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If a dead person once did it, it is obviously a good idea. Case closed!

Unless of course, the death was caused by the contaminated water!

This sounds like the beginning of a Marvel Comic story about AquaHulk

This ordinary man that one day inadvertently bathed in the thermally heated water from an experimental Maximizer water heating system, at the brink of death, suddenly transformed into the words first amphibious super-hero

If a dead person once did it, it is obviously a good idea. Case closed!

Hi @cowdog , The dead guys did this many years ago. I have a water heater from 1895 that does this. Here's a cutaway of a similar one.
These were called contact heaters, where water and combustion products were mixed directly. They claimed to be 92% efficient. I assumed the acidic water cleaned better. You may note that venting wasn't a concern either

Yours, Larry

IIRC, Burnham tried this with their "Opus" gas boiler some years ago. Obviously we never see them now.

Boiler heat exchanger is a frequent point of failure of hot water hydronic heating systems.

Can we avoid using a combustion-water heat exchanger by routing combustion gas through water, making direct contact with water?

Exhaust could be a little acidic from sulfur in fuel, we should add baking soda into the recirculating water to avoid corrosion to metal.

For efficiency we can also add a small heat pump (about the same cost with a portable AC unit) to condense vapor in the humid combustion gas after passing through water.