I want a regular programmable thermostat to turn the heat on/off, but I also want to be able to use the fan to circulate fresh air w/o heat occasionally. The fresh air would NOT be on a program, I would manually control it as needed. What description, what kind of thermostat should I be shopping for?

I have an ordinary natural gas furnace with ducted ceiling vents to each room. The heat is controlled with a wall thermostat. There is no cooling system.

Many thanks for any help you can provide.

If you have a t'stat already installed, just use the FAN switch. It should say ON and AUTO. Push it to ON and it will circulate the air. You can also turn the Heat to OFF and it will not cycle, only fan.

How old is your furnace? If it really old, you may not have the option of running fan manually from stat. If a newer furnace, you should be able to do this no problem. However, you will need a conventional thermostat with a fan on, fan auto setting and at least three wires from the thermostat to the furnace. If you only have two wires to the stat, it just won't work, you will need to pull another wire.

I have an added question... Are you trying to bring fresh outside air in or just circulate the air from within the home? Both current answers are great if your just trying to circulate existing air.