Gas and Oil Home Heating Furnaces - Whats the best solution to clean out drain pans and pipes?

I've been tasked at my company to go through each air handler (4) and clean out the drain pans and pipes. I would like to scrub out the internal drip pan and let the solution drain through the drain pipes to the out side to completely clean everything. What's the best thing to use for this? Bleach? CLR? By the way, the work is being done after hours so the fumes that may spread through the office is not a concern.

I'll use a foaming action alkaline based coil cleaner that are sold to the trade and apply with a pump up pressure sprayer. [like an insectside sprayer] The dirt gets impacted in the depth of the coil and so multipke appliocations and thorough flushing are the keys. The sparyer lets you get the cleaner into the coil depth. Clean from both sides if you can gain access. I use a shop vac to get all the gunk out of the drain pan and shove water and cleaner down the drain line with the hose nozzle. Be sure to check the blower wheel for dirt. Often, I'll scrape the dirt out of the curved surface of each blade and vacuum it away. Be sure to use eye protection and heavy rubber gloves with the cleaner. You also want good ventilation in the work area.