[folivier]

Another thing to check is turn off aqua hot and let it cool down. Then open the cap on top of the metal tank (not the plastic overflow tank) and make sure the coolant level is to the top of the neck for the cap. Might be best to replace the cap, it's a standard radiator cap available at most auto parts stores. If the coolant level is low then top it off with standard 50/50 green coolant and change the hose going to the overflow tank since that probably has been leaking air.
This along with Tom's suggestion are the most common issues for the water not getting hot enough.