You'll want more than your average CO alarm for this
While everyone should have a UL-listed CO detector to sound the alarm if "imminent hazard" CO concentrations are present, your situation (a combustion appliance mounted in close proximity to inhabited space) merits extra precautions. First, I would get a low level CO monitor in addition to your normal CO alarm if I were in your shoes. This extra unit will tell you if there are issues with your appliance emitting CO before it reaches the imminent hazard point, which is important because chronic low level CO exposure can be a significant threat to vulnerable populations, or simply make you miserable all the time with no idea what's going on.
ABS does not a gas vent make
Furthermore, using DWV materials such as PVC or ABS for venting is wrong, and potentially fatally so. The issue is that while the manufacturers can provide an appliance that keeps flue gas temperatures low enough to avoid deterioration of the flue when it is in new condition, scaling and other efficiency-hampering factors cause the flue gas temperature to rise over the appliance's lifetime, leading to plastic flues deterioriating from overtemperature and eventually failing outright.
Avoiding this is simple, though. All one needs to do is follow the Codes and use a UL 1738 listed Type IV special gas vent system; uniformly, these systems are made of stainless steel instead of plastic, allowing them to withstand the exhaust temperatures of just about any gas appliance made without taking any damage whatsoever. Furthermore, the use of highly corrosion-resistant stainless steel alloys such as AL29-4C allows a properly installed stainless steel vent system to last a long time (lifetime warranties are common), no matter how nasty the condensate gets.