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?
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
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.
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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