Blocked Heater Core?

My 2008 Prius just passed 100k miles, so I changed the engine coolant this weekend (with some help from these forums). Things seemed to go pretty smoothly. I got about 6 quarts out of the three drains, put about 4 quarts in, ran the engine, and then added another ~2 quarts. One thing was puzzling: I ran the heat while I was waiting for the engine to warm up and the air never got hot, even after the radiator fans came on. otherwise everything seemed to be fine and the coolant level in the reserve tank has been steady, so I assumed everything was ok.

It was cold this morning so I turned the heat up on my way into work and got nothing but cold air, even when I eventually turned the temp up to max. I think this means that the heater core is blocked with sediment or maybe an air bubble? Cabin heat was working fine as recently as last week. More worrying is that just at the end of my ~25 minute commute the red triangle of death came on. I turned off the heat and it went out immediately, then flickered on for an instant a few seconds later just before I pulled into my parking space. I didn't see any other warnings so I have no idea what fault it was trying to indicate, but I'm afraid it means that I overheated the engine.

The Haynes book says I should flush the cooling system with water from a hose, but the advice on these forums is not to do that because the heater core doesn't drain properly and I'll end up with water trapped in the heater core (regular non-DI water, no less). Also, I gather that getting at the heater core to determine if it's blocked is not the easiest thing to do.

Any suggestions on how to proceed? Do I need to take it to a shop and have them power-flush the system for me?

 

Thanks for the advice, Patrick. How easy is it to hear the air bubbling over the engine revving and the blower on high speed?

I had the heater wide open for about 10-15 minutes while driving 65+ mph on the highway this morning, which seems like it would have accomplished the same thing. I didn't hear any bubbling sounds, but I wasn't particularly listening for them either.

 

Thanks guys. I ran the engine as suggested and got hot air to blow intermittently. I then realized that the coolant level had dropped. I added another pint or so and now have hot hot heat once again.

Any suggestions for checking the level in the reservoir? The container is so opaque I can't really tell where the fluid level is. I have a hard time even seeing the high-low markings.

 

I would be surprised if the reservoir is not stock, but I'm not the first owner so who knows? Anyway, translucent is a relative term. I can see the fluid faintly, but have a hard time determining where the level is. This is not made easier by the radiator cover and the general crampedness of the engine compartment. Looking in the filler cap isn't very helpful as there is a horizontal divider just below it. I guess a flashlight is the answer, although I saw someone in another thread recommend darkening the high/low lines with magic marker, which I think I will try.