I don't know if this has anything to do with efficiency either (though I suspect it does), but I've noticed that with the heat exchanger connecting my gas boiler and wood-fired boiler, it's a momentum thing. In other words, it takes longer to get the system up to temp, but once it's there, it tends to hang in there longer. The Achillies heel of that arrangement is that any inefficiencies in your piping and elsewhere in the system (insulation, etc.) are magnified the more times you circulate the same hot water. Plus, the heat differential is less, which is contrary to efficient operation of a heat exchanger, at least as I understand it. Life is full of trade-offs, no?