thermostat indicates engine is warm, but heater not working.

Hi, thank you for your time.I recently noticed this problem with the cooler weather setting in. It doesnt happen every time I use the heater, however it has happened on two recent occasions weeks apart. Thermostat reads around 240 but Im only getting outside air temp even with heat all the way up. Both times this has occured, the heat eventually came on as I was driving for some time, maybe 5 miles or so.Ive also noticed a slight smell of antifreeze after arriving to my destination. This isnt coming into the vehicle thru the vents, but when I get out of the truck, I can smell it. Again, thank you for your time. Hopefully this wont cost alot to fix...