When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
assuming the fan is blowing air, I would let the car warm up all the way then put your hand on each hose going to the heater core and see if they are both hot. I had to back flush my brothers with a garden hose hooked up to just the heater core and it took all the pressure it had to blow the crap out.