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..... I have an oil-fired boiler at my vacation home. I employ several methods to monitor the heating system & temperature of the house.
I recommend (for those with a modern Honeywell R7284 controller on their burner) a little-known Honeywell Alarm Module (#W8735S3000). It closes a contact if it detects any system faults on the data bus. It's pretty cheap (around $45) and gives you a few hours advance notice before the house temperature drops. I have mine integrated into the house alarm system which notifies me of any number of adverse situations. Solid and reliable.
The Carlin controller (#70200) has the contacts already built in; making life a little bit simpler.
J