flushing a heater core

Also, if your hoses are not the same size, you can usually tell if they are not hooked up right. You can run the coolant through in either direction, but may get some leakage if you have a big hose on a small nipple (that didn't sound right). Check that your heater core is sealed up (no gaps or cracks to let cold air into the plenum) and also that all the doors are open to let the heat into the car. Also, if you have a new core, but used the old hoses, they may have "swelled" inward and reduced the diameter and flow. If you haven't flushed your cooling system in a while, you may have crud restricting flow.