Direct Vent Heater – A direct vent heater has a sealed air intake system and exhaust system creating an air-tight, closed-loop combustion chamber to produce heat. Horizontal piping passes through an outside sidewall to draw in combustible air and expel burnt exhaust gases. This arrangement avoids the necessity of having a vertical flue through the roof, prevents back-drafting of dangerous gases into the house, and results in a more efficient use of fuel. The piping system can either be two separate pipes, requiring two separate holes through the outside wall, or it can be co-axial, where one pipe is inside the other, creating the necessity for only one hole in the outer wall. Many people prefer this type of heater since it does not rely on inside air to support combustion in the chamber, using one of its pipes to pull air in from outside.
B Vent – B Vent Heaters vary from their direct vent counterparts in that these heaters use a conventional flue system to vent gases up a chimney, something similar to the design of a wood burning stove or fireplace. The flue expels the burnt gas through a hole in the wall to keep any dangerous gases (most commonly carbon monoxide) from remaining in the home. These heaters can be connected to an existing flue or chimney, to make switching from your old fireplace or woodstove a breeze. Compared to Direct Vent Heaters, there is only one pipe, meaning these heaters utilize inside air to support combustion.
Vent-Free – Vent-free heaters do not have a ventilation system, but there is some differing opinions on the safety of these heaters. Vent-free heaters still burn gas emitting harmful fumes (carbon monoxide) during combustion. In a well ventilated home, it's rare the emissions would exceed a lethal level, but it is still possible. Taking precautions to allow some venitlation into the home is highly recommended, especially if the heater is used for extended durations. Vent-free heaters are generally more efficient in fuel use and in heating than Direct Vent and B Vent heaters because no heat is lost in ventilation, and remains radiated throughout the room. Their higher fuel-efficiency helps reduce your heating bills as well.