My gas supplier told me to turn it to the minimum to economise on gas use. However speaking to the Bosh technical support he said its a internal thermostat on the boiler. At the minimum setting it will shut down the boiler at a lower working temperature. And in my small terraced house I had best keep it set to the maximum setting, as this would make it more economical.
why would the manufacturers provide this option? Would be any advantage on having the boiler shut down at a lower working temperature?
I'm installing the same boiler at the moment (24Ri) My gas safe man advised getting a model that was just big enough for my house on economy grounds instead of my preference which was for one with some spare capacity. The implication of this advice is that it will spend more time flat out and so is similar to the advice you had. However, my overall understanding is that longer running periods at lower flue temperatures should keep the thing in condensing mode for longer and hence better efficiency. Once I have it installed and approved, I plan to log some data on return water temperatures and flue temperatures to try and assess when it is actually condensing. I'm rather surprised that the Ri boilers don't seem to manage the optimisation of condensing mode.
My cynical side suggests that the efficiency figures are obtained with a near 100% heat sink connected to the boiler under test so that the return water is quite cool and hence get best figures. In practice especially for a retrofit, the radiator loads are what they are and it could be that the boiler is rarely in condensing mode.
Boilers are more likely to work in condensing mode at lower return water temperatures.
On the other hand, if you're using the boiler to heat your hot water, it may never attain the desired temperature set by the cylinder stat and the boiler will cycle indefinitely.
There is actually a requirement (under Building Regs, I think) to size boilers appropriately for the building they're supplying, and several years ago I was required to do a training course in learning how to apply the whole-house boiler sizing method (which I've written up on the wiki somewhere - I think there's a link to a spreadsheet I knocked up for doing it).
And - yes - the reason is that a boiler is more efficient (and economical) running within its design load range than endlessly stopping and starting because someone's put in some frack-off great big oversized boy-racer appliance with go-faster stripes that can't modulate down to the actual load required most of the year ;)
The implication of this advice is that it will spend more time
Yup, you can turn down the wick to have it run cooler and more efficiently during milder weather. However I don't think the Ri has any way of telling it when you want HW rather than CH so if you turn down its thermostat too much you won't get hot enough flow to heat your HW cylinder.
This is a current deficiency of the W-B range IMO: there's no way to electronically regulate the boiler rate. Many other appliances have OpenTherm interfaces which I think can do that so that you can, for example, link them to third-party weather compensation controls.
With better controls you could probably coax higher efficiency out of these appliances, and since it's probably a matter of software and perhaps a few quid's worth of hardware for them to implement I give W-B nil points for not doing so. However I doubt the energy and cost savings amount to anything vast so I'd tend to stick with them as a generally reliable, reasonably-priced and well-supported brand rather than go for bells and whistles in something much more expensive or without the track record.
(Also remember that "high-efficiency" boilers are more efficient than "standard efficiency" even when they're not condensing, because they extract more heat from the combustion gases since they don't have to leave a safety margin to avoid condensing at all costs as s-e ones do.)
I have a 24Ri - installed 2006. Again this is a replacement and radiators have not been "upgraded" to give larger surface area. So in practice, my boiler temperature is set to give responsive space heating rather than max condensation. This results in radiator temperatures that won't burn, but you don't want to leave your hand on (the kids are old enough to compensate now!). Presumably you're Ri is also heating hot water cylinder. So you want a decent flow temperature to enable the cylinder to recover in reasonable time frame. The boiler temperature dictates the maximum radiator surface temperature, so I would be inclined to set your starting boiler temperature based on your preferred max permissible radiator temp. This will involve some trial and error - my dial is set at about 2/3 round the scale, but your model may differ. If you have young kids the temp. will have to be lower than mine. Make sure you give the system chance to heat all the radiator contents to maximum. See how this works in colder weather wrt heat up time and comfort.
I suspect that if you set the stat to maximum, the surface temp of the radiators would be at least uncomforably hot if not dangerously so.
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