It is possible that your coolant does not have the proper protection level and that you have a blockage (frozen water) in one of the heater hoses, or in the heater core. You will need to get the car to a warm place.
You can buy an antifreeze tester that looks exactly like a battery tester that will tell you your protection level.
i have a new blend of antifreeze and i put in 3 diferont tstats. when the car is at idle the motor temp will rise, but when i start to go (usallu on highways) the gage will fall towards cold and then i will have no heat. talked to a local guy and thay sayed that it might be a head gausket or the head it self is thei possable?? i only have 40,xxx mile son the motor.??
i doubt its a headgasket. are you sure you are installing the t-stat correctly? are you bleeding the system properly afterwards?? my temperature goes down significantly on the highway but i always have heat. it runs around 160*F when im on the highway and its cold out (granted not negative temps either).
I'd try bleeding the system again. My car always warms up in the morning, then gets nearly cold again on the highway. Once I drive the car a bit and the entire motor gets heat soaked, it stays warm no matter what speed. I think it's just waaaaaaaaaay too cold where you are for the motor to heat up properly. I wouldn't even go outside if it was that cold.
Head gasket leak seems unlikely but can be checked. Talk to a shop that can test for combustion gas in the antifreeze.
Your car will definitely run cooler on the highway when it is -15F. Normally the extra flow through the heater core due to higher rpm will still give plenty of heat unless the heater core is restricting flow - then higher antifreeze temps during standing idle will produce more heat. I continue to think the problem is with heater core.