2002 Ford Ranger 2.3 Heater

It's been chilly lately by Tucson standards, nights in the 30's, and in the A.M. I find that when I'm standing somewhere at idle for more than a minute or two, the heater blows cool-to-cold air. (As long as I'm driving it performs normally.) Best I can recall this wasn't the case in previous winters -- idle speed still generated enough heat to keep things comfortable. My limited knowledge tells me it's probably some kind of thermostat or temperature sensor issue, but I'd appreciate some more specific guidance before I try to do anything about it. (1) is it in fact abnormal, (2) will it do any damage to the engine, and (3) if I need to fix it, how?