I had a car that I could not get heat when it was idling. It finally came to a head and my low coolant light would come on. Filled it up and 10 minutes later it was low on coolant. Car ran fine too. I could not find a leak and because it lost fluid so quick, I suspected a bad head gasket. Checked compression and found 2 cylinder lower.
Changed out my head gasket and she is running great and I have heat. Just relating my experience for some where this could be the case for them. I bought the car not running hoping to put a motor in it, but got it running because of a bad fuse and 9000 miles later I finally had to do the HG.