What goes into a T, must come out of that T
The late Gil Carson, who was a pioneer of modern hydronics and inventor of pri/sec piping, would stand before a gathering of engineers that he was lecturing. He would begin his presentation the same way: "What goes into a Tee, must come out of that Tee." Profound. Then he would repeat himself: "What goes into a Tee, must come out of that Tee." Though he was a true genius and excelled as an engineer, he never let theory override common sense. Men are prone to do that as they get caught up in technical things.
"What goes into a Tee, must come out of that Tee." If that is true, and common sense tells us it is, then this must also be true: nothing more than what goes into a Tee can come out of that Tee.The logic of that is irrefutable.
While Gil may have been referring specifically to flow, it applies to btu's as well. You can't get anymore btu's out of a Tee than what is put into that Tee. The universal hydronics formula, BTU = Delta T x (500xGPM), proves this also.
High temp baseboard has almost always been design and installed in North America with an average water temp of 170* (180* supply, 160* return). That's a 20* Delta T. That's what's out there and that's what we have to work with in the field. I'm not talking about designing a new system; I'm talking about what's existing. You cannot widen the Delta T on a system like that without also causing a lower average water temp and starving the rads farthest down the line for btu's.
Likewise, you cannot expect any more btu's to make it to the emitters just because it's piped p/s. " Nothing more can come out of a Tee than what goes into that Tee" - my words, not Gil's, but proved above.
The Navien has a maximum return water temp of 160. That means that with a maximum supply temp of 180* and a circ. that's limited to 5GPM, you only have 50k on the btu train. It doesn't matter how many more btu's the burner can produce, that's all that can fit on the train. And that's all that's gonna make it to the station (emitters) no matter how it's piped because that's the design of the system. Piping it p/s doesn't make it produce more Btu's. There's no magic in a Tee that would cause that to happen. "You cannot get any more out of a Tee than what goes into that Tee".
If we widen the Delta T on the system, then the Delta T on the primary would begin to widen proportionately and then we could get more btu's. But we can't do that on an existing system due to the obvious reason stated above.
Thank GOD for men like Gil that were great thinkers because their imaginations were tempered with common sense.