing even worse.

i think the conventional wisdom is wrong on this one.

Interesting topic. But unfortunately you have at least a half century of building experience going against you. It'd probably be hard to prove otherwise. That's a giant hill to climb.

Thankfully, I won't be building a house of my own for a loooonng time. But when I do, I want to know the best place to put my vents. :thumbup:

Hi,

Moving air cools us off, and it would be perceived as drafts. It will also cause a noticeable temperature stratification -- the cooler air will stay settled nearer the floor and the warmer air will stay up at the top. It would not be a comfortable room to be in.

remember this all changes when you use a forced hot air system, but nonetheless. I agree that keeping the coldest part of the room warmer is nice for living in, but it IS far less effective at keeping the rest of the room warm since a good chunk of the heat is escaping directly out of the window.
and the heat cycle sort of DOES die when this happens, the cold air falling on the radiator gets heated and flows up to the window where it cools and falls. SOME of the air gets into the rest of the room, what passes by this cycle that is. its not a good way to do things, its there to eliminate drafts as was said. I have some education in this (and an ABET-Ass. to show it).

Now basically, if you want to pay less to heat your house, it works well to have that window area drafty, fun part is if there is no furniture there, who cares if its a few degrees colder? plus we are too soft lately as a people, must have fully even temperature, must have cell phones, must not do work or have any inconvience at all. bah, bah I say!

that window is a hole in the wall thermally. I dont care if you have the new super awsome R-5 windows or even R-10 windows which I'd sure like to see! a 2x4 wall has an R value of 19, a 2x6 wall with layer of foamboard on the outside as well is better than R-30. the roof should be R-50 or more. so that 0-8 R rating on the window, is a thermal hole.

how to fix your existing window for better heat retention in the winter? easy, get the shrink film, put that on, now drape a blanket over the window when you want to keep heat out.

I skipped to the bottom but you know it must work because haven't you seen the way places like Costco put the great big top down heaters at there entry points

This has gotten me into thinking. If one were to design a window, double or triple-paned, with a vacuum between the panes, would this result in a satisfactory increase in the R value?

It would increase the R-value over using the inert krypton or argon gases that are used in the fancier high-tech windows. How much, I don't know, myself. But even with those inert gases, the gap between the panes they fill is eventually breached over time.


Understanding Energy-Efficient Windows - Fine Homebuilding Article

Snippet from the site:
Like the isolated gasses, a vacuum between window panes will eventually be compromised, too. That's just the result of thermal expansions and contractions over the months and years.

I'd make a semi-educated guess that a vaccuum over inert gas would be chasing diminishing returns.

Best windows available
 

Not to get too far off the original question, but it has been answered well here already but here is a sugestion for "windows"...hang a flat panel LCD monitor on the inside of your wall and a video camera on the outside, leaving the wall in between intact. Build some Faux windows on the outside. yes it sounds crazy but here, a very good well insulated window in 36"x48" can cost upwards of $500 to nearly $1000 and be a giant "thermal hole" with an R-value of about 4 vs your wall at R-20 to 30. The energy lost through the window panes and weatherseal etc. is roughly equivilent to the power required to run the monitor, and the monitor is only on when you want it, as opposed to 24 hours a day 365. Lifetime costs ~20 years you are far better off with the monitors and you can change the "view" to your liking

The TV would be good in the wintertime, but in the summer you wouldn't want to run it - the worst insulated windows would be better than turning on a 100-200W LCD heater on the wall.