I just did this flush a few days ago. Heat is better, however, watch out for your ignition coil below that lower hose. I fried my coil (it was cracked and crappy anyway) by getting some water on it.

*If you have a side room or garage above freezing, roll the garden hose up with the ends together and leave it in there for a few hours or overnight and let it thaw.

Now I have a issue with blend door/vaccum as my heat only blows out my defrost and nothing else.