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Tankless Water Heater Venting

Although tank-style water heaters can be vented through the wall of a house, tankless hot water heaters require special venting to blow hot exhaust gas outside, where it dissipates. Tankless hot water heaters provide greater energy efficiency and more controllable capacity than their predecessors, but they also demand strict adherence to local building codes and manufacturer recommendations for proper installation and safe operation. Tankless gas units require connections to an exterior vent and need new ventilation installed if they replace a traditional electric model.

Tankless Water Heater Venting Requirements:

  • Tankless units must be mounted on a stable base with at least 3 inches of clearance on the top and sides.
  • Tankless hot water heater venting must be done in accordance with local and national building codes and manufacturer’s installation instructions.
  • Tankless gas heaters may require special inspections, called “sanitation” or “opex,” before they can receive an operational permit from the city or county government.
  • Tankless vents must be at least 1-1/4 inches in diameter and made of either stainless steel or PVC (polyvinyl chloride).
  • Tankless units are vented to the outside via a protruding metal pipe that has an opening on the side pointing away from the house. The pipe should always point downward to allow condensation to drain back into the unit.
  • Tankless water heaters are equipped with internal heat traps to help purge condensed water from the system.
  • Tankless unit vents should slope downward at least 1/8 inch per foot, and include a screen at the opening to keep out insects.
  • Tankless hot water heater venting must be as short as possible and not share any portion of its path with other exhaust vents or appliances such as wood stoves or fireplaces. Vents that share a common trunk line with another appliance must be positioned at least 4 inches away from anything else in the wall.
  • The unit’s manual provides specific recommendations for connecting to existing gas lines, usually via flexible tubing.
  • Tankless gas heaters almost always require dedicated circuits and electric outlets for outdoor use, so they can only replace an electric water heater if there is a separate circuit available.
  • Tankless units that replace electric heaters can vent through the house wall, but need special gas hookups at the exterior to connect with the water heater.
  • Tankless hot water heater vents less than 30 feet long must be equipped with a backdraft damper or “flue collar.” This component helps prevent exhaust from leaking into a home when the unit is turned off and traps any gases inside the venting system so they do not escape into a building’s interior.
  • Tankless models over 30 feet in length have no such requirements.

Tankless Water Heater Venting Permitted:

  • Tankless hot water vaporizers may only be vented as described above, which includes the use of a backdraft damper.
  • Tankless vents that do not include these devices cannot legally be installed or operated in some areas and may violate building codes.
  • Tankless units without flue collars must be turned off during periods when the home is unoccupied to reduce energy loss and fire risk, and can only operate with combustion air from an opening vented to the outside.
  • Tankless hot water heaters must also be inspected by a qualified HVAC contractor any time they are serviced inside a garage, crawlspace or attic to ensure there is no combustible vapors present in the exhaust system before it is turned on.
  • Tankless water heater venting systems that contain PVC piping or stainless steel fittings can last for decades, so long as they are free of dents, rust or other damage that would restrict airflow.
  • Tankless water heater vents made with galvanized steel can deteriorate more quickly if they are exposed to acidic elements like rainwater, which is why some codes specify the use of aluminum or copper chimneys in some areas.
  • Tankless exhaust systems must slope downhill toward the outside at a minimum of 1/8 inch per foot, and all piping joints should be sealed with gas-resistant tape or heat-proof putty.
  • Tankless unit installation manuals usually include specific instructions for attaching any necessary fittings to existing gas lines.

Tankless Water Heater Venting Not Permitted:

  • Tankless hot water vaporizers cannot share venting components with furnaces or other combustion equipment.
  • Tankless water heater vents must not be connected to another water heater, even if it is a gas tankless unit.
  • Tankless hot water vaporizers cannot be upgraded with venting devices designed for use with conventional water heaters, and piping can only attach to standard gas supply lines in three places: the home’s exterior main shut-off valve, halfway between the house and where the gas meter connects to the building, and at the location where natural gas service enters the home.

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