On 16/02/2013 15:28, Onetap wrote:

I still don't agree with that. Please note that I'm not saying it won't happen, I'm saying it wouldn't anticipate it happening.

The convection just generates a pressure difference due to the different masses of the H&C water columns.

With BBOE and no convection (air, rad, water at the same
temperature) water will still flow up the first channels and
down the last, the flow rate being such that the frictional pressure
loss equals the pressure difference across the inlet & outlet
provided by the pump.

I wouldn't expect that. The vast majority of the water will go whistling
straight through the bottom tank and out of the other end. To get to the
top of the rad, it would have to fight the restriction of one of the
channels, both up and then back down. in BBOE, it needs convection to
have a reason to do that.


With BBOE plus convection, it works as above, but pump and
convection forces work together.

Actually, the pump would work against convection with BBOE, but the
effect is very small because the velocity of water in the rad due to the
pump is so low. Convection easily overwhelms it.



With TBOE (wrongly connected, flow in bottom & return out opposite
top) the pump and convection forces are opposing one another. The pump
will IMHO overwhelm the convection forces and ther will be little
difference. It will be a parallel flow heat exchanger and the air
movement will be reduced.

I think you're over-estimating the velocity of the water inside the rad
due to the pump. For example, a 1kW rad might need about 1.2 litres per
minute at "normal" temperatures. The top and bottom "tanks" might be
similar in size to 28mm pipe, so that's just 3ish cm/sec even where
*all* of the water has to flow through one of the tanks (near the inlet
or outlet). Once it splits up and goes through the numerous channels,
it's just barely drifting along. Convection will have no trouble
reversing any flow pattern the pump might tend to cause inside the rad.


Domestic hot water cylinders are connected F in bottom, R out the
top, but no-one has ever claimed it stops them working.

The coil in mine has flow into the top. The flow velocity due to the
pump is much higher than in a radiator in any case.

Cheers,

Colin.