Actually, this requires a system analysis to determine if it's a problem of the water heater, or a problem of your system needing a expansion tank.
For instance, if your water supply is provided with a check valve or a pressure reducing valve/regulator on its input, a water heater full of cold water will cause expansion, and without an expansion tank or an open connection to a supply (which a check or regualtor is not) to relieve pressure, pressure in the system will rise as the cold water is heated and expands.
You may be seeing this more in the winter as your water supply may be colder, so it has more potential to expand.
The valve is a temperature and pressure relief valve, so EITHER overheating OR overpressure will cause it to vent. I'm going to guess that it's more likely a pressure issue, and would probably be easily resolved by adding a small expansion tank; but I can't be sure about that without more details about your water system, or actual measurements of the temperature and pressure when it's venting.
No, the 100 Amp breaker feeding the sub-panel is not the reason for the water heater not working.
Has the heater ever made hot water, has this started recently?
The 100 Amp feed is wrong for other reasons
Others will likely talk about 'code violations,'but without knowing when and where your system was installed, those are just guesses at this time.
If the lights are on, and the buttons and dials work correctly - we will take a leap of faith and say the electronics that are not directly connected to the heat elements are working.
I am also going to suppose that the heater is wired correctly.
Also that the hot and cold water pipes are connected to the rest of the plumbing correctly, and that the needed valves are open.
So that when you turn on the water faucet in a sink the cold water to the house flows into the cold water port on the water heater, then flows out of the hot port on the heater, and comes out the sink.
This leaves at least:
power wiring to and from the connection points for the heater
flow sensor - 1
temperature sensor cold side - 1
temperature sensor hot side - 1
heating elements - 3
high temperature cut off switch - 3
scr used to control the cycling of the heating element - 3
If only 1 set of scr, high temp, and heating element work, then some hot water will be produced.
--- I will work on this some more, within the next few hours.
Electronics that are connected to the heating elements
Best Answer
You don't necessarily need to elevate it to protect it from the dirt. They make a water heater drain pain http://www.homedepot.com/p/24-in-Plastic-Water-Heater-Drain-Pan-15021/204834496 for this purpose. (I have no affiliation with home depot, they just have a conveniently searchable site). They're 1) cheap 2) solve your problem and 3) have a hose fitting to attach a line to your sump pit or floor drain.