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[LittleJohn]

Sprinter, I think your math is a little off on Pex S&R - cause remember you can run Pex close to 8'/sec and not have to worry about erosion.  With that being said, assuming 41GPM required in worst case scenario on coldest day;

2" Pex is capable of 52gpm @ 8'/sec       (with head loss of about 11.5 feet/100')
1.5" Pex is capable of 30gpm @ 8'/sec    (with head loss of about 15.4 feet/100')    Would required 2 S&R
1.25" Pex is capable of 21gpm @ 8'/sec  (with head loss of about 17.9 feet/100')    Would required 2 S&R
1" Pex is capable of 14gpm @ 8'/sec       (with head loss of about 22.5 feet/100')    Would required 3 S&R

However sprinter I do agree all this is for naught, depending how the system is designed/zoned.
Also, if 40+ GPMs are required would recommend some, primary-secondary piping, so main loop has the least amount of restrictions

NOTE:  If your system requires +400k btu/hr  what do you feed the beast (OWB), and how often!?!

[Sprinter]

No you shouldn't run 8' per second. Erosion is one thing, but more importantly look at how the head climbs exponentially as you exceed 4' per second. Either way there isn't a designer anywhere that would recommend that kind of velocity. Trunk slammer design, just because it can doesn't mean you should. My numbers are the same numbers found in any hydronic design book or program. So instead of using a economical 007/15-58 you now need a much larger $300-$400 circulator that is beyond design, burning several times more electricity , and masking wrong pipe size. No wonder we see so many circ failures. Not to mention many hydronic devices are not intended to see those velocities. That's enough to push open zone valves, premature failure of air vents and so on, and create cavitation. A lot goes on when you start and stop fluid at that speed.
It starts a chain reaction, a lot of it unseen until problems start showing up, and they can't figure out why. But somebody will find something to blame.

[Sprinter]

Yes, little John, I was being facetious with the first post. That would be true if everything is calling at the same time. However when broken down into a pri/sec off a small buffer tank and zoned. You can properly feed each zone. Just like on controllers with a priority zone for indirect water heaters, it will temporarily shut everything else off and just feed the tank, mine takes about 15-20 minutes at 180-200 deg. Then that's done for the day, cuz that's a 120kbtu load and would take a couple hours with other zones calling. Also if a outdoor reset curve controller is used much lower temps can be used, further decreasing the load. I have unfortunately seen some like this in one big loop or two.  Have you read the glitch and the fix in plumbing and mechanical magazine? It's written by another John, Siegenthaler. Pretty good stuff for troubleshooting and design.

[LittleJohn]

Sprinter, I also apologize  - what I should have said was, you COULD run at 8'/sec not you CAN or SHOULD.
Again it gets back to the math, and yes - you run at or near the recommended limits of a product but there are certain consequences is doing so; and with hydronic system higher velocities equals higher head pressures.