During a home inspection I conducted in Denver Colorado in September 2013, I discovered this hot water heater and the loose vent connection which is pictured here. Hot water heaters, gas fireplace inserts and gas forced air furnaces all produce carbon monoxide as a by-product of the combustion process involved in burning natural gas to produce heat. It is essential that these appliances be vented correctly in order to have the hazardous gasses emitted from them exit the home. If the venting is not installed and attached properly, a condition known as back-draft can occur, in which the carbon monoxide gasses flowing from the combustion chamber of an appliance do not flow effectively into the exhaust ductwork and outside. A home inspection should involve checking for this condition around any gas appliance, and as a homeowner, or tenant, it is important to have properly installed and functional carbon monoxide detectors in your home. It is also a good idea to inspect the gas appliances periodically to make sure a situation like this is not present. If you discover something which causes concern regarding any gas appliance, contact a licensed contractor to inspect and advise you regarding any needed repair to ensure its use is safe.