2001 Ford Ranger heater won't heat when temp low

I have a 2001 2.3L that just had a similar problem. After the engine was warm, the temp. Needle seemed to be where it normally was - in the middle; but when the truck was idling (at a red light for example) the engine would not blow warm air. The fix I found was that the coolant level was low. I added coolant and it seems fine now. I guess if the coolant is to low, then at idle speed, the coolant doesn't make it to the heater core, so no heat can be picked up to circulate.