I have a honeywell dial thermostat and when I turned it up the other day the heat came on for about 5 min then shut off. Now the furnace doesn't go on anymore.I took the casing off and the mercury looks good. How can I tell if it is the furnace or thermostat. Also I noticed late this summer that when I turned up the air ,it came ont only stayed on for a few min and went off also.I didn't think much of it at the time but now I wonder if I have a real problem. Is there any test to perform for checking a thermostat. The furnace is a carrier furnace about 20 years old also.I do not know how old the air is due to moving in about two years ago and everything worked great till a little time ago. Thank you

I posted a link to a site that appears to be for a do it yourselfers

HomeTips DIY: Forced-Air Heating and Furnace Repairs (http://www.hometips.com/cs-protected/guides/furnacefix.html)

I think having a professional look at before it gets cold and breaks

Down would be a good idea, if it hasn't been serviced very often

I think there is a much better site at https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html

For one thing it gives a very quick, simple way of sorting out furnace and thermostat problems.

If the furnace worked right in the past, I would think it is more likely there is a problem in the furnace than with the thermostat. Between the sticky and the good people here, likely you can find and fix the problem yourself.