Hello Glort, i've read a few of your stories.
I'll just add a little to my story, I live in city center in a very old house, 1700/1800 ? maybe even older.
So for exhaust noise I build a labirinth box about 1,50m long and 50 by 50cm inside covered with rockwool there is no noise only some fumes.
For heat recovery exhaust I used a marine brass pipe oil cooler pipes about 8mm , it works like a dream. Exhaust comes out at cooling water temperature.
The house is three floors, street level is a full garage 12m/5m then second floor living kitchen, third floor bedrooms etc.
To make this happen I needed a soundproof box, took some time but I came up with a cool/freezer cell. these are insulated with pur so good for heat but not for sound. I patched it up with rockwool. Result 90%. After 5 hours next to it you had enough, it's a men cave the garage, but a succes clean looks nice and you have a good solid door.
Only thing it heats up inside so ventilator in and out, blowing hot(smelly) air outside waste of heat. Coolingwater heats up so exhaust gas cooler is not condensating the exhaustgasses anymore it starts condensating in my labyrinth box. Thats not 100% waterproof so it's dripping black water.
I did run the engine on natural gas with diesel, lpg with vedge oil etc. But always after 300 hours? the cooler starts clogging up and it's a black disaster again.
I finally got a solution I gues a heat exchanger from a condensing natural gas heater, but I think I have decided to skip the chp project I proved my point but it never reached the point were it can run for a week without attendance. But there was enough water from the fuel I even made a connection to add water and to flush the cooler with chemicals after use.
Ok thats it for now,
regards Bernhard