Tankless water heater pressure relief valve


I understand that some building jurisdictions require that pressure relief valves be installed on tankless water heaters, but other jurisdictions do not require this. The manual for my Weil-McLain boiler does not call for one. I can understand a pressure relief valve on a vessel such as a boiler, but why put a pressure relief valve on the cold water pipe going to a tankless heater? A tankless heater is just a coiled pipe with fins. What could possibly raise the pressure to the point that the pipe explodes?
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A failure to turn off when the water stops running?
Basically, the same thing (failing to turn off the heat source) is the reason the relief valves are put on conventional water tanks.
Potentially, a tank full of superheated water is a BOMB than could take out a good sized piece of your house.
Superheating water in a pipe might also cause a mechnical failure but the potential for harm is much less because the volumn of superheat water is much less.
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Expansion. The scenario that gives birth to pressure relief you describe, is an isolated volume of fluid at a nominal temperature with no means of expansion relief (100% fluid w/no air chamber) then raising the temperature of the fluid (which increases the volume) without increasing the physical space this fluid occupies. Something's gonna give.
-zero
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wrote:

Yes, something will give when the pressure eventually exceeds the strength of the materials. However, in this system, the burner heats the boiler water and the hot boiler water heats the tankless heater which is simply the portion of the cold/hot water piping system that is coiled inside the boiler. The coil is always under elevated pressure from the heated water. Since the boiler has a pressure relief valve, that should limit the temperature of the water in the boiler and therefore limit the temperature and the pressure of the water in the tankless coil. The small volume of the coil can stand a lot more pressure than the large volume of the boiler, so how can the pressure in the coil get high enough to rupture the coil?
I suspect that tankless heater pressure relief valves are meant for systems where the heating is done by heating elements rather than by hot boiler water.
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