I have a janitrol fh-100-15 oil burner.It lights up and runs for a few moments,shuts down,ignites again.(This may be normal for fuel oil?)Anyhow,the high limit on the flue trips during the night and in the morning-nothing.

Check to make sure you don`t have anything causing the exhaust/vent/flue clogged or closed off,, first thing to check,, also check filter,and look for possible closed or covered vents, supply and return

It probably could stand a good tune up and efficiency check and adjustment to start with. The item on the flue is probably a stack safety control instead of a high limit and that also indicates the need for a tune up.

I really appreciate your advise.Any idea how old this unit is w/o knowing serial?

Another thing,I have an ICP furnace.It ignites,runs 4-5 minutes then shuts down.The flame roll-out on the burner assbly trips.I've replaced it,cleaned a-coil,cleaned blower wheel,checked flue.Even replaced igniter assbly.It still ignites,runs 4-5 minutes then trips.