Furnaces that have a combustion blower, which may be located either upstream or downstream from the heat exchangers. If the blower is located upstream, blowing the combustion air into the heat exchangers, the system is called a forced-draft system. If the blower is downstream, the arrangement is called an induced-draft system. Power combustion systems have been commonly used with outdoor furnaces in the past; however, more indoor furnaces are being designed using this concept.