Gas Water Heater Venting Options

Proper venting of gas water heaters is serious business. The products of combustion from gas-fired appliances can be dangerous, and even fatal to the occupants of a home, if they are not properly handled. The more you know about proper venting procedures, the better. Here, we’ll cover the basic venting categories, describe the appliances in each group and give some tips to help keep your next installation safe.

Category I

The first type of gas-fired appliance is appropriately called Category I. Water heaters that fall into this category are considered to be “standard vent.” These skyward metal stacks can be seen atop buildings leaving a trail of grayish vapors in the sky. They vent various types of Category I appliances like boilers, water heaters, and faithful old furnaces, keeping folks warm on frigid winter days.

Components of Category I

Each unit comes with a steel draft hood that is ready for connection to heavy-gauge galvanized steel pipe. This type of piping system is assembled on site by the plumber. The pipe is cut and the proper fittings are selected. The entire piping system is joined together using sheet metal screws and rigid hanging straps where they are needed.

The flue gases on this type of appliance are extremely hot, often above 400 degrees. They move slowly through the venting system due to the natural draft created by these high-temperature vapors. This type of venting system is primarily vertical; the vent stack can run through multiple levels of a building before it terminates above the roof.

Safety Precautions

Because of the high temperature of the flue gases, the vent pipe is very hot. To safely install the pipe it must be kept back a full 6” from any combustible material, like wood or cellulose insulation. For example, a 4” single-wall flue pipe would need a 16” pipe chase to maintain the proper clearance. This is not always possible, so vent stacks are often constructed of a double-layered style of piping.

This pipe maintains the same inner dimension as the single-wall version, but an outer diameter that is about 1” larger, leaving an air space between the layers of the pipe. The air space drastically reduces the surface temperature of the outer layer. This double-wall vent pipe only requires a 1” clearance from combustibles making the vertical pipe chases much smaller and safer.

While these appliances have had their place in the industry for many years, their lack of efficiency has relegated most of them to the past. Higher operating costs and high carbon emissions have brought about their inevitable removal by a unanimous vote. While they can still be purchased to replace existing installations, there are many incentives for replacing these appliances with more efficient types.

Category IV Venting System

The next group of appliances utilizes a Category IV venting system. Direct-vent and power-vented water heaters are included here.

Components of Category IV

A power-vented water heater is a gas-fired appliance with, what we call, induced draft. A blower assembly with a 115V, fractional horsepower motor is typically mounted on top of the tank directly above the flue opening. This motor forces the flue gases through the venting system. It positively pressurizes the vent piping allowing for long horizontal runs that can be terminated out the sidewall of a structure.

While the inducer forces the flue gases outside, it also pulls excess air into the burner compartment. Some of this air is used for proper combustion, but most of it is considered dilution air as it mixes with the flue gases, cooling them down, thus permitting the venting system to be crafted using plastic pipe and fittings.

Safety Precautions

White, Schedule 40 PVC pipe is the most common and readily available material for this category of venting. However, some systems, when there is an expectation for higher temperature combustion gases, will utilize gray Schedule 80 CPVC pipe and fittings for venting.

The flue gases are typically only around 100 degrees, and if you know anything about the H2O in flue gases, you know that it will condense before it even gets outside. The fittings, therefore, need to be solvent welded and watertight.

Each manufacturer will give specific instructions on the pitch of the piping and any drainage connections for the condensate that develops inside the venting system. They will also specify the pipe size, the number of elbows, the maximum length, and any special fittings or bird screens to be used to terminate the vent outside.

Birds often see these open pipes as an invitation to settle down and raise a family, but it never ends well for them. You may discover such a tragedy on your next service call for “no hot water.”

Direct-Vent vs. Power-Vent

The venting for a direct-vent water heater is very much the same as the power-vent. It is also draft induced, but rather than taking the air from the room where the water heater is located, it incorporates either a second pipe or a concentric pipe design, bringing in fresh air from outside for combustion.

Using this two-pipe system will limit the overall length of the venting system. Since the inducer motor is moving both fresh air coming in and flue gases going out, it now needs to overcome the pressure drop for both sets of pipe and fittings. The concentric style also limits the length of the vent.

This “pipe within a pipe” configuration typically uses the inner pipe for discharging flue gases and the outer pipe for collecting fresh air. Within the concentric vent, these fluids flow in opposite directions, but they never mix. The advantage of this type of water heater is it can be located in a closet or small mechanical room with no need to ventilate the room.

Tankless Water Heaters

Tankless water heaters don’t always fit into one category. Many of these appliances are Category IV, condensing with induced draft. Some fall into Category III, induced draft with higher temperature non-condensing flue gases.

These units utilize special piping, which in some cases, is only available from the water heater manufacturer. This venting system must follow strict requirements for overall length, fitting usage and will incorporate a special vent hood where it terminates outside.

Having hot water in our homes is a luxury that we often take for granted. Gas-fired water heaters can provide us with an almost unlimited amount of water for our bathing pleasure. A proper venting system is a critical piece in making sure that the installation is safe and trouble-free. To learn more about proper installation techniques and safety considerations, check out SkillMill’s Gas Water Heater Operation course.


Frank Garro

Interplay Learning Plumbing & HVAC Expert

Frank is Interplay’s resident plumbing and HVAC expert. While he began his career in the plumbing trade, his passion for HVAC led him to become an Associate Professor and HVAC Program Chair, at Ivy Tech Community College. Frank is a Licensed Mechanical, Plumbing and Electrical Contractor, and owns his own mechanical services company. As an educator, Frank is a firm believer in teaching the fundamentals and has a gift for simplifying complex concepts.