Bought a house with a 20+ yr old oil boiler and attached tankless water heater. (Needs replacing for efficiency reasons, but that's another story...) Every few hours, the boiler will turn on for somewhere between 30 sec and a minute, even though it's not in use (no water running, too warm for the heat to be on)

Any ideas what this could be? I figure it must be either a faulty thermostat, or the sensor for the tankless heater thinking that water is running.

How would I go about debugging this?

Thanks for any help!