If it is smoking it needs either the air tweaking or a new burner tip which is probably a 20 dollar part at worst you need a new swirl plate.

also 4 grand on a boiler even in the uk it would cost well less than that.

keep it if it dies it dies but don't get a new one from some one that wants to sell you a new one
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P.Allison fixer of big engines Mk2+Mk2a signed by God / Hacked by the Lord Aberdeen Scotland