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I happen to have a few small 3/4" F&Ts that are in great shape but for failed thermostatics. A Tunstall capsule and some gasket material and we're good. I really appreciate all the guidance.

How big is a box? Actually, that's a good life for a boiler — but a well cared for one, particularly an older one, can last longer than that. Sometimes a lot longer. On the other hand, a more modern, more efficient boiler — with lighter and thinner castings — operating on poorer water quality, or with leaks in the system, may only last half as long.

Steam. You're in luck… it's rare for a boiler to fail catastrophically (piping can, but rarely a boiler). Usually they will give you some warning — sometimes a lot of warning — and what you will see is an increase in water usage. You may also see steam escaping from the chimney.

Now this isn't really good — but the good news is that steam systems can run on absurdly low pressure, so even if you are losing a lot of water and the leak is really gross, you can still run the old thing and get enough time to organise a replacement.

You don't really want to do that if the leak is from the steam side to the boiler room — the humidity can do some damage — but even then it's not an instant no heat situation.

Now you can evaluate what size boiler you need pretty much any time, at your leisure. That's because steam boilers are sized to match the connected radiation, and that — usually — isn't going to change. So take some time any time and write down what you have for radiators and their ratings and add them up.

Replacing a boiler is an excellent time to work on odd piping — but keep in mind that all steam boilers have a recommended minimum piping arrangement for the near boiler piping, so you are really only concerned about weird things elsewhere.

It doesn't hurt to establish a relationship with a good steam boiler man in advance. You need one anyway, to maintain what you have correctly. Might as well start now… but there's no need to get a quote on a replacement until you need one. Prices — and available equipment — just change too fast.