My current boiler is more than 70% efficient so it's not possible to
save a third of my energy costs.

Plus, the payback period you calculated ignores the cost of money. I
can get at least 3% interest now in a bank and that will eventually
rise. And I may sell the house in a few years...

If it breaks I'll replace it. With any luck not on a 1 degree day
(the coldest it's gotten here, by the way, in the last decade is 3
degrees). I own three 1500 watt space heaters.

Of course a new one can break on a 1 degree day too. My parents'
house has the original boiler. From 1948.

But thanks-- I have sent an email to an insulation company to ask
about using fiberglass to insulate the boiler.