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Got a really old heating system, a Burnham V3 which might be circa 1982, at least that's the date on the manual that was in the basement when we bought the place. It's set up to use 1GPH with maybe a 20 year old Beckett AFG. Chamber was replaced a few years ago a day after annual servicing - will never know if they broke something but it doesn't matter. Except for that, it's never let us down. The house is 109 years old, balloon frame construction that's recently had cellulose blown in and is reasonably well sealed per blower door test.
Burning about 500 gallons of fuel a year, at $4 gallon that's $2000. If a new system will save 30%, it'll save $600 year which means it will take almost 12 years at present value to recoup just the $7000 cost of the new system.
Why bother upgrading a clunker? Is it only financially viable for a system that uses a lot more fuel?