On 16/02/2013 07:16, harry wrote:

The factors governing heat exchange are.

Time (spent in the heat exchanger),
Turbulence,
Temperature (difference)

You're forgetting probably the most important one - the effective
surface area of the heat exchanger. If the rad has a big cold patch,
then its effective surface area for heat exchange is obviously much
smaller than it should be.

The more you get of any of them the greater amount of the available
amount of energy is transferred per unit of water.


So in fact if the water spends longer in the radiator, it will come
out cooler.

However the total heat emitted will be greater as the flow increases.

If the outlet is at the top, then increasing the flow will increase the
output power slightly, but then you'll end up with a badly unbalanced
system. The rad in question will be a huge flow-hog and the boiler won't
be able to achieve its rated output because of all the still-hot water
coming back on the return.

(The temperature difference will be less between inlet and outlet so
the average surface temperature will be greater.)

The average surface temperature isn't the average of the inlet and
outlet temperatures. With the outlet at the top, it'll be well below
both of them.

Cheers,

Colin.