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  • geordie joe
    • #2
    • 18th Sep 08, 6:11 PM
    • #2
    • 18th Sep 08, 6:11 PM
    I've got the foil behind the radiators to help bounce heat out, will a radiator shelf also hep to reflect heat out into the room (rather than up the wall).
    Originally posted by meames
    It will only reflect the heat to the edge of the shelf, then it will go straight up again.

    Also, I have one radiator that won't fit a radiator shelf (double radiator), would a floating shelf above it help?

    Thanks

    Michelle
    Originally posted by meames
    No, radiator shelves are a waste of time, and if your radiators are below a window they will do more harm than good.
  • mech
    • #3
    • 18th Sep 08, 8:01 PM
    • #3
    • 18th Sep 08, 8:01 PM
    I don't see they can do any harm. If the curtains are hung above the radiator so that the hot air currently goes between them and the window then a shelf would seem to be a good idea.
  • geordie joe
    • #4
    • 18th Sep 08, 8:30 PM
    • #4
    • 18th Sep 08, 8:30 PM
    I don't see they can do any harm. If the curtains are hung above the radiator so that the hot air currently goes between them and the window then a shelf would seem to be a good idea.
    Originally posted by mech
    No, what happens is the warm air in the room gets cooled by the window. This cooler air falls to the floor and drags more warm air down to the window. This warm air then gets cooled and falls, dragging more warm air to the window to be cooled. It's a continuous cycle which gradually cools the room.

    If you have a radiator beneath the window, and it's on, it breaks this cycle and prevents the window cooling the air in the room.

    Putting a shelf above the radiator will stop the radiator sending heat up to the window and the cycle will start again.

    It's similar to having your fridge door open, the cold air falls out, dragging warm air from above into the fridge and making the fridge work harder to cold this air.
  • A fiend for life
    • #5
    • 19th Sep 08, 1:26 AM
    • #5
    • 19th Sep 08, 1:26 AM
    Interesting thread this is one I am working on at the moment so I had to do a search on it.

    The article below is from some green architects and might be of help.

    Search for this phrase: 'which would mean pelmets' in this link (read the whole paragraph the whole article is pretty informative though):

    http://www.eco-renovation.org/articles/view.php?viewid=266&W21ID=157

    According to the article, what you need is a closed pocket of stationary air space between the curtains and windows. Also if you have cavity wall insulation it seems that radiator reflectors may well be a waste of money. Shelves are also apparently not really effective (as mentioned above).

    A key question first of all might be whether the curtains are a good fit (eg does it feel colder between the curtains and window once the room has warmed up)

    Don't take my word on it though it seems to make sense to me but I am no expert.

    You might also get a better response on the DIY board.
    Last edited by A fiend for life; 19-09-2008 at 1:39 AM.
  • grannybiker
    • #6
    • 19th Sep 08, 7:51 AM
    • #6
    • 19th Sep 08, 7:51 AM
    Most of my radiators are below windows, but I thought, (hopefully,) I'd overcome the problems as I always tuck the curtains between the radiator and the wall, or, if they're too short, tuck them up onto the window sills.
    Have I been wasting my efforts doing the above?
    The space between window & curtain is always much colder once the room warms up.
    (Old cottage, thick stone walls/ windowsills.)
  • A fiend for life
    • #7
    • 19th Sep 08, 9:22 AM
    • #7
    • 19th Sep 08, 9:22 AM
    Hi grannybiker,

    I have a radiator underneath my window in the living room and bedroom. In the living room I have a 2" overhang on the window sill so I have put two overlapping lengths of net curtain wire underneath the sill. When I close the curtains I pull them in front of the wire and tuck them underneath and up behind the wire. This seems to heat the room a bit quicker.

    In the bedroom I have a foot recess in the window so will fit either removable shutters (not sure about material and aesthetics though) or thicker curtains. Getting a good seal seems to be the way to go so I am going to try that and keep an eye on humidity.
  • meames
    • #8
    • 19th Sep 08, 12:47 PM
    • #8
    • 19th Sep 08, 12:47 PM
    Thanks for replies

    My radiators aren't under windows, some are on other walls, others are situated next to the window (but not below).

    Thanks

    Michelle
  • mech
    • #9
    • 19th Sep 08, 1:35 PM
    • #9
    • 19th Sep 08, 1:35 PM
    No, what happens is the warm air in the room gets cooled by the window. This cooler air falls to the floor and drags more warm air down to the window. This warm air then gets cooled and falls, dragging more warm air to the window to be cooled. It's a continuous cycle which gradually cools the room.

    If you have a radiator beneath the window, and it's on, it breaks this cycle and prevents the window cooling the air in the room.

    Putting a shelf above the radiator will stop the radiator sending heat up to the window and the cycle will start again.

    It's similar to having your fridge door open, the cold air falls out, dragging warm air from above into the fridge and making the fridge work harder to cold this air.
    Originally posted by geordie joe
    That doesn't make any sense. You can't reduce heat loss through a window by making the air in front of it warmer. Quite the reverse.

    You want to seal the window off from the warm air in the room as much as possible using the curtains. The best way would be to rest the bottom of slightly long curtains on the window sill. If there's no window sill, a radiator shelf would seem a good idea (or tucking the curtains behind the radiator, but I don't like doing that as it will reduce the performance of the radiator by interfering with convection up the back of the radiator).

    However, for radiators positioned away from windows I can't see shelves being any advantage besides being useful to put things on.
  • Ken68
    Hi Meames, a good thread

    Hi grannybiker,

    I have a radiator underneath my window in the living room and bedroom. In the living room I have a 2" overhang on the window sill so I have put two overlapping lengths of net curtain wire underneath the sill. When I close the curtains I pull them in front of the wire and tuck them underneath and up behind the wire. This seems to heat the room a bit quicker.

    In the bedroom I have a foot recess in the window so will fit either removable shutters (not sure about material and aesthetics though) or thicker curtains. Getting a good seal seems to be the way to go so I am going to try that and keep an eye on humidity.
    Originally posted by A fiend for life
    Last year I made up some removeable exterior polystyrene shutters, to those d.g. windows at the back of my house, north facing.
    Immediate savings of 20 for half a winter, and they cost 20 to make. Not heavy, take them down about 9am and put them back at dusk.
    Another upside was NO CONDENSATION on the inside of the window.
  • geordie joe
    That doesn't make any sense. You can't reduce heat loss through a window by making the air in front of it warmer. Quite the reverse..
    Originally posted by mech
    Sorry, I didn't expand my sentence enough.

    If you have a radiator beneath the window, and it's on, it breaks this cycle and prevents the window cooling the air in the room.
    By this I meant that you prevent the window cooling the air that is in the room. Instead the air rising from the radiator gets cooled. But as this air is hotter than the surrounding air, and it is rising past the window, it gets cooled less.

    Imagine you have a room where all the air is at 25 degrees, and you don't have a heat source in the room. Also imagine that the room is well insulated and is not losing heat any other way but via the window.

    The air near the window gets cooled to 21 degrees. It then falls to the floor and drags air from above it down to the window. The air is at 25 degrees, but cools to 21 degrees and falls to the floor, dragging more warm air down from the ceiling to the window.

    In time all the air that is at 25 degrees will be dragged to the window and cooled to 21 degrees. Then your room temperature would be 21 degrees.

    If you had a radiator on what would happen is this. Hot air from the radiator would rise up past the window. But as it is hotter than the air in the room, lets say 28 degrees, it rises fast and doesn't stay near the window long. Because of this it only cools to 26 degrees. This means it is still hotter than the air in the rest of the room.

    It also prevents the air that is in the rest of the room, at 25 degrees, from getting near the window and being cooled to 21 degrees. You are still losing heat through the window, but that heat is being lost from air that is hotter than the room temperature anyway. So the overall temperature of the room does not go down.

    You want to seal the window off from the warm air in the room as much as possible using the curtains. The best way would be to rest the bottom of slightly long curtains on the window sill. If there's no window sill, a radiator shelf would seem a good idea (or tucking the curtains behind the radiator, but I don't like doing that as it will reduce the performance of the radiator by interfering with convection up the back of the radiator).
    Originally posted by mech
    it won't work, that well. Unless your curtains are made of plastic the cold air will still fall through them, and warm air will be sucked down the gap between the top of the curtain and the wall.
  • A fiend for life
    I quite agree it doesn't make sense. But I think what Geordie_Joe is saying is that with a shelf in place the rising warm air current is further away from the window leaving a larger volume of colder air column against the window. Thus as the warm column of air rises it flows 'downhill' to the cooler air ie towards the window: proportionately more energy energy is transferred towards the window than upwards and into the room. This effect would be compounded on a windy day, or with single glazing.

    So it seems ideally the curtains should be slightly inset from the edge and just touching the window ledge and with a clear air space above the radiator so warm air can ascend unhindered and cycle up to the ceiling and in to the room.

    More importantly it also suggests that if you're going to put the heating on, even during daylight hours, the curtains should be closed. I think I've got that anyway.
  • economiser
    A shelf over a radiator will reduce its output by about 10% due to reduced air circulation. This does not mean a reduction in efficiency just that it will be less effective at heating the space. As most radiators are oversized you probably will not notice the difference except in the coldest weather. Putting furniture in front will have a similar effect and radiator covers probably reduce output by about 20%.
  • mech
    Sorry, I didn't expand my sentence enough.

    By this I meant that you prevent the window cooling the air that is in the room. Instead the air rising from the radiator gets cooled. But as this air is hotter than the surrounding air, and it is rising past the window, it gets cooled less.
    Originally posted by geordie joe
    I'm fully aware that radiators are traditionally positioned under windows to avoid cold draughts, but it has no bearing on heat loss or energy efficiency. I remain to be convinced that the distance of this column of air from the window makes any difference at all. Cold air will always pour off the window, that's just basic convection. And with a radiator there it will always be heated before it pools at the bottom of the room and encounters an occupant. The trick is to minimise heat loss from the window in cold weather in the first place, by screening it off from the room with heavy curtains.

    Imagine you have a room where all the air is at 25 degrees, and you don't have a heat source in the room. Also imagine that the room is well insulated and is not losing heat any other way but via the window.

    The air near the window gets cooled to 21 degrees. It then falls to the floor and drags air from above it down to the window. The air is at 25 degrees, but cools to 21 degrees and falls to the floor, dragging more warm air down from the ceiling to the window.

    In time all the air that is at 25 degrees will be dragged to the window and cooled to 21 degrees. Then your room temperature would be 21 degrees.
    If it was 21 degrees outside I wouldn't have my heating on at all. :-)

    If you had a radiator on what would happen is this. Hot air from the radiator would rise up past the window. But as it is hotter than the air in the room, lets say 28 degrees, it rises fast and doesn't stay near the window long. Because of this it only cools to 26 degrees. This means it is still hotter than the air in the rest of the room.

    It also prevents the air that is in the rest of the room, at 25 degrees, from getting near the window and being cooled to 21 degrees. You are still losing heat through the window, but that heat is being lost from air that is hotter than the room temperature anyway. So the overall temperature of the room does not go down.
    So basically you're saying that without a radiator the room would cool down. So having a shelf over a radiator is the same as no radiator?

    it won't work, that well. Unless your curtains are made of plastic the cold air will still fall through them, and warm air will be sucked down the gap between the top of the curtain and the wall.
    Not really. Unless they are net curtains or ludicrously thin. Cold air will pool behind the curtains. You can feel it. The curtains don't have to be airtight for most of the potential benefit to be had. Think about it: Which is warmer? A plastic mac or a wool coat?
  • geordie joe
    I'm fully aware that radiators are traditionally positioned under windows to avoid cold draughts, but it has no bearing on heat loss or energy efficiency.
    Originally posted by mech
    We are not talking about heat loss, we are talking about where the heat is lost from.

    Without the radiator on under the window the hot air from the middle of the room is dragged to the window and cooled down, and eventually make it's way back to the middle of the room.

    With the radiator beneath the window, and on, it's the hot air that comes off the radiator that gets cooled. And because it starts off hotter than the air in the middle of the room it doesn't get so cold.

    I remain to be convinced that the distance of this column of air from the window makes any difference at all. Cold air will always pour off the window, that's just basic convection.
    Originally posted by mech
    Yes but if you have warm air rising close enough to the window it will prevent this. Putting a shelf over the radiator takes this stream away from the window.

    And with a radiator there it will always be heated before it pools at the bottom of the room and encounters an occupant.
    Originally posted by mech
    No because it is falling past the radiator. You can test this for yourself, just place a thermometer on the floor underneath your radiator, when it's on, then place it above the radiator and see the difference.

    The trick is to minimise heat loss from the window in cold weather in the first place, by screening it off from the room with heavy curtains.
    Originally posted by mech
    The trick is not to confuse AIR with HEAT. We are not talking about heat as such, but the movement of air around the room. Specifically, the movement of air that is warm and in the middle of the room where it is useful, to it moving to the window and cooling down.

    If it was 21 degrees outside I wouldn't have my heating on at all. :-)
    Originally posted by mech
    Oh yes you would! If you knew me you would know I was born in the late 50's and my year at school was the last to be taught the old systems. I was never taught the metric system, nor anything about Centigrade, except how to convert Fahrenheit: into Centigrade but told I would never need it as we don't do Centigrade in this country.

    Then I left school and the government decided we all had to start using funny money and funny rulers etc.

    When I talk degrees I talk Fahrenheit:, so if your room was 21 degrees you would have more than your heating on!

    [quote=mech;14324587]So basically you're saying that without a radiator the room would cool down. So having a shelf over a radiator is the same as no radiator?

    Not quite. What I am saying is that without a radiator all the air in the room would (eventually) be dragged over to the window and cooled down. Therefore the room would cool down.

    But with a radiator the only air the window would cool down is the hot air rising from the radiator. This air starts off hotter than the rest of the air in the room, and doesn't cool down as much, so the overall temperature of the air in the room doesn't go down.

    Not really. Unless they are net curtains or ludicrously thin. Cold air will pool behind the curtains.
    Originally posted by mech
    To some extent yes, but the window will continue to cool the air near the top of the window, and this air will continue to fall. This air must find a way out, if it can't then the pressure would build up and it would eventually blow the curtains out of the way.
    You can't have cold air continually falling onto your windowsill and not finding a way out. Even if it find a gap 100 inches above the sill, it will go through there. Even if there are no gaps it will simple seep through the curtain material.

    You can feel it. The curtains don't have to be airtight for most of the potential benefit to be had. Think about it: Which is warmer? A plastic mac or a wool coat?
    Originally posted by mech
    Sorry, but again you are mistaking air movement with heat loss. I'm not disputing that heat is lost through a window, just pointing out where that heat can come from.

    Without a radiator warm air from the middle of the room is dragged to the window, cooled and then pushed back into the room, making the room colder.

    With a radiator the air that gets cooled by the window is air that is hotter than the air in the middle of the room, and because it is rising past the window fairly fast it doesn't get cooler than the air in the rest of the room. Therefore the temperature of the room doesn't go down.

    It's not the heat loss that counts, it's the circulation of the air and where the lost heat comes from that counts.
  • A fiend for life
    Thanks for replies

    My radiators aren't under windows, some are on other walls, others are situated next to the window (but not below).

    Thanks

    Michelle
    Originally posted by meames
    Generally it seems shelves can reduce radiator effectiveness depending on their size and clearance from the radiator. But they are unlikely to improve heating if that is what you are looking for. If you have a well insulated room: double glazing cavity wall insulation etc. it might not be so much an issue. If you do go with it I would ensure you have sufficient clearance for the air to circulate up and around the shelf.

    This one seems a reasonable site:

    http://www.diyfixit.co.uk/diy/centralheating/position_radiator/position_radiator.html

    The DIY board might be worth a try too.
  • mech
    We are not talking about heat loss, we are talking about where the heat is lost from.

    Without the radiator on under the window the hot air from the middle of the room is dragged to the window and cooled down, and eventually make it's way back to the middle of the room.
    Originally posted by geordie joe
    What made you think anyone was advocating removing the radiator? Of course the room will cool down without any heating!

    Yes but if you have warm air rising close enough to the window it will prevent this. Putting a shelf over the radiator takes this stream away from the window.
    The stream is already away from the window. Most windows have windowsills ranging anywhere from 8 inches to 18 inches deep. If it doesn't have a windowsill then you definitely want a radiator shelf to stop all the hot air going between the window and the curtains.

    There shouldn't be much reduction in radiator effectiveness if you can mount a shelf 3 or 4 inches above the top of the radiator.

    No because it is falling past the radiator. You can test this for yourself, just place a thermometer on the floor underneath your radiator, when it's on, then place it above the radiator and see the difference.
    I thought your argument hinged on the idea that air next to a window over a radiator can only go up? You seem confused. The air above a radiator would be warmer than the air below it even if the radiator was in the middle of the room and there were no windows, curtains or radiator shelves.

    Oh yes you would!

    (snip)

    When I talk degrees I talk Fahrenheit:, so if your room was 21 degrees you would have more than your heating on!
    Your example makes even less sense in Fahrenheit. 21 degrees is well below freezing. Your radiator is now inoperative as the pipes froze and burst while you were away during the winter.

    So basically you're saying that without a radiator the room would cool down. So having a shelf over a radiator is the same as no radiator?
    Originally posted by mech
    Not quite. What I am saying is that without a radiator all the air in the room would (eventually) be dragged over to the window and cooled down. Therefore the room would cool down.

    But with a radiator the only air the window would cool down is the hot air rising from the radiator. This air starts off hotter than the rest of the air in the room, and doesn't cool down as much, so the overall temperature of the air in the room doesn't go down.
    Yes, a radiator is a heating device. What has this got to do with radiator shelves?

    To some extent yes, but the window will continue to cool the air near the top of the window, and this air will continue to fall. This air must find a way out, if it can't then the pressure would build up and it would eventually blow the curtains out of the way.
    Don't be silly. It doesn't have to be completely airtight. Even if it was, the difference in air density is tiny. Just enough for air to rise and fall. It can't blow heavy curtains off a flat surface.

    Without a radiator warm air from the middle of the room is dragged to the window, cooled and then pushed back into the room, making the room colder.
    Again, who advocated having no radiator?
  • Dave2112
    Good thread as I was thinking about shelves myself.

    I've got double glazing, mid weight curtains and thermal linings. I make sure that the curtains are tucked up on the window sill when the radiators are on and the air between the curtain/window is noticably colder than the air in the room. Can't get a reading as the heating isn't on yet!!! I don't think that shelves would make a difference in this scenario. The only time they might make a difference is if they stop the warm air going behind the curtains - the curtain would have to sit on top of the shelf for this to happen. On non window radiators they would provide no benefits.

    Re silver foil which was mentioned. I had some left over from a previous house so have just fitted it behind radiators on external walls. Internal ones are free to be heated up as a thermal mass. Now I have cavity wall insulation and one of the links said that the foil wasn't usefull on these. I can only guess it's because the external walls can act as a better thermal mass as they're insulated from the outside.
  • SAMMY350
    Hi,

    For anyone looking for radiator shelves, I bought mine from www.saveonheat.co.uk and Im quite happy with it, fits in with my decor, the price was quite reasonable as well. Worth the 2 weeks wait. Recommended.
  • ged1980
    Hi,

    For anyone looking for radiator shelves, I bought mine from www.saveonheat.co.uk and Im quite happy with it, fits in with my decor, the price was quite reasonable as well. Worth the 2 weeks wait. Recommended.
    Originally posted by SAMMY350

    :rolleyes:


    good site they doing christmas offers early or not being updated not a good sign
    If you dont like me remember its mind over matter, I dont mind and you dont matter

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