Thank you. Would you consider using 1/2" tubes with the Ultra-Fin plates 16" on center instead of 1/2" tubes with Uponor Joist Track heat plates 8" on center?
Ultra-Fins are convectors, and require higher water temps, and the response time is on the sluggish side to boot. The heat is drawn from the tubing into the Ultra Fin via conduction, but to get from the Ultra Fin to the floor is only via heating up the air surrounding the Ultra Fin, with an air convection loop in the joist bay cavity. This takes a fairly big temperature difference to deliver the heat at a reasonable rate, but it also has the down-side of higher losses through the joists to the space below, and potential for losses to thermal bypass air movement laterally in the air cavity between the insulation and the sub-floor.
JoistTrak plates do the heat transfer primarily via conduction, which gets heat out of the tubing and into the floor at a faster rate at any given temperature difference between water & floor, and you can snug up the insulation to the Joist Trak, limiting the thermal bypass and joist conduction losses.
You're looking at about 12 BTU/hr per square foot of floor, which through 3/4" plywood + 3/4" oak (R1.7) a you can get with ~95-100F water in the JoistTrak (depending on the R-value of the subfloor + finish floor + floor coverings). See the nomograph on the last page of this document. But takes 120F or more to get the same BTU/foot-hour
with the Ultra-Fin. That's a 20-25F higher operating temp, which has efficiency consequences. If your floors are thicker or more insulating than the 3/4" plywood + 3/4" oak paradigm case, the differences in water temperature requirements grows proportionally- if your flooring stackup is more like R2.5, you'd need 120-125F water with Joist Trak, but out of condensing range with Ultra Fin.
The apps where Ultra Fin might make more sense is with non-condensing boiler, and BTU per square foot requirements would be over the 180F specified operating temp limit of PEX using suspended tube alone, and it would improve the response time of suspended tube.
It goes in quicker & cheaper than extruded plate. But if you wanted to cheap out, some of the better sheet-metal plate systems can deliver 12BTU/ft and higher at condensing temps.
For less money and hassle you can get pretty good comfort out of low-temperature panel radiators, and run it at a single water temp high enough that this micro-zoned heating system won't short-cycle the boiler into an early grave. At 130F out/115-120F return a single
Biasi B-24.71 delivers about a third of the min-mod output of your oversized boiler, and has enough thermal mass to keep the burn times almost reasonable. The fact that your total radiant load is significantly lower than the boiler's min-fire output is a problem when you cut it up into even smaller zones, as discussed on your other thread.
There's no need for the 240ASME- the smallest in the line 180ASME is MORE than enough for both your heating and hot water loads, at Seattle's worst-case incoming water temps. I'm using a non-condensing tankless with only ~150KBTU/hr max-fire (about the output of the 180ASME) as a combi heating boiler (with a buffering external heat exchanger) in my house with nearly 2x the space, in a location 20F colder than yours, and a heat load of nearly 35KBTU/hr, and I could still take endless showers without the tankless breaking 120KBTU/hr firing rate. In practice, with a drainwater heat exchanger and the 48 gallon buffer it never breaks 60,000BTU/hr output (measured), with all zones calling for heat and the teenager stuck in endless shower mode. The 180ASME is already overkill in your application unless you absolutely need to run two full-flow showers simultaneously on the coldest day of the year with all zones calling for heat simultaneously. The 240ASME is just more overkill at best buying you another ~1gpm of hot water performance in mid-winter.