Why does the 2015 Ford Escape heater blow cold while parked and idling?

What is your question?

When warmed up and moving, the heater functions normally. However when parked and idling, the heater blows cold. However, I have found that if I increase the idle RPM to over 1000 by giving the engine more gas, it begins to work normally. What would cause an issue like that. I was thinking, perhaps the thermostat is faulty, but I don't see how, unless some how the coolant is diverted away from the heater coil.

How long has your 2015 Ford Escape had this problem?

A couple of years, but it seems to be getting worse.