If the control line for your furnace is just a 24V loop (or a dry loop), a good hint is that you can keep a traditional thermostat in parallel with your 'smart"

If the control line for your furnace is just a 24V loop (or a dry loop), a good hint is that you can keep a traditional thermostat in parallel with your 'smart" one - so that if either closes the loop, the furnace fires. Then you can aim for "comfortable" with your "smart" one and "safe" with your real one.