On 06/05/2021 13:46, jon wrote:
On Thu, 06 May 2021 13:10:17 +0100, Fredxx wrote:

On 06/05/2021 12:55, jon wrote:

What is this nonsense, heat is dissipated by radiation, convection or
conduction so how come efficiency becomes a problem.

The physical geometry of the "radiator" affects how well it transfers
heat to its environment. Convection is the most important by far.

Shiny metallic "radiators" like chrome towel rails in the bathroom
hardly radiate any heat at all and once you put a towel onto them they
don't do very much to warm up the air in the bathroom either.

For conventional central heating radiators most heat-loss is through
convection. Air is a poor conductor.

It seems there is less overall air flow for a tall radiator than long
for a radiator with the same area and construction.

So hardly nonsense.

So where does the heat loss go other than in the room..?

Hotter water leaves the radiator exit port than would be the case for a
radiator with a more appropriate geometry.

The critical thing is how much area is at each temperature and the
airflow over it. The amount of heat that the "radiator" which actually
loses most of its heat to the room by convection can transfer to the
air. There is an optimum height, width and wall separation for maximum
heat transfer. Insulating the wall behind a radiator improves it a bit
if the wall is an outside one. You can see any radiators mounted on
outside walls with thermal imaging. It matters less on an interior wall.

If you choose three radiators of 1m^2 with very different geometries.

Height x Width
0.5 x 2
1 x 1
2 x 0.5

The first will work best of all since it has a stripe of 2m length that
is at inlet temperature and releases warm air low down in the room.

The second is a bit more practical and would still work well enough.
Most are typically around a 1:2 ratio of height to length.

The last one has a much smaller stripe at the highest temperature and
mostly delivers hot air to the ceiling (where it stays). They are a
designer fad that serves no useful purpose (except to sell more
replacement radiators when they prove to be unsatisfactory).

If you happen to live on the ceiling then they might be useful to you.

--
Regards,
Martin Brown