Is that boiler a closed combustion device, i.e., have an air intake as well? If so, you probably don't want it there. The instruction manual will have the appropriate restrictions. Since a vent may not seal perfectly, even though it is an exit, there are restrictions on potential co intake.
Modcons don't like intake air with stuff like potentially chlorine, lint, or other chemicals in it, either. Those could be coming out of the vent. Again, your best thing is the installation manual and potentially, the local inspector's office, but they usually go by the restrictions in the installation manual.