How do I fix Hardie hot water system C32 that is hissing and leaking?

SOURCE: noritz tankless water heater gives error code 11

This worked for me in the same situation. There is a filter inserted into main gas/ water control valve that senses water flow restriction. The sensor is connected to the gas valve. If it senses inadequate water flow it tells the gas valve to shut down so the water in the heater won't boil. Very expensive part. If you do find the filter clogged, a sediment water filter shoud be install on the inlet side of the water piping.

SOURCE: storage hot water system sounds like its crackling

check your thermostats and see if they are set to highest setting. Sometimes if theya re they will pop and crack when filling back with cold water.

SOURCE: Have a GE GG40T6A gas hot water tank. Leaking

12 year old water heater is getting near the end.
Some tanks last 20 years, but that's the exception.
To find exact age of your brand water heater:
http://waterheatertimer.org/pdf/WaterHeaterSerialNumberDecoder.pdf

Leaking TP valve:
1) Lift TP lever and gently lower back into position to see if that stops leak.
Occasionally something gets stuck in the valve.
2) Sometimes water heater temperature gets turned up because heater is not producing as much hot water as before, and higher temps cause TP to release pressure because of overheating.
Dial temperature down to 120 degrees.
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html

3) If steps above don't fix problem, replace TP valve >> TP valves have 125 or 150 degrees rating >>> older tank shouldn't be running above 125 because higher temps cause more pressure on tank, so thermostat should be lower than 125 degrees TP rating.
Buy replacement TP at any hardware store + a roll of teflon tape to seal pipe threads.
Turn gas control to pilot ... this will keep pilot lit but burner will not fire.
Let water cool, and then drain some water out of tank.
If TP valve is on side of tank, then open drain on side of tank and drain water until it is lower than valve level. Open bathtub spigot to let air into line.
http://waterheatertimer.org/Water-heater-will-not-drain.html
Be careful around water that is higher than 120 degrees F >> you can get burned quickly >> apply cold water to any burn.
Use pipe wrench and unscrew TP valve counterclock-wise.
Install new TP and put teflon tape on threads.
Re-fill tank completely and open tub valve on hot side until water is running >> then turn gas control back to original position.
Do not fire up any water heater before tank is completely full of water or this could damage tank.

To read more preventative information about water heater leaks and age problems:
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html

SOURCE: I have a hot water heater that when running seems

The most likely problem is that the thermostats are set too high. Remove the thermostat cover plates and look to see where the thermostats are set. The range should be between 120F and 135F. Liquids expand with heat so the higher the temp, the high the pressure. Decrease the setting on the thermostat until the leak stops.

The other likely cause is a head of air trapped in the top of the tank. After drawing hot water, it wiill be displaced by the incoming cold makeup water. This will compress any air in the top of the tank. As the water heats, the air will heat and pressure will build - possibly causing the leak. You can purge the tank of air by opening the pressure relief valve until water comes out (which I assume that you have already tried).

SOURCE: I have a 135 litre, 10 year old Rheen mains pressu...

usually on the hot water piping, It starts to rust closed faster than the cold side, due to electrolosis. What you r discribing are the first signs of this happening. only solution is to replace the effected pipes..