so says a ‘coffee expert’ revered by many
well jim, let me tell you that an italian manufacturer in the dim and distant past tried doing what you would like to see, i.e. a heat exchanger with a cold water feed like the ubiquitous E61 design has. it was the Ford Edsel of the lever espresso machine world: an abject failure.
while a heat exchanger with a cold water feed works exceptionally well in the E61 system it is an outright disaster in a lever machine
why? because the flow rate in a lever espresso is so much higher than in an E61 system that a cold water feed on a heat exchanger in a lever machine will send the whole heat exchanger – thermosiphon circuit stone cold every time you pull the lever
for this reason lever machines with thermosiphons, which obviously need a heat exchanger to keep bringing the cooled water that has been through the group back up to temperature before the water is sent around the group again, must be replenished from the boiler
when the L1 is at idle the circuitry behaves exactly like a heat exchanger in an E61; when you pull a shot there is a sufficient volume of water in the TS circuit that you are not making your espresso with water at boiler temperatures
L1 very much has a heat exchanger, albeit with a hot feed. With a 2.3L boiler this guarantees the water in the boiler is always fresh and therefore it will not taint your coffee
To have built the L1 with a cold water feed into the heat exchanger would have been an unmitigated disaster