A CO alarm is most important with open flued or unflued appliances but still well worth fitting with room sealed or balanced flue appliances as should a fault develop or a case seal fail there is still the risk that fumes could leak out.
So I would say fit one.
I have a balanced flue boiler at home that replaced an old open flued one which is just under a year old now and a similar one about two years old in a rental property and I have them fitted at both places, in fact I replaced my home one just this week.
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