The difference is that this is not a heat exchanger and a not a listed device. Modern heat exchangers are kept under negative pressure on the combustion side during operation so any leakage would not leak combustion gases into the domestic air.
The difference is that this is not a heat exchanger and a not a listed device. Modern heat exchangers are kept under negative pressure on the combustion side during operation so any leakage would not leak combustion gases into the domestic air.