Gas is metered in CCF and billed in CCF, therms, or decatherms, not dollars. Seriously- run the fuel use load calc using exact meter reading dates and heating degree-days for those time periods.
Some online
calculators are
much better than
others, and none are as good a professional Manual-J tools used in the hands of a qualified professional. To get reasonably correct results out of LoadCalc or CoolCalc requires being very AGGRESSIVE about the air tightness, R value & U-factor numbers rather than the impulse to conservatively low-ball the R-values and assume high or even moderate air leakage.
An atmospheric drafted or power drafted water heater runs about 80% steady-state efficiency. So 38K-in means it's only delivering 0.8 x 38K= 30,400 BTU/hr into the water. If in it's worst case it's running 40 out of every 90 minutes that's a 4/9= 44% duty cycle. At 30,400 BTU/hr and a 44% duty cycle it's only delivering 30,400 x 0.44= ~13,400 BTU/hr
at most to the radiators. So even if you were only actively heating half the house your heat load is nowhere near 45K.
Replacing the water heater with a stainless steel modulating condensing commercial water heater would improve the overall efficiency, and would be a better choice than any NCB due to it's self-buffering thermal mass making it impossible to short-cycle. HTP's "Light Duty" version of their Phoenix water heaters has a 76KBTU burner with a 3:1 turn-down ratio.
It's not the heat load that is too high, but rather the minimum firing rate that is too high, and the thermal mass too low, leading to potential short cycling with lower condensing temperature water. The minimum firing rate of the NCB-180 is 14K, which is probably still more than half your likely design heat load, the NCB-210's is 18K, which is probably more than 2/3 of the design heat load. That is a big difference if it were on the edge of being a good fit, but neither is is even close to being a good fit for your heating load. But the 150K max firing rate of the NCB-180 really doesn't quite cut it for a 2 bathroom house in a location where the incoming water temperature in winter can drop as low as 40F, (which it can in much of King County).
Even the NCB-210's max of 180K isn't great, but since it's minimum firing rate is the same as the NCB-240's, if you can demonstrate with the napkin-math that it won't short cycle on your radiation that would be the better choice, if you're dead-set on going with a low-mass combi boiler.
To really get the best comfort & value out a modulating condensing boiler, operating it under outdoor reset control and having a modulation range at the low end that's something like 1/4 the design heat load would be more appropriate. Otherwise you might as well go with a condensing water heater.
I'm not going to look up the output specs on all your radiators for you, but
you should. If you want to total up their output at 140F (I believe they all have a 140F spec) and report back we could use that to infer to expect the behavior of the NCBs to be when the average water temp is 110F or 100F, and with a more accurate load calculation we can also use it to get a good starting point for setting the outdoor reset curves.