Actually the problem is that whatever the boiler, it will have a minimum flow rate for it to work properly/at all.
So it pure coincidence that the "minimum flow rate" for, say a Vaillant 418, just happens to be the flow rate necessary to produce the stated output (18kw)with the stated temp differential (20C)?
From the MI: Minimum flow rate 12.9 litre/min (Page 8, table 2.1)
(18kw x 60) ÷ (4.18 x 20) = 12.92 litre/min.
You can do this for any boiler and the answer is always the same: the "minimum flow rate" is that required to meet the specified output with the specified temperature differential. If the boiler modulates down, say to 12kW and the flow rate stays the same then the differential will drop from 20C to 13C. If you want the differential to remain constant at 20C, the flow rat has to reduce to 8.6 litre/min. The same is true if you range rate the boiler down to 12kW, the flow has to be reduced.
Your statement is entirely correct (though I haven't checked the actual numbers you've quoted), but doesn't actually disagree with what I wrote if you take it in context. You are looking at a different aspect of the system to what is being discussed.
Taking the boiler you've quoted figures for, can you tell me what it does if you reduce the flow rate to (say) 1 l/min ? Why does it mater ? Well if the heating load is such that the TRVs only pass 1/min through the CH circuit then unless you fit a bypass circuit somewhere, then that's the only flow you'll get through the boiler.
Say your heating flow temp was 60, and return was 30, and flow rate was 1 l/min (that's 2.1kW). By the time you've mixed 7.6l/m of hot water at 60˚ with 1 l/m of water at 30˚, then your boiler return is now at 56.5˚. So you are running the boiler for extended periods when it's not condensing.
The question was not what the minimum flow rate is to maintain a given delta-T for whatever power the boiler is able to range up/down to, but what is the minimum flow rate before the boiler stops functioning properly/at all. Clearly it
should be low enough that you can get min power out while still maintaining a sensible delta-T - below that then it's case of a) will the boiler shut down because of a low sensed flow rate, b) will it suffer from overheating (or even localised boiling) and shut down.
Historically we've designed the rad loop to maintain a significant flow rate and the discussion hasn't been much more than a curious question to most - and the return temperature issue wasn't an issue in those heady PC days (Pre-Condensing). Now we are expected to fit TRVs and IMO a modulating pump goes well with that - but the question of what the low flow rates do to the boiler gets neglected by most plumbers (I refuse to call most of them heating engineers as some of them like to call themselves).
IMO the ideal setup is to decouple the flows and buffer the boiler output - so you can have the boiler run intermittently at a good flow rate commensurate with it's capacity, and have the CH loop run at a flow rate commensurate with demand.
Just plucking some numbers out of the air, suppose you had 20l of active buffer (the actual tank would be bigger), and you kept the top at around 60˚ and stopped heating when the bottom reached 45˚, and further assume that the boiler will start ranging down when the return exceeds 40˚ (for a delta-T of 20˚
, and we'll assume we avoid mixing. To heat 20l of water, through 30˚ (from 30 to 60) with 18kW would take in the region of 70s. To take that heat out again at 2.1kW would take around 10 minutes.
So in this (vary simplified) scenario - we have no problem of low flow rate through the boiler, we have no issues with flow rate through the CH loop (it can be fully TRV), and the boiler will fire on full power for a bit over a minute every 10 minutes and then completely shut down. When the boiler is firing, it will always be condensing.
Obviously these are all figures plucked from the air and we could argue until the cows come home about what if this, what if that, etc, etc. The simple fact is that unless your boiler can continue running with very low flow rates, then you can't match it properly to a TRVd CH loop without either a bypass or a buffer. A bypass will frequently result in temperatures that don't allow for condensing, and that drops efficiency dramatically. Add in the current fad for fitting grossly oversized boilers in order to try and provide more than a dribble of DHW from a combi, and things are even worse. 18kW is not enough for even a small flat as a combi.