Two separate HX pipes (one for hot water, one for brew water) pass through one boiler (as I understand it). The duration of the trip in the HX tube through the boiler is carefully worked out so the water passing through the tube is heated correctly for its final use, i.e. less than the temperature of the boiler water used for steaming. If you already know this, my apologies.
So both hot water and brew water come directly from the water supply on demand (the volume of water in each HX tube is quite small). The boiler water for steaming sits in the boiler until all the older boiler water is used and replaced by fresh water which might take weeks in domestic use. Impurities in a steam-only boiler may also be concentrated as they will be left behind as "pure" steam evaporates away.
I have never tried an Americano so the attractions of the MiniNova for me are the E61 group and rotary pump (which together seem to be particularly forgiving of duff grind/tamp technique, i.e. mine!), no delay in steaming, plumbed-in water supply and drain tray plus a good reputation for reliability - all at what seems to me a reasonable price. Given that I start from a Gaggia Carezza, the 3-way soleniod will be a time-saver also. The fact that the E61 head is partly enclosed is good too as several E61 machine owners have burnt themselves on the E61 head - and we may have up to 6 cats in our kitchen at any one time. Mind you, the cats are pretty smart at avoiding hot objects until they have cooled down a bit then they lie on top of or next to them and enjoy the warmth.
MiniNova in the UK :
http://www.pennineteaandcoffee.co.uk/mini.html (£699 + 17.5% VAT for the semi-automatic rotary version. BTW, I have no connection to that company.)
Cheers,
Hugh
Grinders : Macap MXK conical for espresso, Mahlkoenig Vario for Chemex, Macap MC6 (spare when our office was closed) for cafetiere, Zassenhaus Knee Mill for cafetiere when working away from home.
La Spaziale Vivaldi S1 espresso machine. HotTop KN8828P roaster. Chemex manual drip for most brewed coffee plus cafetieres and eSantos.