Can a B vent pipe terminate horizontally?
No, a B vent pipe cannot be terminated horizontally. According to manufacturers, all B vent pipes must terminate vertically with a rain cap in place in order to vent properly. It is worth noting, however, that if a category 1 appliance is being vented it may be acceptable to vent it horizontally, but only with a gas exhaust fan properly mounted to pull out the gas from the flue.
The fan must be mounted on the exterior wall. Under no other circumstances may a B vent pipe terminate horizontally, though. An appliance that is approved to vent horizontally is most often not compatible with the use of a B vent pipe.