Can some kind soul help me with my understanding (or lack of it) concerning combi boiler flow rate temperatures please? The reason I ask is that I am using the Stelrads STARS BTU calculator for radiator sizing since I believe it is regarded as accurate (unlike most calculators online which vary wildly!). STARS offers two seperate calculations regarding flow rate temperature (CH that is, not DHW flow), one based on 80degrees and one at 70degrees. I am under the impression that old combi's worked on 80degrees, and modern condensing boilers on 70degrees? Can anyone tell me which value I should be using for my 29Kwh domestic combi? regards, boydell.
If you want your system to be as efficient as possible the return temperature needs to be below 55C (the lower the better). Assuming a differential of no more than 20C, that gives a flow of 75C. Designing for the max flow temp of the boiler will not achieve maximum efficiency. I would choose the 70C option. The rads suggested will be physically larger than the 80C option; but that is because the output is less at 70C.