On Thursday, 6 May 2021 at 13:10:20 UTC+1, 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.
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.

Likely true. But, in many cases, so what?

Let's assume a horizontal radiator is 100% efficient.

But how much of the time does that efficiency matter? I suspect a large proportion of radiators are throttled by a TRV. Further, if the radiator can cope on the coldest days, it probably has plenty of capacity in hand for most of the rest of the year.

Dropping efficiency to, I suggest, 90% would only really affect that small period during which the absolute maximum output is required.