Hello,

Sorry if this is a silly question but why do boilers have two ratings,
e.g. 15-21kW? Is this their output at their lowest setting at output
at their highest setting?

I am looking to buy a new boiler and I can see a couple of potential
models, one is 15-21kW and the other is 15-26kW. If the 15-26 covers
all of the range of the 15-21 and then some, does this mean it is a
better buy as it will give me extra capacity?

Does running a boiler at the low end or high end of its range cause it
to be more or less efficient? I'm sure I remember reading something
about efficiency varying according to load but I can't remember what.

I had a quick read of Ed's boiler choice faq and though it helped with
other questions, I couldn't see the answer to this.

Thanks.