The filter replacement indicator came on, on the LED display of my RTH230B thermostat. What filter is it talking about? Is there some microscopic filter in the thermostat that I'm supposed to replace? Or does the thermostat think it needs to remind me to have my furnace serviced? (Oil furnace with baseboard hot water.) Or is it telling me to replace a filter in a non-existent A/C? The owner's manual says only that the indicator comes on "after 500 hours of use."

Thank you.

Replace the furnace filter. That is what it means. If you do not have a air filter then just reset. Follow the directions in your owners manual for all settings.

Ahh, a non-problem. Thank you for your prompt response.