Geez: The event is caused by the sidearm heating the water in the tank to higher than normal temperatures - and the water expands more than normal. It is recommended that water heater controls be set for a minimum temperature of 130 degrees to control bacterial growth, and my water heater is set at 140 in the non-OWB season. The sidearm has the potential to raise the water heater temperature up to 160, 170, 180?....it all depends on how hot your OWB is and how much domestic hot water you are using.
mimilkman1: The potential for an expansion problem is increased when you have a household that uses lots of hot water. If the family runs the water heater out of hot water - the water heater has ingested a significant amount of 50 degree water from the well, cistern or public water supply. This water will soon be heated up to 160 degrees or so and will expand significantly. If you don't run your water heater out of water and the water inside stays nearly hot - the expansion will not be a problem. It will also not be a problem if you have a well or cistern pump with a storage tank that has an air space to provide water pressure when the pump is not running - as the expanding water can just go back into the tank. It will also not be a problem if you have public water without any pressure regulator in the system - as the expanding water can just to back out into the service line. Expansion is a problem for homes with pressure reduction valves because they act like check valves and will not allow any flow backwards - and expanding water then builds pressure in the house lines and water heater. Sometimes people will find that the valves on their toilets open when the pressure builds too high, and these will start flowing before the T&P valve releases.