Gas Heater Cleaning, Heat Exchanger

This is a typical gas heater installation. While it does work, and has worked well for many years, you can see that there is a safety problem with running the gas line over the top of the heater.
In order to maintain continuity, we will only address the heat exchanger cleaning. Usually, the only tools required for this job are a 1/4" and/or 5/16" nut driver.

This particular heat exchanger is clogged very heavily, and does not allow for the proper flow of heat from the burners to escape to the ventilator.

This is typical heat damage due to a clogged heat exchanger.

Determine which tool is required to remove the top of the heater, and remove the screws that attach the top vent to the main heater housing. Typically, there will be 8 screws. Be careful with the sheet metal, it can cut you!

With all of the screws removed, carefully lift up the ventilation unit it separates from the base. On certain heaters, it may require a bit of force to get it off. If all else fails, recheck to see if there are any screws that you may have missed.

With the top off of the heater, you'll probably encounter something that looks like this... There may or may not be screws attaching it to the top of the heat exchanger. If so, then remove them to gain access to the entire heat exchanger.
This is what you should see, the top of the heat exchanger with the heat containment shields on top. There is usually a wire that holds them in place, (used primarily for shipping reasons), that you can probably just pull off with your fingers or cut with a pair of wire cutters. Once the wire is removed, take note of where the shields are located (for replacement), remove them and put them aside.

Look very closely at the grids (fins) of the heat exchanger, and you'll probably see quite a bit of built up "gunk" inside them. This is what causes the heater to over-heat... it literally chokes itself.

Using a directed pressure stream of water, clean the exchanger thoroughly. This normally will only require about 30 seconds to do. DO NOT USE A PRESSURE WASHER! You could permanently damage the heat exchanger!
Also, keep the stream directed straight DOWN... so to prevent the saturation of the insulation inside the heater. This is quite critical, particularly in older heaters that use conventional hard fire-brick.

Reassembly

Reassembly is simply the reverse of disassembly.
For safety reasons, you should let the heater "dry out" for about 24 hours before putting it back into operation. Because of the possibility of water entering the burners, it can create a fire hazard because the gas will be blocked from its normal path. It is recommended that you use some type of air blower to blow out all water and other foreign matter from the burner area before igniting the heater and pilot light (if equipped) again.