Water heaters often burn fuels — natural gas or propane — to heat water. Burning fuel produces gaseous by-products that must be exhausted out and away from the home. In this article we cover the basics of proper venting and the three most common venting systems: standard atmospheric venting, power venting, and direct venting.
Standard Atmospheric Venting
Hot gases naturally rise. Atmospheric venting takes advantage of this tendency, carrying the gases upward and outside through the vent tubing without forcing them in a direction they don't want to go. Because it doesn't fight nature, standard atmospheric venting needs no fan or auxiliary equipment, which makes it the most common type of venting.
Proper construction matters for both safety and performance. Key considerations include:
- Materials: Exhaust gases are hot and corrosive, so use proper steel or stainless-steel exhaust piping that won't degrade.
- Fasteners: Use appropriate hangers and fasteners to support the vent.
- Pipe size: Follow the manufacturer's recommendations and local codes so the vent can handle your appliance's capacity.
- Vent cap: A cap where the pipe exits the home prevents water intrusion and backdrafts.
Vent ducts must provide a path for exhaust to rise up and away. Run ductwork vertically through the house and roof where possible. Horizontal runs must pitch upward — a good rule of thumb is at least ¼ inch of rise per foot, with the horizontal distance no more than 75% of the vertical distance. Keep turns and bends to a minimum, since each one restricts flow; check local codes for the maximum allowed.
Our Top Picks
Rinnai RUCS65iP Tankless Gas Water Heater
Modern condensing tankless units support flexible direct- and power-venting, often through a side wall with inexpensive PVC.
Final Editorial Verdict
Direct Venting and Power Venting
Direct venting is used in air-tight rooms where there may be insufficient combustion air. Direct vents both push exhaust gases outside and bring fresh combustion air into the room, using special coaxial piping with separate chambers for intake and exhaust. This type can run completely horizontally straight out through a wall, giving flexible installation.
Power venting uses an electric blower to push exhaust gases through the venting. The drawbacks are the added noise of the blower, the higher cost of a power-vented heater, and the extra energy to run the fan. But the advantages are significant: because a fan moves the exhaust instead of natural convection, the vent doesn't need to be vertical. The unit can be installed almost anywhere power is available, the venting can be much longer and fully horizontal, and — because the blower cools the exhaust as it moves — cheaper PVC can be used instead of metal.
Whenever you install a direct or power vent horizontally, make sure the vent turns down toward the ground outside to prevent water intrusion.
Conclusion
Venting choice depends on your home. Standard atmospheric venting is simple and common but needs a vertical path. Direct venting supplies its own combustion air and suits tight spaces. Power venting offers the most placement flexibility at the cost of some noise, energy, and up-front price. In all cases, follow the manufacturer's specs and local codes — proper venting is a safety essential, not an option. Inspect your vent connections once a year for rust, loose joints, soot, or signs of backdrafting, and clear any bird nests or debris from the termination cap. And because gas combustion can produce carbon monoxide if a vent ever fails, install a working CO detector near the water heater and on every level of your home — it is a cheap, life-saving backstop that every gas appliance deserves.