TBOE radiators in new build incorrectly piped

[Jonny boy]

Yes it matters return should be at the bottom so when the water cools it flows into the return

Not necessary at all .
One of the biggest myths of small bore heating . TBOE doesn’t care what the configuration is and the so called “ traditional wisdom” of putting the return at the bottom or lower pipework is flawed.
Correctly as you state , as the hot flow gives up it’s heat it becomes less buoyant more dense and will sink , but there is no counter convective interaction and although the efficacy is improved with TBOE , it would improve further with Hot flow at bottom return at top . Why ? Because it created counter flow , which is far more efficient in mixing flows and therefore gives greater thermal efficiency and efficacy . Heat from the convective sense naturally wants to rise . It will of course give up heat becoming cooler and want to fall back down . If the hot feed is at bottom, it counter flows and blends far better . The remaining heat wants to rise anyway so it’s sensible to return from the top , not forgetting it’s still relatively hot and the useful heat exchange has taken place in the rad.
All large heat exchangers function with counter flow

 

[ShaunCorbs]

How hot would the return be ?

 

[John.g]

Not necessary at all .
One of the biggest myths of small bore heating . TBOE doesn’t care what the configuration is and the so called “ traditional wisdom” of putting the return at the bottom or lower pipework is flawed.
Correctly as you state , as the hot flow gives up it’s heat it becomes less buoyant more dense and will sink , but there is no counter convective interaction and although the efficacy is improved with TBOE , it would improve further with Hot flow at bottom return at top . Why ? Because it created counter flow , which is far more efficient in mixing flows and therefore gives greater thermal efficiency and efficacy . Heat from the convective sense naturally wants to rise . It will of course give up heat becoming cooler and want to fall back down . If the hot feed is at bottom, it counter flows and blends far better . The remaining heat wants to rise anyway so it’s sensible to return from the top , not forgetting it’s still relatively hot and the useful heat exchange has taken place in the rad.
All large heat exchangers function with counter flow

Having the return from the Top certainly made a big difference in the few tests I carried out over 3 years ago

It seems pretty clear that convection just won't work properly in any system with the return on the top. In the BTOE there are 8 readings between 45C and 38C, the flow just seems to flow upwards to the top of the rad and then partially short circuits out the other end whereas in the other two it flows along the top of the rad and then drops down in parallel flow paths and out the return, (makes sense I suppose).View attachment 37636

 

[Chuck]

I think that what Jonny_boy is saying that the conventional flow-top return-bottom arrangement results in a stratified temperature distribution but putting the flow in at the bottom and return out from the top would disrupt this pattern resulting in a higher average temperature of the radiator surface.

While this might be true for some designs of radiators, if you try it in practice with a typical domestic radiator the flow scoots up the inlet side, along the top and straight out of the return. This is the 'conventional wisdom' and @John.g's measurements seem to confirm it.

The flow speeds inside a radiator are really quite low and I have no difficulty believing that convection forces play a significant, maybe dominant, role even in a pumped system.