OP
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Why they are doing this makes no sense. It's a boiler for in floor heat. Incase of an emergency or situation they want to be able to kill power with mushroom head e-stop. At another location we just had 2 single pole switches as "e-stops"I think the problem here is terminology, I suspect what your client wants is 2 latching stops, there is a habit to call these E-Stops regardless but it is the circuit that protects it that denotes whether it is or not even if it is inscribed, having said that modern units that comply to E-Stops are defined by colour distinction and sometimes complemented by symbols.
A red actuator against yellow is classed as a E-Stop designation (UK/EU), if I were you after clarifying with the customer I would get a latching stop button that does not meet that colour distinction therefore will not be acknowledge as an E-Stop.
If a genuine E-Stop system is required then it is likely the boiler will not have the correct circuitry to do this, you would have to design and install yourself as a separate system that gives failsafe functionality and interrupts the boilers controls at 2 or more points allowing normal rundown procedures so as not to stop any overrun features of the boiler like fans or pumps that need to ensure safe shutdown.
I will reiterate, we need to understand what the customer is asking and why, what is the full reason for this to be implemented, if there exist no hazards to be averted and it is simply for convenience then latching 'functional stops' are a simple cheap and prefered option that just break the call signal.