It alternates with the temperature setting. Unfortunately, I lost the manual and the model number. It was installed in 1998. Heat still working, should I be worried?

I lost the manual and the model number

Without the model number we cannot help anyway.

Heat still working, should I be worried?

As long as the heat is working no worry but if you ever need to adjust the setting on the thermostat that could be a problem.

Thanks for help. I was messing around with it an realized it needs batteries, so I replaced them and now it looks like "FL" is gone. :) Also, instructions on programming were on the inside of battery cover so overall a win win situation.

"FL" is the abbreviation for Florida. Your thermostat is telling you it needs a vacation. Or you do. Or I do. :p

Too true and unfortunately makes it impossible to search the web for an answer, all you get is links to Florida. I'd also like to say the duro dyne (parent company of durozone) has a very crappy website.

Glad you got it going!

Maybe FL means "falling level" or possibly "failure looms" :)