overflow valve constantly leaking

It's a boiler, not furnace (just so you know). Overflow ...means pressure relief, probably.

In 1928, it was common to hide the exp tank in an attic space. Whether it's hidden away or there really is none, you need one. Today's exp tanks are quite small and can added right near the boiler.

Jim

There may have been a tank in the attic that has been removed? It was also once common to connect the boiler to a high mounted toilet tank that would take the expansion and drop it to the overflow when necessary. I've found this at times. When the toilet got replaced with a modern unit they just capped the pipe and now there's no expansion provision. It wouldn't be hard to add one.

Dan

Jim

There may be an expansion tank hidden by later construction. If so, it is full and needs to be drained. If you don not have an expansion tank, one should be added. They make them to fit between floor joists above the boiler area.

BB

I am not a heating expert, but I imagine what you call an overflow valve is a pressure relief valve, water expands when heated, and without an expansion tank, just enough goes out the relief to (hopefully) keep the pressure from going too high. There is a forum called "The Wall" on