Peerless gas boiler replacement?

I'm just a homeowner/engineer, not a heating pro. If it was me, I'd ride that horse until it broke. Meanwhile, you have time for planning a replacement.

Number one thing is to know how much heat you actually need. Boilers are often oversized because installers don't take the time to do proper heat loss calculations. You can get a rough idea of how much boiler you actually need by telling us (a) city/state you live in, (b) square footage of your house, and (c ) when the house was built, how much insulation in walls/attic, and what type/vintage windows.

Agree with the above posts. If it has been combustion tested and is burning ok and not leaking the boiler can last at least another 10 years.

You could get a quote, sized a new boiler correctly put the money to the side and run the old one. If your home can be tightened up, insulated and windows replace if needed do that first.

https://forum.heatinghelp.com/discussion/comment/1851435#Comment_1851435

That's good. So your heat loss on a design day is probably 20 BTU/hr/sq ft or less. That's under 40,000 BTU/hr total for the house.

If your boiler is not also making domestic hot water (ie you have a separate water heater that isn't connected to the boiler), you could get away with a smaller boiler than you have now, but the one you have is not grossly oversized. ASHRAE recommends sizing to 1.4x design heat loss, so you would probably end up sized around 60,000 BTU/hr.

Also, be aware that the main efficiency gain of a modulating-condensing (mod-con) boiler happens when you can heat the house with low-temperature water that allows the boiler to condense the flue gas vapor, and that depends on how much radiation you have. A 1960's house with baseboard radiators probably needs high temperature water on cold days, which means that for most of the heating season you might not get the full efficiency benefit of a mod-con boiler. And mod-cons are more complicated and potentially more expensive to maintain and service. So you could wind up with the worst of both worlds: not getting the efficiency gain, but getting the added complexity and maintenance cost.

My brother's small gas HW boiler went 65 yrs! Mad Dog

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I can remember and old forced hot water system that was installed in the 30s or 40s that we had to cut into the supply pipe to change the flow check. This system probably hardly ever had to have water added to it. This was in the 70s so it was 30-40 years old. The steel pipe on the inside was like a brand new pipe. CI should last longer although a boiler is subject to more expansion and contraction.

just make sure there are no leaks in the system so very little makeup water gets added. fresh water with its dissolved oxygen and minerals will kill a boiler.