Do not use draino as it has aluminum in it, which it reacts with the acid to produce hydrogen which is what exploded previously. Also iron(rust)+aluminum+heat (from the neutralization reaction) = possibly a thermite reaction, which would be cool, but may also mess up your A/C system.
First, It's not the acid that reacts with the aluminium in Draino to produce hydrogen gas. It's the lye (sodium hydroxide) which is the MAJOR ingredient of Draino! If you don't believe me put nothing put Draino (with aluminium) in a glass and add water and watch! Aluminium isn't necessary for the operation of Draino, the manufacturer added it because it produces bubbles and it makes the consumer
think it's doing something! If you studied high chemistrey you'd know that lye reacts with fats to form soaps. That's how it clear drains, a primary cause of clogged drains is congealed non water soluble fats. The lye in Draino converts those to water soluble soaps. BTW the reaction product of water, lye and aluminium is hydrogen gas and sodium aluminate, go look it up.
Second, you're not going to get a "thermite reaction"! Thermite is very hard to ignite (speaking from experience!) and you DON'T have a high enough temperature for that to happen. Especially with it wet!!! What you MIGHT get would be would be an electro-chemical reaction due to the difference in the electromotive potential of the different metals. This is the exact same thing that happens when you have dissimilar metals in contact with each other. For example, I'm sure that we've all seen what happens to aluminium in contact with steel when it gets wet, especially when it gets wet with water containing an ionized compound such as the salt in sea water! In other words, any reaction that you get would be exactly the same as that of getting sea water on your engine!