On Tuesday, 16 June 2020 20:56:19 UTC+1, Robin wrote:
On 16/06/2020 17:59, tabbypurr wrote:
On Tuesday, 16 June 2020 17:51:25 UTC+1, Robin wrote:
On 16/06/2020 17:41, tabbypurr wrote:
On Tuesday, 16 June 2020 09:28:35 UTC+1, Robin wrote:
On 16/06/2020 08:55, PeterC wrote:
On Mon, 15 Jun 2020 22:30:39 +0100, Robin wrote:
On 15/06/2020 21:55, tabbypurr wrote:


A rad is 100% efficient because there is nowhere for any waste energy to go. Thus energy input & energy output are actually the same thing for a rad.


That's conservation of energy.

Efficiency involves the concept of usefulness - as in the traditional
definition "the ratio of the useful work performed by a machine to the
total energy expended". On that it's entirely reasonable to consider
how a radiator in a CH system affects the ratio of the energy released
where it's wanted to the total energy expended (i.e. including all the
energy released where it's not - e.g. in the boiler, pump, and pipes
under suspended floors).

Looking at it another way, I'd consider a radiator which took water in
at 70 degrees and passed it out at 65 degrees to be less efficient than
one which passes it out at 55 degrees. The second done is doing a much
better job at putting the energy where it's wanted.

Is that efficiency or is it effectivness?


It's a fair cop that I conflated the 2. My mitigation is that I was
trying to stop the post growing even longer. Slightly longer 2nd attempt:

The second radiator is more effective (assuming it's a good thing to
have at least the option of warming the room faster)

probably, though I don't know what 'second one' you refer to

and (very probably)
also more efficient

no.

in that the CH system delivers heat to the room more
efficiently.

no. You are still not grasping what radiator efficiency is.


Please state your definition.

NT: useful energy out over energy in is the definition.

With a wet radiator there are only 2 places energy can go: into the room or out the outlet pipe. In neither case does the rad lose any energy, hence efficiency is 100%.


The inclusion of "useful" was good. But you then :

a. ignore the fact that not all the energy leaving through the output
pipe is "useful" if the rest of the system has losses (i.e. heat lost
where it's not wanted); and

It's not being ignored, we know there are losses in the boiler & in a minority of cases the pipework running where heat is not wanted. Those inefficiencies occur in those devices, not in the radiator.

It's also not news that often one component's operation affects efficiency elsewhere in a system.


b. assume that all energy released into the room is equally useful when
in many cases it's not because the idea is to make people in the room
/feel/ comfortable.

Measuring (b) is complicated and I don't pretend to understand all the
components. But a simple example is that a pure convector in a room
with a high, cold ceiling is not the best source. And I don't need to
as clever people have written BS 7726 (Ergonomics of the thermal
environment €” Instruments for measuring physical quantities) which
defines an "operative temperature". So my definitions of the efficiency
of a radiator would focus on how much it increases the operative
temperature for (a) a given (net) energy released by the radiator and
(b) the corresponding (gross) energy the system has to provide.

I stumbled across this subject some years ago when looking for evidence
about the claims for tall radiators - which do seem to have benefits for
a given area.

It's sometimes relevant, in a huge domed hall, heat in the dome could be regarded as waste. But in your average domestic semi it's not normally an issue and does not affect system efficiency. It never affects radiator efficiency.


NT