Does a tankless water heater require new venting?
Venting is one of the biggest differences between an old storage tank and a modern tankless water heater, and it catches many homeowners by surprise. That old tank likely vented up a chimney or metal flue that’s been there for years. A tankless unit almost always needs something different.
Why? Because high-efficiency tankless units burn fuel so completely that their exhaust is cooler and chemically different, requiring dedicated venting designed for it. Getting this wrong isn’t just an efficiency issue; it’s a safety one.
HeatLand is TSBC-certified for gas fitting and has installed properly vented tankless systems throughout Metro Vancouver and the Fraser Valley since 1999. Here’s what you need to know about venting a tankless heater.
Why old venting rarely works
Traditional storage tanks are relatively inefficient, so their exhaust is hot. That hot exhaust rises naturally up a chimney or B-vent, carrying combustion gases safely outside. The old venting in your home was built for exactly that.
Modern high-efficiency tankless units are condensing appliances. They extract so much heat from the burning gas that the exhaust comes out cool, and as it cools it condenses into an acidic liquid. That acidic condensate would corrode a standard metal vent or masonry chimney, and the cool exhaust no longer rises reliably on its own.
For these reasons, a condensing tankless unit requires dedicated venting made from corrosion-resistant material, usually specialized plastic pipe, and a way to drain the condensate safely. Reusing an old chimney simply isn’t compatible with how these units work. This is why new venting is the norm rather than the exception on a tankless conversion.
Types of tankless venting
Gas tankless units generally use one of two venting approaches. Direct venting, also called sealed combustion, uses two pipes: one draws combustion air from outside and the other expels exhaust. Because it doesn’t use indoor air for combustion, direct venting is efficient and well suited to tight modern homes and locations with limited air supply.
The alternative uses indoor air for combustion and a single pipe for exhaust, which requires adequate ventilation in the space around the unit.
Condensing units also need a condensate drain to carry away the acidic liquid the exhaust produces, routed to an appropriate drain. The vent typically runs a short, direct path through an exterior wall, which is often simpler than the old vertical chimney route. Electric tankless units, by contrast, produce no combustion gases and need no venting of any kind, one reason they’re common where venting is difficult. Your installer determines which approach fits your unit and home layout.
Why correct venting is a safety essential
Venting a gas appliance is not a place to cut corners. The vent’s entire job is to carry combustion gases, including carbon monoxide, safely out of your home. Carbon monoxide is colorless and odorless, and exposure can be fatal, so a properly designed and installed vent is a genuine life-safety component.
Using the wrong vent material, improper routing, inadequate slope for condensate, or a poorly sealed joint can all allow exhaust to leak indoors or cause the unit to malfunction. This is exactly why venting must be installed by a licensed, TSBC-certified professional and inspected, never improvised.
A working carbon monoxide detector near your gas appliances is also strongly recommended. If a CO alarm sounds, if you smell exhaust, or if you feel unusual headaches or nausea near the unit, leave the area and call for help immediately. HeatLand installs and verifies tankless venting to code so your family stays safe, and we service all major brands across Metro Vancouver and the Fraser Valley.
Quick Checklist
- Confirm whether your unit is condensing (needs special vent)
- Plan corrosion-resistant venting, not an old chimney
- Provide a proper condensate drain for the exhaust
- Choose direct venting for tight or air-limited spaces
- Have venting installed by a TSBC-certified technician
- Install a carbon monoxide detector nearby
| Unit Type | Venting Requirement |
|---|---|
| Condensing gas tankless | Dedicated corrosion-resistant vent + condensate drain |
| Non-condensing gas tankless | Dedicated heat-rated vent |
| Direct-vent gas tankless | Sealed two-pipe (air in, exhaust out) |
| Electric tankless | No venting required |
Frequently Asked Questions
Can I reuse my old chimney for a tankless heater?
Generally no. Modern condensing tankless units produce cool, acidic exhaust that would corrode a chimney or standard metal vent and won’t rise reliably on its own. They require dedicated corrosion-resistant venting and a condensate drain. A licensed technician confirms the correct venting for your specific unit.
Do electric tankless heaters need venting?
No. Electric tankless units produce no combustion gases, so they require no venting at all. This is one reason they’re popular in condos and locations where exterior venting is difficult. Their main requirement is a substantial dedicated electrical circuit and adequate panel capacity instead.
What is direct venting on a tankless heater?
Direct venting, or sealed combustion, uses two pipes: one brings combustion air in from outside and the other carries exhaust out. Because it doesn’t draw indoor air, it’s efficient and well suited to tight, energy-efficient homes and spaces with limited ventilation. A licensed installer determines if it fits your setup.
Reviewed by the HeatLand Heating & Cooling team — licensed HVAC contractor serving Metro Vancouver & the Fraser Valley since 1999. This article is for general guidance only. If you smell gas, suspect a carbon monoxide leak, or are unsure about any electrical or gas component, stop troubleshooting immediately and call a licensed technician or 911.