Tankless Water Heater Venting: Choosing the Right System and Installation Tips

Tankless water heater venting is a crucial aspect of installation that ensures the system operates safely and efficiently.

Proper venting, along with professional installation, ensures the system runs safely and effectively, preventing issues like backdrafting, carbon monoxide leaks, and ensuring optimal performance.

In this guide, we’ll explore the essentials of venting for tankless water heaters and why it’s vital for both safety and efficiency.

Do Tankless Water Heaters Require Venting?

Yes, gas-powered tankless water heaters require venting to safely expel harmful exhaust gases produced during operation. These units burn natural gas or propane to heat water, releasing byproducts such as carbon monoxide and nitrogen oxides (NOx). Without a proper venting system, these toxic gases could accumulate indoors, posing serious health risks.

Electric models, however, do not produce exhaust and therefore do not require venting, making them a simpler option for installation in homes without venting infrastructure.

Gas tankless water heater venting systems are designed to direct exhaust gases outdoors, ensuring safe and efficient operation.

Indoor heaters typically use either a power vent or a direct vent system, depending on whether they draw combustion air from inside or outside the home. Outdoor models, on the other hand eliminate the need for venting entirely since they release exhaust gases directly into the open air.

These are some of the reason why choosing the right venting setup based on your unit type and installation location is critical for both safety and performance.

Types of Venting for Tankless Water Heaters

There are several types of venting systems for tankless water heaters, each designed to suit different installation environments and ensure safe, efficient operation. Understanding these options helps you choose the right system for your home.

Direct Venting

Direct vent systems can utilize two separate pipes or a concentric pipe (pipe within a pipe).

The two-pipe system uses one pipe to pull in outside air for combustion and another to release exhaust gases, creating a sealed system that is ideal for maintaining indoor air quality. Such a setup ensures combustion gases do not mix with indoor air, enhancing both safety and efficiency. However, it requires more complex installation due to the need for two heat-resistant pipes.

The concentric system features an insulated intake pipe surrounding the exhaust pipe. This setup keeps the vent cool to the touch, minimizing burn risks and offering a safer, space-efficient option. Even if the exhaust pipe leaks, toxic gases remain contained within the concentric vent.

Power Venting

Power venting uses a single exhaust pipe, with combustion air drawn from the room where the heater is installed. A fan helps expel the exhaust gases, making it suitable for spaces with good ventilation. This system is easier and less expensive to install than direct venting but requires an electrical connection to operate the fan. While effective, it can be slightly noisier and less efficient in tightly sealed homes.

These models need adequate airflow and should be installed in large rooms or connected spaces with provisions for fresh air. If installed in a confined space (less than 50 cubic feet per 1,000 BTU/hour), two air openings are required, positioned at specified distances from the floor and ceiling.

Indoor vs. Outdoor Models

Indoor Models

Indoor tankless water heaters are designed for installation inside the home or in sheltered areas like garages. Such models must vent exhaust gases outdoors to prevent safety risks like carbon monoxide buildup. They require a dedicated venting system that directs exhaust through a pipe to the outside. Installation needs to account for proper airflow and clearance to ensure safe operation. Some indoor models can be adapted for outdoor use with special kits, though this typically involves more complex setup and higher costs.

Outdoor Models

Outdoor models are built to be installed outside, eliminating the need for additional venting systems. These units feature built-in vent slots that allow exhaust gases to exit directly into the open air. They are ideal for warmer climates and are a great space-saving option since they don’t take up room indoors. However, they may require weather protection to shield them from the elements, especially in regions with colder temperatures where freezing could damage the unit.

Related: Outdoor vs. Indoor Installation

Materials for Tankless Water Heater Venting

Choosing the right venting material ensures safety, efficiency, and compliance with building codes. The most common materials include stainless steel, PVC, CPVC, and polypropylene, each with specific applications.

Stainless steel is a durable and heat-resistant option commonly used for gas-powered tankless water heaters. It handles high exhaust temperatures effectively, making it ideal for direct and power vent systems. While reliable and long-lasting, it tends to be more expensive and requires professional installation due to its rigidity.

PVC (Polyvinyl Chloride) is a lightweight and cost-effective material suitable for high-efficiency (condensing) water heaters. These systems produce cooler exhaust gases, allowing PVC to be used safely. Although affordable and easy to install, PVC is not recommended for systems with high-temperature exhaust.

CPVC (Chlorinated Polyvinyl Chloride) offers slightly better heat resistance than PVC, making it a good choice for venting systems that operate at moderate temperatures. It is often used in mid-efficiency systems but shares similar cost and installation benefits as PVC.

Polypropylene is a flexible and heat-resistant plastic designed specifically for modern condensing tankless water heaters. It resists corrosion and is easy to work with, offering an excellent balance between durability and cost. However, it is less common and may be pricier than PVC.

Additional Considerations

Non-condensing models require Category III or Category IV stainless steel vents due to high exhaust temperatures. Condensing models allow the use of PVC or PP pipes, which are more affordable and easier to install.

Each manufacturer specifies the maximum vent lengths and limits the number of elbows, as these shorten the allowable vent distance.

If one tankless heater cannot meet your home’s hot water needs, multiple units can be combined using a manifold to share a single vent system. This minimizes wall openings, saving time and money during installation.

Typical Vent Installation

When installing a tankless water heater, proper venting is essential to ensure safe and efficient operation. The most common vent sizes for these units are 3” and 4” in diameter, which connect to an adapter included with the heater. Proper vent sizing is important to handle the exhaust gases effectively and prevent any performance issues.

• Horizontal Installation: For horizontal venting, the vent pipes should slope downward, directing exhaust gases away from the heater and out through a sidewall. A 90-degree elbow directs the gases outside, with another elbow at the terminal to finish the venting system. This setup helps to maintain a smooth flow of exhaust while keeping everything properly sealed.
• Vertical Installation: In a vertical installation, the vent pipes slope upward to carry the exhaust gases to the top of the building. These systems include a condensation trap and often a rain cap or elbow at the end to prevent water from entering the pipe and damaging the system.

For non-condensing units, the vent should slope downward to prevent acidic condensate from pooling and damaging the pipes. In contrast, condensing units are designed to have vents that slope toward the heater, with a condensation trap to manage the liquid that forms during the heating process.

Tip: It’s always a good idea to install vent screens at the terminal to keep debris, such as leaves or insects, from entering the system and causing blockages. This small step can save you from potential issues down the line and help maintain your heater’s efficiency.

Common Venting Issues and Troubleshooting

Blockages or Obstructions in Vent Pipes

One of the most common issues with tankless water heater venting is the buildup of debris, dust, or soot inside the vent pipes. This can block the flow of exhaust gases, leading to poor performance and even increasing the risk of carbon monoxide backdrafting into your home.

To prevent this, check your venting system regularly, especially in areas prone to dust or animals. If you notice a drop in hot water output or hear unusual sounds, inspect the vent pipes for blockages. Cleaning or replacing them as needed will help keep your system running smoothly.

Corrosion or Damage to Vent Materials

Over time, heat and moisture can cause venting materials, like PVC or metal piping, to corrode. This damage can lead to leaks, which not only reduce the heater’s efficiency but also risk allowing exhaust gases into your home.

Regularly inspect your venting system, especially in humid environments or areas prone to corrosion. If you spot any corrosion or visible damage, replace the affected parts right away to prevent safety hazards and restore your heater’s full functionality.

Improper Installation Leading to Backdrafts

Unprofessional installation of your vent system can cause dangerous backdrafting, where harmful exhaust gases, including carbon monoxide, get pulled back into your home instead of being expelled outside. This can happen if vent pipes are the wrong size, routed poorly, or lack proper clearance. If you notice unusual smells, reduced heater performance, or condensation around the vent, it may be a sign of backdrafting.

Check that the venting system follows the manufacturer’s guidelines and local codes, and consult a professional if backdrafting occurs to ensure everything is installed correctly for safe operation.

Key Venting Guidelines

• Avoid dips or sags in horizontal venting by using proper supports.
• Keep vent paths as straight and direct as possible, with minimal bends.
• Ensure airtight seals on vent connections to prevent gas leaks.
• Do not connect the venting system to an existing chimney.
• Follow manufacturer guidelines for vent distances from doors, windows, and other objects.
• Combustion air must remain free of corrosive chemicals.
• Check the vent system annually for leaks, blockages, and damage.
• White smoke from exhaust vents is normal.

Warning: Improper tankless water heater venting can lead to carbon monoxide poisoning. Always prioritize safety.

Photos: hotwatertalk.com