Boiler Room Configuration

I'm designing a mechanical room for a radiant floor system. 3700 sq ft home retrofit. I've mostly settled in on the design below, but wanted to get it sanity checked by some folks with more experience than I have. There is a full diagram attached. As always, thanks for sharing your knowledge and experience with the group. We genuinely appreciate it.



Assumptions/Facts:

<ol><li>48k BTU heat loss</li><li>15 Degree Delta T</li><li>105 Degree Supply Temp</li><li>20K BTU "Backloss"</li><li>6 Zones (5k - 10k BTU each)</li><li>All zones are 2-2.5 ft of head</li><li>Polaris water heater hacked to supply 140-155 water</li></ol>





Components

<ol><li>Polaris 130K 34 Gal</li><li>Sentry zone valves</li><li>Alpha system pump (on constant pressure) </li><li>Bumblebee on heat exchanger loop (at 105 setpoint, manually adjusted for outdoor temp)</li></ol>



Questions



<ul><li>Given that the system should always be pushing more flow than the heat exchanger loop, am I right to have the system as primary and the heat exchanger as secondary? Or should they be switched?</li><li>I could use a taco 007-vr on the polaris side, and loose the bumblebee and secondary loop all-together, but the VR is 3X the cost of the bumblebee. Is the added complexity penny wise and pound foolish? Is there a cheaper circulator that does setpoint variable speed and is brass/stainless?</li><li>I've got both the alpha and bumblebee oriented vertically. That adds a few bends to the system, but it's my understanding it will help ensure air bubbles travel toward the air eliminator. But with both pumps pulling water downhill (and thus pulling air bubbles down too) does it really help anything?</li><li>Are there any other obvious flaws here that are making me look stupid (I've got thick skin, I can take it).</li><li>What should be done about backloss? My understanding is that some of that heat is truly lost, but that much of it is coming right back at you in the form of thermal mass. Is there a rule of thump for how much you get back? I've got slab, crawl space, and second story all in the mix, if that helps any.</li><li>Polaris is available in 34 and 55 gal models. Without the hack, either model will cycle like crazy, taking less than a minute to overcome the 3 degree delta built into the system. With the hack in place, is the extra energy storage of the 55 Gal model worth the extra cost and space?</li></ul>