More and more homeowners are seeing the benefits of gas water heater venting options over standard storage-tank water-heating units. Tankless products bring drastic improvements in efficiency, cost, and nonstop performance when compared to their tanked counterparts.

But while they see the benefits, questions or misconceptions about installation may keep some homeowners — and their plumbing contractors — from making the upgrade to tankless.

“What has been a challenge is the need to vent,” says David Federico, brand director of tankless manufacturer Rinnai America. “Depending on where the water heater is installed, it can add cost and complexity to a project.”

Early units, he says, required stainless-steel venting, which greatly upped installation costs. But today, advances in tankless designs, including the introduction of condensing units, afford contractors greater flexibility in venting materials and venting designs.

Learn the answers to these five frequently asked questions surrounding venting tankless water heaters so you understand these opportunities — and can recommend a propane tankless upgrade with confidence.

Q. How do you determine which venting materials to use?

A. In most cases, PVC makes a great venting material. It’s the least expensive option and is easy to install. And it’s a good reason to consider a high-efficiency condensing unit.

“Higher-efficiency condensing units usually allow plastic vent materials, while mid-efficiency non-condensing products typically require metal,” says Trevor Pinto, product manager for water heater manufacturer Bradford White. “You always want to refer to the installation and operation manual that shipped with the appliance and your local authority having jurisdiction (AHJ) just to be safe.”

The reason for the difference is because higher-efficiency units reuse heat from the exhaust gas, making the exhaust cooler. Lower-efficiency units have higher-temperature exhaust that’s above the limit for PVC. “The limit for PVC pipe is 149 degrees Fahrenheit,” says David Hoskyn, national training manager at Navien. “If the exhaust temperature is higher, you must address venting in another way.” And check your local code: In some areas, PVC is no longer allowed, Hoskyn says.

Hoskyn adds that if exhaust exceeds allowable temperatures, PVC will start to break down, compromising the integrity of the venting. He recommends a tankless unit with a built-in safety mechanism that powers down the appliance just in case temperatures go above the limits for PVC.

The Rinnai Sensei tankless water heater offers 14 possible venting configurations and can use PVC, chlorinated polyvinyl chloride (CPVC), or polypropylene venting.

Chlorinated polyvinyl chloride (CPVC) piping and polypropylene venting eliminate this worry. They tolerate higher temperatures up to 194 degrees and 230 degrees Fahrenheit, respectively, and can be used with condensing tankless units. However, polypropylene offers more corrosion resistance and is fully recyclable. Its lighter weight and greater flexibility make it a suitable choice for many applications.

Q. What pipe diameter is required for venting?

A. Depending on the efficiency, most manufacturers approve multiple diameters for their appliances. But the overall length of the intake and exhaust may make it more cost-effective to go with a smaller vent size. It is always important to review the appliance’s installation and operation manual to ensure you select the appropriate vent size to avoid future problems. A manufacturer, for example, may allow a choice of 2-inch or 3-inch venting diameters but then limit a 2-inch pipe to 60 feet and 3-inch pipe to 150 feet.

Q. How can I reduce penetrations through the wall?

A. National code mandates that gas-fired appliances have 50 cubic feet of combustion air per 1,000 Btus. If the room where the water heater is installed has enough combustion air, an intake vent is unnecessary. But if the area lacks adequate combustion air, contractors can choose a direct-vent unit with both an exhaust vent and an intake vent to pull in air from outside.

The intake and exhaust pipes must be placed far enough apart to avoid pulling exhaust gas back inside. “Units need good, clean combustion air to run properly,” Hoskyn says.

Concentric vent kits can keep penetrations to a minimum by allowing both intake and exhaust vents to pass through a side wall together. “It’s basically a vent inside a vent,” Federico says.

This type of venting has a larger pipe diameter. “But instead of making two smaller holes, you make one larger hole,” Pinto says. “This can be an advantage in a building constructed of a material that is difficult to cut through. It is more efficient and cost-effective to cut one hole versus two.”

Contractors may be able to use an existing chase or sleeve to run exhaust out through old venting. However, Pinto says this works only if the tankless water heater is approved for the same vent material, pipe dimension, and minimum and maximum lengths, and is approved by the local AHJ.

Concentric vent kits can keep penetrations to a minimum by allowing both intake and exhaust vents to pass through a side wall together.

Q. If multiple tankless water heaters are used, is a separate intake and exhaust vent required for each one?

A. Multiple units can be common vented. This means contractors can combine the intake and exhaust vents into two common pipes.

“If I have four units and I couldn’t common vent, I’d have eight penetrations through a wall,” Hoskyn says. “But with common venting, I can put all of them into one exhaust pipe and one intake pipe and have two penetrations.”

The more units, the larger the common vent’s diameter. Contractors may need a 6-inch pipe instead of a 3-inch one. Contractors can save money by using smaller diameter pipe until the venting connects to a common vent.

If aesthetics are a concern, contractors can run the intake common vent out one side wall and the exhaust common vent out another. Not all manufacturers approve this, making it important to check installation requirements. Contractors can also tie the two together with a concentric vent kit.

Common venting is not approved for all tankless appliances. “Make sure you investigate ahead of time and know what your product can and cannot do,” Pinto says.

Q. What are the clearance considerations for piping leaving the building?

A. “I am a proponent of roof venting because it gets exhaust gases up and away,” Hoskyn says. But roof venting can be difficult and expensive, especially if a unit is installed in the basement. In those cases, it can be beneficial to vent through a side wall, using the unit’s combustion fan to blow exhaust from the unit horizontally.

Locating the venting with proper clearances ensures exhaust does not make its way back into a building. Exhaust vents should not be installed under a bank of windows or where they are blocked by vegetation or snow. In these cases, contractors may want to run the vent up a little higher or through the roof. Consult the manufacturer recommendations and national and local codes for the minimum clearances.

With the right venting solution for your application, you can plan for a propane tankless water heater upgrade that’s fast, safe, and affordable for your clients — so they have the opportunity to enjoy their lower energy bills and endless hot water.

Top photo courtesy of Rinnai.

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Ronnie Wendt