What do you mean you ran it for half an hour and never saw the thermostat open?

If the temp gauge doesn't indicate overheating, and the engine runs fine, and it's cold outside, and I'm only idling it with no load in the driveway, and I've got the heater on, I wouldn't expect the radiator to feel hot. You're not generating much heat, and the heater core can disipate it.

Go drive it on the highway for 10-20 minutes, pull over, and feel the upper and lower radiator hoses. The top one should be quite hot, and the lower one warm.