8 Facts to Know About Venting Tankless Water Heaters

If venting concerns are keeping you from installing tankless water heaters on your projects, these tips might help you find a simpler, less expensive solution.

For many of your customers and projects, selecting a gas tankless water heater is a smart decision. Propane tankless water heaters are one of the most efficient water heating options available, allowing homeowners to reduce energy costs by up to 50 percent and reduce greenhouse gas emissions by up to 61 percent compared with standard electric water heaters. But in some cases, misconceptions about the venting needs of gas tankless units might keep construction professionals from recommending them. Before you decide against a tankless water heater for your next project, consider these eight facts about venting.

1. Gas tankless water heaters don't have to use indoor air for combustion. Tankless water heaters can be vented in two ways: power-vent or direct-vent. Power-vent units use indoor air for combustion and simply vent the exhaust to the outside. Direct-vent units pull in air from outside the house, so they have two vents for intake and exhaust. While power-vent units require only an exhaust vent, they create additional placement concerns: They must be placed in a large-enough room or a room with vents or louvers so that they have adequate make-up air for the gas combustion. Because direct-vent units use outdoor air, they can be placed in smaller spaces, such as an attic, closet, or small mechanical room.


In this typical water-heating system replacement scenario, workers install a two-unit propane tankless system in place of an aging and under-sized electric storage water heater.

2. You don't always need two ventilation pipes, even for direct-vent units. Direct-vent water heaters can use two separate pipes for intake and exhaust, but some manufacturers offer concentric venting, a single pipe that contains an inner exhaust vent and an outer intake vent. Concentric vents provide a couple of advantages, Trey Hoffman, global product manager for Rinnai, says. First, with only one pipe, installers only need to make one penetration in the wall or ceiling. Second, unlike exhaust vent pipes, which are hot to the touch and thus require clearance to avoid contact with the wall, concentric vents are cool to the touch because they keep the warm exhaust air on the inside. So while concentric vents are larger—typically 5 inches in diameter, versus 3 inches for a single pipe—they don't require additional clearance through the wall. Concentric vents also offer a safety benefit, Hoffman says. If the inner exhaust vent develops a leak, the exhaust air will stay contained within the intake pipe and can't enter the home.

3. You don't have to go through the roof. Traditional (tank) gas water heaters vent through the roof using galvanized steel B-vents because they work through natural draft, allowing the hot exhaust air to rise up and out of the house. By contrast, tankless water heaters' vents can terminate on a side wall because their combustion fan blows exhaust from the units horizontally. That fact is particularly helpful to remodelers or professionals replacing electric tanks (which don't require venting) with propane tankless units, Hoffman says. "If you had to go through the roof every time, it would make the venting part of your installation much more expensive and difficult," he says. "The fact that you can go out a side wall means that you have a lot more flexibility in where you put the unit. Now all you have to do is move the plumbing around a bit to accommodate the new position of the water heater."

Tankless water heaters, such as this unit from Rinnai, can be easily vented through a side wall.

4. With an outdoor unit, you don't need to vent at all. In warmer climates, it's easy to install a tankless water heater outdoors, with no additional venting required. Tankless units are designed to withstand below-freezing temperatures through self-warming capabilities that prevent freezing and cracking. (Because the heating elements run on an electrical supply, however, tanks can freeze in very cold climates where electrical outages occur, making indoor installations a better option for those locations.) Replacing a tank water heater with an outdoor tankless unit can even free up indoor floor space, Tommy Olsen, Rheem's market manager for tankless and specialty products, says. "You pick up 9 square feet on the floor, but you also pick up that floor-to-ceiling cube," he says. "One of the guys I work with replaced the tank in his closet with a tankless unit outside, and now he's got additional pantry space."

5. With a condensing tankless water heater, you don't need metal venting. Non-condensing tankless water heaters typically transfer to the water only about 80 percent of the heat they generate. The remaining heat creates a hot exhaust gas that requires metal venting, typically stainless steel or thick aluminum. Condensing units, on the other hand, are typically about 95 percent efficient, so the temperature of the exhaust gas is lower — around 110 to 120 degrees. That means they can be vented with a less expensive plastic, generally PVC or polypropylene. In fact, the price difference in the venting can even offset the cost of the higher-efficiency unit, Olsen says. "The overall installed cost of a high-efficiency unit is typically equal to or lower than that of a mid-efficiency product, so it's an easy up-sell at that point."

6. You don't have to have a box stuck to your wall. For new-construction applications, some manufacturers offer recess boxes to keep the tankless water heater inside the wall. At 14 inches wide, non-condensing units can fit between conventional studs; 18-inch-wide condensing units may require more creative framing. "Now your water heater is flush with the outside of the house," Hoffman says. "It makes for a very neat and tidy solution."

7. You don't always need separate vents for multiple water heaters. While commercial and large residential applications may use multiple tankless units, they don't necessarily require two vent penetrations per unit. Rinnai is developing a common-venting system that uses a manifold to share the same exhaust and intake vents for up to eight tankless units, a useful option for projects where pros want to avoid extra penetrations in the building envelope for practical or aesthetic reasons.

8. Your venting system doesn't have to be ugly. Several manufacturers have designed aesthetically pleasing vent options. "You've got people that really like the tankless option, but at the end of the day, they don't want that pipe sticking out of the wall," Olsen says. With attractive pipe covers and termination points available, beauty's no longer a reason to shun tankless water heaters.

Check out these courses at the Propane Training Academy to learn more about the advantages and attributes of tankless water heaters:

A Comparative Analysis of Residential Water Heating Systems (Available in on-demand webinar or PDF format)

Condensing Tankless Water Heaters: Using Propane for the Most Efficient Water Heaters on the Market (Available in PDF format)

Water Heaters: Retrofitting from Standard Electric to Gas Tankless (Available in on-demand webinar format)


8 Facts to Know About Venting Tankless Water Heaters
 
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