The amount of delivered heat measured in BTU/hr is dependent on the water temperature, flow rate and the temperature drop in the heat emitters (radiators, baseboard, air coil, etc.) Here's my previously posted graph of the relationship for a supply water temperature of 140 deg F.

In any case you need a certain flow rate (gal/min) for design conditions. The pipe size and pump characteristics determine how much flow you can have. In a closed loop hydronic system resistance to flow is only determined by the pipe and other components that the water flows through. There is no static head. For a given pump and piping system the intersection of the "pump curve" and the "piping resistance" determines the flow rate. For example look at the graph my previously posted family of pump and piping curves. There are three different Taco pumps (green curves). There are three different lengths of PEX-AL-PEX (black curves) and two different lengths of copper (red curves) pipe. The intersection of the pipe resistance curve with the pump curve determines the water flow rate. Once you know the flow rate you can then look at the heat transfer curves described in the paragraph above and determine the heat transferred.

Obviously there are a lot of variables and design tradeoffs. You can have different pipes (PEX, PEX-Al-PEX, copper, iron) alone or in combination. You can have low, medium or high head pumps. You can have lots of radiant area like in floor tubing. You can have marginally sized copper baseboards that require high supply temperatures, etc. I hope this gives you a better understanding of what a design engineer or a trained plumber considers when designing a heating system. Engineers like me, tend to design in detail, using equations and graphs. Plumbers design using rules of thumb provided to them by design engineers. Properly applied both methods can design a satisfactory heating system.

What sting is saying in "In a trouble free design" is his design rules of thumb are conservative. One inch PEX can carry more heat than 80,000 BTU/hr but the application requires detailed analysis.

Notice you get more heat transfer at greater flow rates. You also get more heat transfer if there is a larger temperature difference between the supply water temperature and the return water temperature. A large difference would occur when for example your house is cold and the heat emitters are hot. Less, transfer when the house warms up. I plotted this graph for 140 deg F water. That's about the lowest one would use in any radiation design system. Hotter supply temperatures would provide proportionally more heat transfer.