It is highly recommended that new furnace installations be accompanied by a double pipe system with combustion air taken directly from the outside. This venting system provides more controlled combustion as ambient air in the mechanical room is no longer relied upon for combustion. Furthermore, any holes in the building enclosure that were previously used to provide combustion air can possibly be sealed (if they do no affect other naturally aspiring combustion appliances), decreasing air leakage heat loss and increasing occupant comfort (reduction in unwanted drafts). These changes improve overall home energy performance and increase the service life of the new furnace equipment. A two pipe system can reduce several common heating/ventilation issues including air leakage, incidence of unwanted smells, and equipment performance problems related to building depressurization.