Your numbers for efficiency look roughly correct. I understand the Lister is quite efficient, on the order of about 37-38%. So the rest is almost all heat. 1 gallon of heating oil contains 139,000 Btu, so roughly 62% of that is heat and noise energy. Some of the heat will be lost in belts and other friction lost heating the room where the engine resides. I would figure overall to expect to cheaply recover 60% of avaible heat. Keep in mind that heat transfer depends on difference in temps. When your cooling water is cold, the rate of recovery is great. I can leave my hand om ny exhaust pipe after the heat exchanger when the water is cold. But at 190 degrees water temp, the rate of recovery is much less. Diesel exhaust is much cooler than on a gasoline gasoline, an effect realized by their greater efficiency. But you can recover the heat, and rightly you should. The biggest problem is carbon build up on the transfer surfaces. I use a domestic copper coil containing water over which exhaust passes and noticed drop off in transfer rate quickly. There is one really good way to recover nearly all the heat, and that is using a wet scrubber. I plan to further investigate this option. However, you must then deal with all the crud that builds up in the water. The exhaust gets nearly perfectlt clear of particulates as a result, all or nearly all the particulates get filtered out, but you need to do something with them in the water. I see marine heat exchangers of this type advertised that boast of a max 10 degree difference between water temp in the unit and final exhaust gas temp. This is truly remarkable. But they are simply dumping the water. I need to recirculate it through my hydronic system. I believe the key here will be using a separate water loop from my hydronic water, which allows the matter to settle out separately, for occasional cleaning, and a copper coil from the house heating water through this hot water. More complexity but it satisfies the requirements. If anybody has already designed a working solution, I would greatly enjoy reading about it to save some aggravation.
Thanks