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What you're describing sounds like a probable draft pressure problem. Did your guy ever put a manometer on the exhaust?
The only way to diagnose this problem is when the unit doesn't fire up. If you have a multi-meter I can tell you which terminals to probe to test the voltage
That's a distinct possibility. It sounds like your draft may be borderline. Which could be due to a weak fan or an obstruction in the exhaust or intake. The procedure is to probe the NO terminal on the pressure switch and B2 on the L8148 (the aquastat relay). If you don't have 24 vac there then the draft differential pressure must be checked with a manometer.
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