some times home heating unit blows cold

Same here. My furnace fan continues to blow after heating up and then shutting off. The technician says that that it does that because it continues to blow out the hot air. But the air gets cooled off too soon and so at the end it comes out cooler.

I also have the fan on at all times year round.

If your fan is on, the air will feel cold even if it is room temperature. Think of a ceiling fan blowing on you. 77F air will feel cold to your 98F body if you blow it hard enough. What type of heating unit do you have? Heat pumps are very common in warmer climates, but lose efficiency rapidly if the outside temperature gets down below 40F. At 20F, the heat pump will do well just to hold the temperature at 70F.

And not directly related to what you asked: You keep your winter temperature at 77F?! You would cut your power bill by a significant amout if you dropped that to 68-70F.

77, holy crap, turn it down a bit

The fan comes on and turns off according to the temperature of the heating plenum. The house thermostat has an anticipator that shuts off the heat source, gas or electric or heated fluid, before the house reaches "set temperature".

The fan continues to blow until the plenum lo-range shutoff temperature is reached, which as Engineer mentioned, will give air temps that feel cool to the body even though they are still supplying "heat" to the house.

Your fan not only circulates warm air, it cools the furnace. If the fan shut down as soon as the fire went out, there would probably be some internal overheating. Notice that some gas furnaces won't work if the fan fails, or there is loss of electricity.

I don't think plenum temps ever reach such high temperatures, Roger. I'd say that the fan continues to blow as an energy saving feature, extracting as much heat as possible for the living areas of the house.