Radiator under window sill

[JohnW2]

Hi folks,
This question has been asked before in one way or another, but I'd be grateful for current views...

I'm trying to make decisions about placement of CH radiators in a house that currently has none. A couple of the rooms have internal sills (or is it cills? ) that protrude about 100mm inboard of the wall It's a bit messy at the moment, but pic below gives an idea.

The curtain will be above the sill (as is the net curtain in piccie), so will not get in the way of the radiator. Is it adequate to mount a radiator on the wall, in the usual way, with a gap between the top of radiator and the sill (I've seen gaps of both 50mm and 100mm recommended) or would it be desirable/better to move the radiator out further from the wall?

Thanks for your advice,
Kind Regards, John

 

[JohnW2]

I'm trying to make decisions about placement of CH radiators in a house that currently has none. A couple of the rooms have internal sills (or is it cills? ) that protrude about 100mm inboard of the wall It's a bit messy at the moment, but pic below gives an idea.

I forgot to post the piccie. Here it is ...

 

[ericmark]

You raise a very good question, the pictures seen in books, so the circulation was biased around heat loss at windows, but today our windows are double glazed, and the losses are much reduced, however the same point exists, why heat a window?

Mothers house the radiator was 90º to the window wall, on an internal wall, and I noted the huge difference in temperature around the room. At the time it was causing me a problem, due to position of the thermostat, and sun through the bay windows, in winter near to bay window I could record 32ºC in the sun, but opposite wall 18ºC and I had to consider where in the room we sat, was looking for 20 to 22ºC where we were seated.

The main problem was the large radiator would get hot, sun would come out, and although the TRV had closed, there was too much energy stored in the radiator, the cure was to use electronic TRV heads, and allow the boiler to modulate, so instead of the radiator heating to 70ºC then cooling again, it would remain at around 30ºC until the sun came out, so it could turn off and cool down quicker, still got an over shoot in the sun, but much reduced, instead of the 32ºC was getting 26ºC as the TRV was now faster acting. And centre of room where we sat, was now at 23ºC.

The whole problem started due to the caravan, pre-caravan there were apple trees shading the sun when low in the sky, these were removed to allow the caravan to be moved around, as my sister got a larger caravan, and of course apple trees loose their leaves in winter, seems odd but the bedroom above the living room, did not have the same problem, smaller window, and radiator under the window.

This house hills mask sun until 11 am. So once we get the sun, it is higher in the sky, so it does not heat the house as much, but still the patio doors and windows do cause a raise when the sun comes out, so again using electronic TRV heads as they act faster.

 

[denso13]

The curtain will be above the sill (as is the net curtain in piccie), so will not get in the way of the radiator. Is it adequate to mount a radiator on the wall, in the usual way, with a gap between the top of radiator and the sill (I've seen gaps of both 50mm and 100mm recommended) or would it be desirable/better to move the radiator out further from the wall?

As far as is reasonable below the cill, 100mm is fine.

 

[oldbuffer]

1. Sill or Cill - either spelling.
2. The important gap is at the bottom, try for a minimum of 150mm off the floor, and avoid going below 100mm.
3. Gap below sill is less important, but go for more rather than less.
4. Spacing the radiator off the wall to clear the sill would be a waste of space, and might also look rather odd.
5. Radiators are badly named. Standard ones work by creating convection currents of hot air, rather than through radiated heat. Figures for convection to radiation ratio vary but if you work on 80 : 20 (Convection to radiation) you won't be far out. The convected hot air rises to meet the ceiling, then "rolls" across the ceiling and down the wall on far side of the room. Putting a slight obstruction like a narrow sill just above the radiator means the current will just go round it in its ascent.
6. In your case, where you don't have much height to work with, I'd:
6.1 Do the heat loss calculation for the room to determine the heating requirements.
6.2 Measure the vertical distance between floor and underside of sill. Then deduct 250 mm from this to get the optimum height of the radiator.
6.3 Measure the width of the bay, and deduct 150 mm to allow for valves.
6.4 Determine (manufacturer's / supplier's web sites) if there is a standard radiator size which will give you the heat output required. Probably looking at type 22 radiators (double with 2 lots of fins). If not, go for lower and narrower until you get close to what you want. If a long way adrift, consider adding smaller radiators in the bay returns to increase heat output. These can be joined to the central one to avoid extra pipes through the floor, but this is neither a trivial planning exercise nor a trivial plumbing connection.

 

[JohnW2]

As far as is reasonable below the cill, 100mm is fine.

Thanks. There's about 700mm available below the sill, so with a 100mm gap above, a 500mm high radiator should be OK. If my calcs are correct, a 500 x 1,400 mm radiator (which would fit) should be adequate for the room I depicted.

Although it would obviously not be ideal, it would be easy enough to move the radiator a bit further from the wall (i.e. less, if at all, beneath the sill), but I presume your answer means that such would not really be necessary?

Kind Regards, John

 

[JohnW2]

Many thanks for your interest and your detailed reply!

2. The important gap is at the bottom, try for a minimum of 150mm off the floor, and avoid going below 100mm.
3. Gap below sill is less important, but go for more rather than less.

Fair enough.

4. Spacing the radiator off the wall to clear the sill would be a waste of space, and might also look rather odd.

As I've just written, that was my thought.

6. In your case, where you don't have much height to work with, I'd:
6.1 Do the heat loss calculation for the room to determine the heating requirements.
6.2 Measure the vertical distance between floor and underside of sill. Then deduct 250 mm from this to get the optimum height of the radiator.
6.3 Measure the width of the bay, and deduct 150 mm to allow for valves.
6.4 Determine (manufacturer's / supplier's web sites) if there is a standard radiator size which will give you the heat output required.

Per my recent post, I've essentially do all that, although I deducted 200mm, rather than 250mm to get a figure for maximum height of radiator.

If I allow more than 200mm (in my case it would be about 300mm) that would move me to a 400mm high type 22. At 1,400mm wide, that would (per my calcs) be just about adequate, but not with as comfortable a margin as a 500mm high one. That leads to two questions:

1... Given the 700 mm available below the sill, which would be better - 500mm high with less gaps or 400mm with marginal heat output?

2... If I went with 400mm tall radiators, how would you suggest that the 300 mm toital gap be split between top and bottom.

There is another option, although a bit odd and certainly more expensive - if I used a 400mm wide one, I could also additionally have two 400 x 400 ones along the 'side walls of the bay.

Kind Regards, John

 

[Agile]

My advice would be to go for 500 mm high rad.

You can oversize a rad with no problems but cannot undersize.

I would suggest about 200 watts per square meter if you have solid walls and two external walls. Or 150 watts psm if you have only one external wall.

If you are struggling for heat output then you could fit three rads in series with rad valves only on the two inner connections. Doing that would enable a longer center rad.

 

[JohnW2]

My advice would be to go for 500 mm high rad. ... You can oversize a rad with no problems but cannot undersize.

Thanks for your interest. That would certainly be my inclination but it obviously would reduce the gaps above and/or below the rad. Given the ~200mm I would have to play with if I used a 500mm rad, what would you suggest - about 100mm both above and below the rad?

I would suggest about 200 watts per square meter if you have solid walls and two external walls. Or 150 watts psm if you have only one external wall.

That room is about 3m x 3.5m, with just one external wall, but with the large single-glazed window you've seen. At 200W per m² that would be about 2,100W but my calculations had come up with an answer around 1,800W (i.e. about 170 W/m²).

If you are struggling for heat output then you could fit three rads in series with rad valves only on the two inner connections. Doing that would enable a longer center rad.

As I wrote, there is room for two 400 x 400 mm ones at the sides', but that would be a little odd, and certainly significantly more expensive. I suppose that can be left as a Plan B to keep 'up my sleeve' in case the radiator initially fitted proves to be inadequate.

Kind Regards, John

 

[Agile]

100mm above and 100 mm below should be fine.

I thought you had said the window was double glazed and it looked as if it was in your photo..

The big advantage of the three rad solution is that you can use a longer centre rad if you do not need rad valves on it.

If the window is not double glazed and you have no plans to add secondary glazing then it is quite important to ensure enough heat output.

What are the rad outputs you would get with a single rad and with the three rads?

 

[JohnW2]

100mm above and 100 mm below should be fine.

Thanks. That seems consistent with what everyone has said, so that's probably what I'll go with.

I thought you had said the window was double glazed and it looked as if it was in your photo.. ...

No, currently single-glazed. However, not in brilliant condition (probably 'original', hence around 130 years old!), so may well be replaced (or, as you go on to mention, have some secondary glazing added).

The big advantage of the three rad solution is that you can use a longer centre rad if you do not need rad valves on it. .... If the window is not double glazed and you have no plans to add secondary glazing then it is quite important to ensure enough heat output.

I've been doing a bit of experimenting with fan heaters. Admittedly, the current ambient temp is not that low (about 11-12 °C) but a 2kW fan hear is a lot more than enough to heat the room - and that's with none of the adjacent rooms heated.

More to the point, I'm not sure why I was talking about 1,400 mm wide (I blame 'old age'!). When I thought,it didn't sound right, and having just re-measured, it looks as if an 1,800 mm one (with valves) would probably fit. A 1,800 x 500 type 22 alone ought to be around 2,700 W, so appreciably more than your 200 W/m².

Kind Regards, John

 

[pcaouolte]

A significant consideration for me is your seating position, I would always want the radiator to be in front of me.

If you look at Eric's diagram of heat circulation it exactly represents what happens in our lounge, the radiator is behind our seating position, although we do not have a sofa tight up to the radiator. The heat goes up from the radiator, across the ceiling, down the far wall and then cool air at low level flows back towards the radiator. That cool air hits our legs when sitting on the sofa and feels like a cold draft even though there is no draft. When the radiator goes off because the room is up to temperature the cold draft ceases.

 

[JohnW2]

A significant consideration for me is your seating position, I would always want the radiator to be in front of me. ... If you look at Eric's diagram of heat circulation it exactly represents what happens in our lounge, the radiator is behind our seating position, although we do not have a sofa tight up to the radiator. The heat goes up from the radiator, across the ceiling, down the far wall and then cool air at low level flows back towards the radiator. That cool air hits our legs when sitting on the sofa and feels like a cold draft even though there is no draft. When the radiator goes off because the room is up to temperature the cold draft ceases.

That all makes some sense. I was brought up with the idea that a rad should be positioned on the coldest wall - which, even with double/triple glazing, is usually the one with a large window.

Whilst, as I said, what you say makes sense, what I don't really see is why the situation would be significantly different if the radiator were in front of you. Either way, coolish-air will be travelling at low level from the wall opposite the radiator to the wall with the radiator. The only major difference I can see is that if the radiator is in front of you, the 'cold draft' you describe will be travelling under (or around) the sofa and hitting the back, rather than the front, of your legs?

 

[pcaouolte]

the 'cold draft' you describe will be travelling under (or around) the sofa and hitting the back, rather than the front, of your legs?

With a radiator in front of the sofa the cold draft hits the back of the sofa and I cant feel it. The draft doesn't hit the back of my legs because they are protected by the sofa. The small amount of radiated heat from a radiator in front of me also helps to keep my legs warm.

With the radiator behind the sofa the cold draft hits the front of the sofa and makes the front of my legs feel cold. I spent hours searching for the source of the draft until I noticed that it is only present when the radiator is hot.

 

[JohnW2]

With a radiator in front of the sofa the cold draft hits the back of the sofa and I cant feel it. The draft doesn't hit the back of my legs because they are protected by the sofa. The small amount of radiated heat from a radiator in front of me also helps to keep my legs warm. .... With the radiator behind the sofa the cold draft hits the front of the sofa and makes the front of my legs feel cold. I spent hours searching for the source of the draft until I noticed that it is only present when the radiator is hot.

Fair enough. An interesting observation and, I must say, something I've never thought about before. For what it's worth , the two rooms in which I most often sit have their rads behind the sofa I sit on (and, thinking back, the same was true in my previous couple of houses) but I've never actually been aware of such a phenomenon.

If one does experience the issue you describe, and perceives it as a problem, I imagine that it could probably be addressed with a ceiling fan or suchlike?

I suppose one downside of having the rad in front of one's sofa is aesthetic - I'm not sure I would want to be staring at a radiator! I would be much more likely to be looking at a TV, and above a radiator is not really the ideal place for that.