I've spelled out a lot of it in
this recent blog piece but I'll review it specific to your case here.
To analyze the cast iron you need to estimate three numbers first:
1: The total square feet of
equivalent direct radiation.
2: The total water mass/volume in the system, including the distribution plumbing and radiators. (For half inch copper it's about 1 gallon (8.34lbs) of water per 100' of pipe. )
3: The thermal mass of the cast iron in the system.
In an ideal system the ratio of the output of the boiler at minimum fire will be less than 50 BTU/hr per square foot EDR. At 70 BTU/hr per foot the thermal mass of the system may keep it from short-cycling and still run at reasonable efficiency but it won't magically turn it into a right-sized boiler that modulates.
You may have to look up the water volumes and shipping weight of
new radiators of similar size & shape to figure out the water volumes. The relevant number for both is the weight/mass. The specific heat of water is 1 BTU/lb per degree-F, the
specific heat of cast iron is 0.11 BTU/lb per degree-F. So to convert the mass of the iron to water-thermal-mass equivalent, multiply it by 0.11, and add it to the water mass.
As an example case, say you have a zone with a half dozen Sunrad or Burnham Radiant type radiators that add up to 225 running inches, and have 80' of total supply & return plumbing, all half-inch. According to Burnham's specs there is 1 square foot EDR per running inch, and 0.15 gallons per every 2.25" section, which is comparable to the
OCS Cast Ray. Shipping weight on the Cast Ray is 5.1lbs per 2.25 section, so we'll use 5lbs/section as iron weight.
Plumbing water mass: 80' of pipe /100' per gallon is 0.8 gallons, x 8.34 = 6.7 lbs.
Radiator water mass: 225"/2.25" per section is 100 sections, x 0.15 gallons/section = 15 gallons, x 8.34lbs/gallon= 125 lbs.
Radiator cast iron thermal mass: 100 sections x 5 lbs/section is 500lbs of radiator, x 0.11 = 55 lbs water-equivalent.
Total thermal mass: 6.7 + 125 + 55=
187 lbs of water-equivalent.
Total sq.ft. EDR: 225" x 1 sqft/inch= 225 square feet EDR.
Ideally the boiler's min-fire output would be under 50 BTU/ft-EDR, which would be 225' x 50= 11,250 BTU/hr, in which case you'd be able to run at mid-90s combustion efficiency continuously, without cycling the boiler on/off. If that's the case the boiler is sized reasonably for the radiation, and you should be able to tweak the system to get it to run high
But let's say it's the NCB 240, which has a min-fire input of 18,000 BTU/hr, and at 95% efficiency would be delivering 0.95 x 18,000= 17,100 BTU/hr. (That would be a ratio of 17,100/225' = 76 BTU/hr per foot EDR.) The rads are only emitting 11,250 BTU/hr which means there is (17,100 - 11,250=) 5850 BTU/hr of excess going into the system. When that happens the temperature will rise until the boiler's internal controls sense that it's over the outdoor reset setpoint and quench the burner, and wait for the rads to emit enough that the temperature drops back into the right range before re-firing the burner.
A typical mod-con will have somewhere between a 5F and 10F difference in temp between the burner-off and refire, and this is where the thermal mass of the system comes into play. It takes 1 BTU to raise the temp of a pound of water 1 degree F, 5 BTU/lb to raise it 5F. So the BTUs to raise the 187lb thermal mass of the system 5 F is then 5F x 187lbs= 935 BTU. At the min-fire excess of 5850 BTU/hr that takes about 935/5850= 0.16 hours, or 9.6 minutes.
That is plenty long, not even close to a short-cycling situation, and would be plenty of radiation + mass to get 95% efficiency out of it. But you can see that as radiation EDR shrinks, the excess BTU is more, and the thermal mass less, and with much less radiator than that the minimum burn times would shrink an order of magnitude. You need burn times of at least 3 minutes and fewer than 5 burns an hour to be in the "reasonble" range. If it looks like 10 burns/hr or more at min-fire it's going to take a toll on equipment longevity and efficiency.
If you run this napkin analysis on your system and it's close, be sure add in the water volume inside the boiler as well.