Radiator under window sill

[ChrisJP]

Rads are traditionally sited under a window because glazed areas will usually have significantly greater heat loss than walls and the cold air against the glass will create a downdraught that can then be counteracted by the rad and the resultant turbulent mixing will lessen the convection current effects already discussed in this thread.

The other reason for putting radiators under windows is that it avoids putting radiators on walls that otherwise can have other furniture on them - like bookcases, side units, sideboards etc. Having radiators on any blank wall effectively prevents it being used for anything else.

 

[JohnW2]

The other reason for putting radiators under windows is that it avoids putting radiators on walls that otherwise can have other furniture on them - like bookcases, side units, sideboards etc. Having radiators on any blank wall effectively prevents it being used for anything else.

Indeed so.

Do you have any thoughts about the question I've recently posed?

 

[Jurassicspark]

Do you think that the winner is the swing or the roundabout?

Hi John,

it’s a trade-off between more heat and potentially reduced convection vs better efficiency with less heat. If you feel the room would benefit from that extra 20%, the 600 mm one could be the "swing" winner. However if the slightly less heat is manageable and you value the better airflow and space around the radiator, the 500 mm would be the "roundabout" option.

 

[JohnW2]

Hi John,

it’s a trade-off between more heat and potentially reduced convection vs better efficiency with less heat. If you feel the room would benefit from that extra 20%, the 600 mm one could be the "swing" winner. However if the slightly less heat is manageable and you value the better airflow and space around the radiator, the 500 mm would be the "roundabout" option.

Thanks.

The problem is that, in this situation, "20% extra radiator" does not necessarily mean "20% more heating capacity" (or necessarily any more heating capacity. My aim would be to maximise the amount of potential heating (primarily convection) - so the ('trade off') question is whether the 600 rad one would, in practice, actually result is more (or less, or the same) potentially available heating, given that the 'extra 20% of radiator' would be somewhat countered by the gaps above and below it being only about 50 mm (as compared with 100 mm with the 500 mm rad).

 

[Mister Banks]

Thanks.

The problem is that, in this situation, "20% extra radiator" does not necessarily mean "20% more heating capacity" (or necessarily any more heating capacity. My aim would be to maximise the amount of potential heating (primarily convection) - so the ('trade off') question is whether the 600 rad one would, in practice, actually result is more (or less, or the same) potentially available heating, given that the 'extra 20% of radiator' would be somewhat countered by the gaps above and below it being only about 50 mm (as compared with 100 mm with the 500 mm rad).

Radiators should be installed with 150mm between bottom of radiator and the floor in order to maximise the convection capability.

 

[JohnW2]

Radiators should be installed with 150mm between bottom of radiator and the floor in order to maximise the convection capability.

Yes, we know about the 'ideal', but we are discussing 'the least of the evils' of what options are available.

To have 150 mm below my rad and anything more than 50 mm above it (under the deep sill) would limit me to a 400 mm high rad, but it's the 'lesser of the evils' of a 500 mm and 600 mm one that Ii am asking about.

Most other people seem to have suggested that 100mm above and below a rad is adequate.

Do you believe that a 400 mm high rad with 150 mm below and above it would result in more convection heating that a 500mm one with 100mm above and below and/ or a 600 mm high one with only 50 mm above and below?

 

[ChrisJP]

It seems to me that, as long as the bottom of the radiator is not skimming the floor, they'll be no significant restriction on convected airflow. Air molecules are small and get through very small gaps - otherwise we wouldn't have drafts

 

[Harry Bloomfield]

To have 150 mm below my rad and anything more than 50 mm above it (under the deep sill) would limit me to a 400 mm high rad, but it's the 'lesser of the evils' of a 500 mm and 600 mm one that Ii am asking about.

I would suggest, that the 50mm above, would be the pinch point for air flow. I would attempt to improve it by making it 75mm at the top, 125mm at the top. If it proves inadequate due to inadequate air flow, there would be the ugly option, of adding a curved strip of something, along the top of the rad, to help prevent air getting trapped above the radiator, to help funnel it out.

 

[JohnW2]

It seems to me that, as long as the bottom of the radiator is not skimming the floor, they'll be no significant restriction on convected airflow. Air molecules are small and get through very small gaps - otherwise we wouldn't have drafts

That's true, but it's more the gap at the top (between rad and deep sill) that concerns me a bit, so that, if things are tight, it is probably best to have a greater gap at the top than the bottom - or perhaps, as Harry has suggested, install a 'curved something' at the top (under the sill) to aid convection currents - and I think the latter could be almost invisible, so not particularly 'ugly.

 

[JohnW2]

I would suggest, that the 50mm above, would be the pinch point for air flow. I would attempt to improve it by making it 75mm at the top, 125mm at the top. If it proves inadequate due to inadequate air flow,

Do ytou mean 75 mm t top and 125 mm at bottom? If so, as I've just suggested, I would be inclined to go with the opposite of tht, since I think it's the gap at the top which is more crucial to optimal convection.

there would be the ugly option, of adding a curved strip of something, along the top of the rad, to help prevent air getting trapped above the radiator, to help funnel it out.

Yes, I thought of that possibility and,. as I've just written, think that it could be almost invisible, hence not particularly ugly.

However, you seem to be assuming a 500mm high radiator, whereas my question was whether I should be consdiering a 600 mm one. That would mean 50 mm gaps at top and bottom (or maybe even 75 mm top and 25 mm bottom). Whilst that would presumably appreciably impair convection, I don't know to what extent that would be trumped by there being "20% more radiator".Any thoughts?

Kind Regards, John

 

[Harry Bloomfield]

However, you seem to be assuming a 500mm high radiator, whereas my question was whether I should be consdiering a 600 mm one. That would mean 50 mm gaps at top and bottom (or maybe even 75 mm top and 25 mm bottom). Whilst that would presumably appreciably impair convection, I don't know to what extent that would be trumped by there being "20% more radiator".Any thoughts?

Yes, sorry, I misread it. With such a small space to play with, I would make the gap even, top and bottom - plus there would be even more need for the curved 'what-ever'

 

[JohnW2]

Yes, sorry, I misread it. With such a small space to play with, I would make the gap even, top and bottom - plus there would be even more need for the curved 'what-ever'

All agreed. However, it leaves me having to decide what size radiator to go with.

My heat loss calculations suggest that, even after adding 25% 'for good luck', I need only about 1,700 W. On the basis of dT50 (which I think is what I may well have to stick with!) a 600 x 1,800 one should provide about 3,100 W, a 500 x 1800 mm one (with 100mm gaps top and bottom) should provide about 2,700 W and even a 400 x 1,800 one (150mm gaps top and bottom) should provide about 2,300 W. Hence, even if I moved to a somewhat 'nicer' dT than dT50, it might not be worth even considering the 600 mm one (with its inevitably small 'gaps')?

 

[Harry Bloomfield]

All agreed. However, it leaves me having to decide what size radiator to go with.

A too small radiator, could be improved, by improving the air-flow. Either a fan heater set to just blow cold, or for a more permanent set up - a series of CPU fans, via a small PSU.

 

[Mister Banks]

How would it look if you screwed a couple of wooden battens to the wall for the brackets to attach to, would that be sufficient for the top of the radiator to then clear the sill?

 

[JohnW2]

A too small radiator, could be improved, by improving the air-flow. Either a fan heater set to just blow cold, or for a more permanent set up - a series of CPU fans, via a small PSU.

As I indicated, having a "too small radiator" is the last thing I'm contemplating. Assuming dt50, even a 500mm one would have about double the calculated required heat output (excluding by 25% 'overage').