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Its wired incorrectly or you bought the wrong model. Or another device such as an aquastat is calling for heat, or has a set of electrical contacts inside fused closed (burnt contacts).... there can also be problems with any printed circuit boards or their integrated circuits (chips).
Sorting out which can be a challenge.
Tell me a lot about your technical background and I will let you know what I think about our chances of walking you through a diagnosis... that will not be an answer at this point. So please hold your rating unless or until I manage to get this resolved for you.
If we go further I will need photo's of all the relevant diagrams in the system, attach those using the paper clip icon at the right end of the tool bar.
Or you can tell me which direction you are from the center of the nearest city and I can see if there is a good boiler controls tech close by.