A modern sensor would be a mass airflow sensor, but a sail switch that you can adjust the amount of surface area that it hits so it alerts if the amount of air isn't high enough it doesn't activate when the furnace is running.
this post was submitted on 23 Mar 2025
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Home Assistant is open source home automation that puts local control and privacy first. Powered by a worldwide community of tinkerers and DIY enthusiasts. Perfect to run on a Raspberry Pi or a local server. Available for free at home-assistant.io
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If you have your thermostat in home assistant you only need to track the run time and then have it alert you or trigger a sensor when it's been running for a set number of hours.