Look in your manual or search this forum and read up on the REST feature, maybe you are using it somehow and do not realize it.
It is basically a pump that pumps the coolant through the heater system core when it is turned on, using the dash button, with the ignition off. Nice feature, not being one to read instruction books I found out about it on this forum. I have learnt a lot on this forum and hope to learn more, lots of info from smart contributors.