Ford Repair: car heater, car heater, heater core

Question
Transmission fluid was leaking into the radiator on my 2000 Taurus with 158k miles. I recently replaced the radiator which solved that problem, and also replaced the thermostat and flushed the system. I waited until we got cold weather before I added coolant, but only 1/2 gallon into the reservoir (too lazy to drain the radiator). But now the heater doesn't blow hot air only warm.
I'm going to drain the radiator and add 1 gallon of coolant and hopefully that will solve the problem. If not, can you suggest anything else I should do?

Answer
The heater core does not care if it flows coolant or water or beer, if there is no heat won't make any difference. A lot of Taurus heaters around that year had serious cooling system problems, and you might have a dirty or clogged heater core. Especially after mixing coolant and ATF. You can try backflushing the heater core, or blow compressed air thru it, see if it will clear. I did a core on one recently that was completely packed with rust.

The other problem could be the blend door motor, but first check for a change from hot to cold, if that works then it's most likely a clogged heater core, or lack of coolant flow in the system. We have seen water pump impellers broken or rotted off the pump. And you should never run plain water in that car, always a 50/50 mix coolant. The different metals used in the engine and systems can cause electrolysis problems, and only coolant/antifreeze in the right concentration can protect the system to any extent.