If this is a safety device for a boiler or unvented HW then given you need to ask what's needed to fit it, can I suggest you get a professional to install it. If not fitted correctly it may fail to operate properly and leave whatever it is protecting potentially dangerous to you and yours.
My apologies,
It was late and I treated it as a simple query about connections, rather than questioning the competency of the OP to replace C/H safety devices.
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