CO detectors have little purpose unless you have old fashioned open flued appliances or open fires (of ANY fuel ).
Your new boiler is room sealed and unless severely wrongly assembled will not allow gas products to enter the room.
Tony
Yes, unless severely wrongly assemble...In our last home, we had a new boiler and system installed incorrectly. After about 18 months of behaving strangely, a 3rd party H.E noticed that the inner tube of flue, the end of which sits right on top of the boiler, was a 'plastic' type- for condensing boilders. They'd fitted a part from a condensing boiler (cooler gases) into our 'normal' non-condensing boiler. The tube melted and deformed, gases mixed and we got a perpetual flame uplift problem.
It's a long story, but it gave me the fear.
We also have a woodburner, so 4 Co detectors is quite reasonable