Heat loss/resistance in CH system

For the sake of future readers who may know that you are an expert, I take it you are using "proportional" in the colloquial sense of "depends on". As you know. it is actually proportional to quite a high power of the internal diameter, the exact relation depending on whether there is any turbulence.

Since I'm doing corrections, I suppose it's reciprocal resistance, flow per unit pressure rather then pressure per unit flow, that goes up with diameter!

Perhaps proportional is a poor choice of word since that normally implies that you can swap in an equal sign and a constant and you have yourself an equation. The influences on flow resistance are more complex than simple variables than you rightly state. Perhaps flow resistance would be better described as a function of pipe length and diameter and other variables. :-)

The thrust of what I was getting at was, yes a reduction in diameter will restrict flow, however a very short length reduction in diameter will have much less effect than a longer run at the same smaller diameter. So using a pipe insert that reduces a nominal 15mm^2 to 10mm^2 for 20mm will not be anything like as "damaging" as a 5m run of 10mm^2 micorbore pipe.