This 8x8 finned tube heat exchanger while being a total challenge to implement doesn't add substantial solution temperature (only 10° with my insane flow wand, 15° with an 8 flow) however it has attenuated heat exiting the exhaust air and namely dampened sound. Exhaust temps are reduced 30° to 120° and it's 75 decibels next to it and a very tolerable 60 @ 15'.
The vac exhaust heat exchanger idea is nothing new for sure, but the sound and heat suppression is a pretty kickass side effect. Cfm's are reduced more by the muffler than the heat exchanger. Overall my system takes a 15 CFM penalty through the exchanger and muffler vs reading off the stack.
In an ETM situation a couple of these together would be very effective in preheating incoming water before the main heater and substantially quieting the exhaust noise especially when running on cord power.
Again, nothing new or groundbreaking, but worth the trouble for me. Cheers!