No go on tell me when it is actually allowed with a certain part fitted within th boiler or have your google searches not highlighted a basic requirement
No, I'm not playing that game. You are the one who implied that a buffer tank (or low loss header, or whatever) is not required to allow use of a modulating pump - and in the context, the implication is driving a rad loop that is fully TRV and hence arbitrarily low flow rates. Yes I know some boilers have modulating pumps, but I am equally certain that they cannot modulate down to arbitrarily low flow rates - it's just not possible to modulate the gas flame down to match, and absent a thermally massive heat exchanger (I know there's one manufacturer, forget who now, that uses a big alli casting rather than the usual finned job - but I wouldn't class that as thermally massive, and the manual still has the minimum flow rate set to produce a max delta-T of 20˚) it's just not possible to safely reduce the flow rate below a certain level.
So, for a full TRV rad loop, the only option without a means of decoupling the rad loop and boiler is a bypass - either internal or external to the boiler. And a bypass is an efficiency killing function - it artificially raises the return temperature (reducing efficiency, massively if it goes far enough to stop condensing) when it should be going down (increasing efficiency).
I don't need Google (or any search engine) for that, it's basic engineering - perhaps you should it sometime.
As to your "will always be firing up" claim, Lets put some real numbers to that - and yes, these are real numbers.
When I put the store in the flat, I did some observations and measurements. One was that with the hysteresis in the cylinder stat, the boiler would run for about 90 seconds "topping up" under light load (I think the flat heating was on at the time). The boiler is rated for a tad under 30kW. In 90s that means 1/40th of 30kWH, or 0.75kWH of heat each time. Standing losses were measured at about 80W, so that's a bit over 9 hours to lose 0.75kWH. So perhaps 3 times day - that's hardly what you imply with "your boiler firing up throughout the day".
OK, there's some room for quibbling over details, but the gist is, from real observations and measurements, my setup does not have the boiler "firing up all the time".
And lest you go on about wasted heat from those standing losses - it's not wasted when the heat goes into the envelope of the building - except for a short period when it's hot weather (it's not wasted at the time I'm writing this in mid July !)
Contrast with a combi. SWMBO could have the combi firing up several times a minute when doing the washing up - dozens of times in (say) half an hour. Tap on to rinse, boiler fires up; couple of seconds later, tap off, boiler shuts down - might not fire up every time, but still "lots" of times.
And while on the subject of minimum flow rates ...
That also applies to the DHW function. Thanks to her new boiler being inferior in that respect to the one it replaced, and a long pipe run to the bathroom, my elderly mother will no longer take showers. The shower is fine for a while, then it goes stone cold - and it's not just her I know of with that problem. At the low flow rates, the boiler runs out of ability to modulate down the heat input, so the output temperature rises. As the water temperature rises, the thermostatic valve in the shower (before combis, we could have showers without thermostatic valves
) mixes in less hot water, reducing the flow rate further, so the temperature keeps going up, the flow rate keeps going down, until eventually the boiler shuts down. At this point, the water in the long pipe run is very hot, so the flow rate stays too low for the combi, while cold water from the mains flushes the heat from the boiler and pipe run. Eventually, the cold water reaches the shower which turns cold, and the mixer valve will demand hot water - only then will the boiler fire up, but it then has to heat the water at a high flow rate, and after an uncomfortable couple of minutes the shower will warm up again. The cycle then repeats ... Someone else I know of with that problem "solves" it by leaving a hot tap on at the sink, deliberately wasting hot water so as to keep the flow rate high enough to stop the boiler shutting down - so any saving by eliminating the waste and inefficiency of a store completely blown away by deliberately running hot water to waste..
Another problem my thermal stores do not have.
I will admit to one huge problem with a thermal store. I find that RGIs are completely flummoxed by them, and even when I show then the "as fitted" manual and the diagram showing all the plumbing, they (or at least, most of the ones I get sent) still struggle with the very idea. From experience I cannot leave them alone with it or they'll do something stupid and/or that they've been told not to do. I've come to the conclusion that a lot of the younger ones simply don't know anything but a combi.
