The easiest way to determine if you have a conventional furnace is to check if

your furnace has a draft diverter, Conventional furnaces

have a draft diverter that assists in venting the combustion products

 up a chimney.

Similar to conventional furnaces, mid-efficiency furnaces vent the combustion

products up a chimney, except they do not have draft diverters, as can be

seen when comparing the furnaces. Instead of a

draft diverter, mid-efficiency furnaces have induced draft fans which force air

up the chimney when the furnace is running. This reduces the possibility of

back-drafting occurring through the furnace when the furnace is not running.

High-efficiency furnaces do not vent

the combustion products up a chimney.

Instead they vent the combustion products

out a side wall of the building usually

through a black (ABS) plastic pipe,

High-efficiency furnaces often have a

second black plastic to provide air

from outside (combustion air) to the

furnace (blue circle). The other large

difference between high-efficiency furnaces

and the previous two types is

that they will always have a drain system

of some kind to allow

the moisture that condenses during the

burn cycle to be routed to the drain

(usually a floor drain).