I been working over the last 10 years mostly with condensing boiler and I have the believe that becouse of the way they are connected to the commercial buffers they just ocasionally achieve condensation. The way I see it, the problem comes from the fact that hot water returning from the boiler mixes all the time with cold water coming from the system. This mixing of let's say 180F water from boiler with 110F from the system, resulted in the water mix temperature return to the boiler close to 145F. We all know that water returning to the boiler have to be below 135F (CO condensation temperature) to allow for condensation. The lower the better to secure condensation.
The question is how can I minimized the degree of mixing in the buffer to allow 110F cold water from the system to go to the boiler where it is needed cold and hot water from boiler 180F to go to the system at that temp without mixing?.