8 things to know about venting tankless water heaters
Don’t let these venting misconceptions hold you back.
For many customers and projects, selecting a propane tankless water heater is a smart decision.
Propane tankless water heaters are one of the most efficient water heating options available, providing a reduction in energy costs of up to 50%. In addition, these systems can reduce greenhouse gas emissions by up to 61%.
But in some cases, misconceptions about the venting needs of gas tankless units might keep professionals from recommending them. 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.
1. Propane 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 the outside, 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.
2. Tankless water heaters 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. 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 vs. 3 inches for a single pipe — they don’t require additional clearance through the wall. Concentric vents also offer a safety benefit, too. If the inner exhaust vent develops a leak, the exhaust air will stay contained within the intake pipe and thus cannot enter the home.
3. Tankless water heater venting through roof is not necessary. Storage tank propane 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 facility. By contrast, tankless water heaters’ vents can terminate on a side wall because their combustion fan blows exhaust from the units horizontally. This fact is particularly helpful for professionals replacing electric tanks — which don’t require venting — with propane tankless units.
Having to go through the roof every time makes the venting part of installations more expensive and difficult. The ability to vent through a sidewall means you have a lot more flexibility in where you put the unit, then you just have to move the plumbing around to accommodate the new position of the water heater.
4. Outdoor tankless water heaters don’t need venting. 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. Plus, replacing a tank water heater with an outdoor tankless unit can free up indoor floor space.
5. With a condensing tankless water heater, you don’t need metal venting. Noncondensing tankless water heaters typically transfer to the water only about 80% 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% efficient, so the temperature of the exhaust gas is lower — around 110° to 120° F. That means they can be vented with a less expensive plastic, generally PVC or polypropylene. The price difference in the venting can even offset the cost of the higher-efficiency unit. According to one manufacturer, 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. Placement for tankless water heaters doesn’t always have to be a box stuck to a wall. For new-construction applications, some manufacturers offer recess boxes to keep the tankless water heater inside the wall. At 14 inches wide, noncondensing units can fit between conventional studs; 18-inch-wide condensing units may require more creative framing.
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. One manufacturer developed 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. Many people really like the tankless option but at the end of the day, they don’t want that pipe sticking out of the wall. Luckily, with attractive pipe covers and termination points available, that’s no longer a reason to pass on tankless water heaters.
For more information on tankless water heaters, visit Propane.com/Water-Heating.