I'm looking at eventually replacing my Ideal Icos heat-only boiler
which currently supplies a sealed S-plan system with an unvented HW
cylinder. It works well, or at least as well as an Icos-driven system
can, but I know that beast isn't going to last forever. I am therefore
considering one of Vaillant's ecoTEC offerings, primarily for expected
benefits of the brand, weather compensation and low(er) modulation
capability.

Confusingly, the Vaillant ecoTEC 415[1] is described as an 'open vent'
boiler, however digging deeper it seems to be equally suitable for
both open vent and sealed systems. Hence, I'd prefer to call it a
'heat only' boiler - but why don't Vaillant?

I perceive some benefits in keeping a heat only boiler, particularly
with regards to spreading the components around so as not to put all
my eggs in one basket. The way I see it I have far more flexibility
and ease of repair by having the pump, expansion vessel, PRV and zone
valves outside of the boiler. Having previously owned a combi in my
old house, whose repairs was tantamount to open-heart surgery, the
central heating in this new house is something of a joy to maintain
given it is all spread out.

However, does anyone know what am I missing out on by considering the
415 rather than the 615[2] system boiler? Are there advantages with
having the aforementioned components inside the boiler? Are the 415
and 615 otherwise essentially the same boiler, save for the location
of these additional components? There's a ~£100 price differential but
if there's a consequential functionality/capability cost of making
that financial saving I'm keen to know what it is.

Regards,

Mathew

[1] 415: http://www.vaillant.co.uk/installers...plus_open_vent
[2] 615: http://www.vaillant.co.uk/installers...EC_plus_system