Balancing / microbore / new radiator / radiator removed

15 Jul 2016
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United Kingdom
A quick question. Formerly we had 2x downstairs radiators (one in hall, one in front room) fed by 8mm microbore from the same 15mm feeder / return (i.e. 15mm feeder -> 2 x 8mm microbore -> 2x radiators -> 2x 8mm return -> 15mm return). We've removed one of these radiators, and the other has been replaced with two smaller rads linked in series (can give reasoning, but not critical to query) - however, the 8mm has been retained, so now we effectively have 15mm feeder -> 1x 8mm microbore -> radiator -> radiator -> 8mm return -> 15mm return. Now we're trying to balance the radiators and the second radiator in the series in cooler than the first (to be expected) - however, the lock shields on virtually every other radiator have to be almost off (less than 1/4 turn) for the new radiators to get hot. I am wondering whether removing one radiator has somehow reduced the flow through the retained microbore (although pressure should've increased?) as previously we had 1 x 15mm going to 2 x 8mm (so effectively 176mm2 cross section -> 100mm2 cross section) whereas now we have 176mm2 cross section -> 50mm2 cross section) the previous greater x-section of the 2 x 8mm allowed water to flow through the 15mm easier...
Or is this nonsense?
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Surely you've answered your own question: you in effect now have smaller pipework, so the flow is reduced.
Thanks...yes...I think I'm confusing flow rate and pressure...presumably the pressure in the 8mm will increase vs previous because it's gone from 2 x 8mm to 1x 8mm - but the flow rate through the 15mm will be less as there's less pipe for the water to flow into when it leaves the 15mm? Or is the pressure constant because that is maintained by the pump? I wish I'd paid attention in GCSE physics...
Thanks for replying, much appreciated. Just wondering whether to replace the 8mm with 15mm now (although I've balances the rads so they all warm up, so perhaps not necessarily worth the hassle)
You've got to think of the rest of the pipework in the system though. Imagine (wrongly, but it illustrates the point) that the pump will make a certain fixed flow rate flow through your entire heating system. How much of that flow goes to each radiator is inversely proportional to the resistance offered by the rad, valves, pipework of that segment. Therefore, if you increase the resistance of one segment, it will experience less flow - and the other segments will get more flow (the balance of the flow, in fact).

This is of course a significant oversimplification but it suffices to demonstrate more or less what's going on.

You've achieved better balance by restricting the other radiators. You've probably reduced the overall flow rate in the system, which will (potentially) have a detrimental impact on the performance of it.
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Thanks very much for the good explanation - really helpful. Much appreciated!
I don't think it is a very good explanation.

Suppose you consider that the pump provides a certain pressure across its output of say 4.0 m head of water.

The flow through each parallel rad circuit will be inversely proportional to its resistance.

But unless its a smart pump it will not produce an exact constant pressure. But the pressure is dependent on the total flow which is the sum of each individual rad circuit.

So when considering any single radiator circuit, variations in just that circuit will not have a very significant effect on the pump pressure which depends on the total flow.


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