In fairness, years before I was qualified, I used a mixture of 22 Cu, 15 Cu and 10mm Hep to make a manifold heating system where I was living. The 15 was for a DPDC rad that needed more than 10. The 10mm Hep was for everything else. The 22 was to the manifold (which was, itself, made from 22), and the cylinder.
According the the pipe sizing method of the Copper Developmnent Association or whatever it's called (as previous post) that I used (a slightly simplified version to the City and Guilds Level 3 textbook method IIRC), 10mm was excessive for some of the rads, but I wasn't going to buy yet another roll of pipe for the smallest rads.
It actually worked very well indeed. Since there was minimal water content in the pipework, the rads warmed up quickly. Since the system was to some extent balanced by the resistances in the flow and return runs being roughly equal prior to any fine tuning using the lockshields, the system balanced quite easily and, since none of the lockshields needed cranking down massively, the system ran quietly. The pump was an old UPS 15-50 on the lowest speed to allow a 10°C drop. This was in a three-bedroom, 8 radiator house.
If I'm working on an existing system, using 15mm gives me a margin of safety that allows me to know my pipework is not restrictive. This is useful if a larger or additional radiator is later substituted, or where I do not know how the runs are made, how much available head there is, etc. But, if I were starting from scratch, I have no problems with microbore - I have seen it work very well. The only problems IMO is that the system needs to be kept clean, that microbore is more vulnerable to damage than half-hard 15mm, and that, visibly connected to a fat old iron radiator, it might look a bit strange.
I maintain that flow rates should not be a problem if the pipework is sized as it should be.
Take
Classic 3 Column Cast Iron Radiators 500mmRRP 17 and use a 500 high by 630 wide radiator (10 section) as an example. Output is 1800btu, which is about 0.5kW. Conveniently (saving me looking it up properly), this link (
http://copperalliance.org.uk/docs/l...eating-systems-pdf.pdf?Status=Master&sfvrsn=0) suggests 5kW at an 11°C drop needs 109ml/s which is 6.6l/m. So a 0.5 kW radiator needs only 0.66l/m: a mouse's fart. On 10mm, the loss of head for a 0.5kW radiator running at an 11°C drop, is a mere 0.006 metres per metre run of copper pipe. In fairness, we tend to use a 20°C temperature drop these days, and there are other factors involved, so this is only an approximation, but you see my point. By all means shove more water through the radiator than its output requires to allow a fast warm-up, but be aware that the condensing boiler will not then run in condensing mode, and the pump will be using more electricity than it needs to.
You'll then use the lockshield valve on the radiator to reduce the flow to the 11ml/second that the 0.5kW radiator actually needs... and the radiator will take LONGER to warm up because of the slightly increased water content of the system.
As others have said, the point is to size the pipework properly. Too fat a pipe will be less of a problem than too thin a pipe, but the ideal is a properly sized pipe.
