The worst-case scenarios happen, but are pretty rare. The vast majority of leak scenarios start out as a dribble or unexplained loss of system water (going up the flue instead of on the floor), not an inundation as the first clue. Even in the latter scenario, as long as the fill-valve is turned completely off it limits the amount of spillage to the volume of the heating system, (which has substantially less volume than the Holden Reservoir.
) In a typical baseboard type heating system that's less than 25 gallons, often less than 15.
Auto-feed filler valves are really more of a convenience than a necessity. If the system develops a leak the sizzle bang pop from the boiler as the system pressure drops becomes evident LONG before there's any safety concern. Check the system pressure (~12psi when tepid or cool), and just turn the isolation valve that's between the auto-fill and potable water completely off. Problem solved.
There are plenty of cast iron boilers that go well beyond 50 years that are not in imminent danger of leaking, but they're also not in imminent danger of becoming reasonably efficient. If yours isn't showing signs of a melt-down, it's worth figuring out well ahead of time. When you do, size the boiler for the space heating load, which is much smaller than what you need for a tankless coil, and size the indirect for the hot water load. The smallest boilers out there still make more hot water than the typical 40-5o gallon standalone tank.
It's probably worth installing an indirect on the existing system now though, and reconfiguring the controls on the boiler to idle at lower temp. Tankless coils usually require standby temps of 160F or higher to deliver reasonable hot water performance. That raises the standby losses to ridiculous levels, typically making the boiler room the warmest spot in the house, often by more than 10F. With an indirect you can drop the low-limit temp to 140F for an oil boiler, 130F (or even cold-starting) for a gas-fired cast iron boiler.
A 3x oversized boiler that was rated 83% AFUE is typically delivering no more than 78-80% when new, and maybe 75% at age 25 if it's in good shape and has been regularly maintained. If you replace it with something that's no more than 1.7x oversized (the AFUE test presumption) it'll pretty much hit it's numbers. ASHRAE recommends ~1.4x oversizing as the best compromise between comfort, efficiency, and reasonable recovery rates from deep setback.
The average single family home in MA in a statewide survey 5-6 years ago was bout 13-14kilowatts (44-48,000 BTU/hr), but that includes a lot of completely un-insulated older stock housing. A typical tightened up 2500' older house in Worcester will come in between 35-40,000 BTU/hr @ +5F (the local
99% outside design temp.) At 1.4x oversizing that "typical" house wouldn't need more than 50-55,000 BTU/hr of boiler output to have a very responsive heating system, with plenty of horsepower to spare for Polar Vortex events. What's the D.O.E. output rating on that 20-something Weil-McLain? (I'll bet it's over 100K, might even be 150K.)
With a couple of mid to late winter fuel bills and the exact meter-reading or fill-up dates & quantities it's possible to calculate with simple arithmetic the size of the replacement boiler that's guaranteed to cover your space heating load. Most boilers with tankless coils are at least 3x oversized for the space heating load, and that ends up cutting into both comfort and efficiency.