You can install a bypass line with a metering valve in front of your heat exchanger, routed to the exhaust
, to slow down the rate of water passing thru the exchanger.
I had the same problem. I even went to a 190º t'stat and it still wouldn't get above 120º. But I'm in the colder climates right now and may end up closing the bypass when in the tropics.
BTW- Is this the same Sergy that I talked to on the phone
a few years back about Clippers?